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Chapter 8. Marine Photochemistry of Organic Matter
Abstract
Dissolved organic matter (DOM) is the strongest light-absorbing component of seawater, especially in coastal regions, and therefore it plays a dominant role in marine photochemical and photophysical processes in surface waters. This critical review focuses on the impact of DOM photochemistry on marine biogeochemical processes, highlighting and evaluating recent advances and areas for future work. Specific topics reviewed include: (1) coupling photochemical and microbial processes; (2) photochemical dissolved inorganic carbon (DIC) formation and oxygen consumption; (3) carbon monoxide photoproduction; (4) role of photochemistry in the sulfur, nitrogen, and phosphorus cycles; (5) mechanisms of DIC photoformation and DOM oxidation; (6) particle and sea-ice photochemistry; (7) photochemical transformations of siderophores and toxins; and (8) modeling photochemical rates. Finally, several avenues of future work are discussed including the need for mechanistic studies, photoproduction, and air-sea exchange of important atmospheric trace gases, DOM marine food web dynamics and trace metals, photodissolution, and photoflocculation of particles, and improved quantification of photochemical rates.
... A dynamic pool of carbon in flux -After being long considered a static pool of refractory carbon, DOC is now recognized as a dynamic component of the carbon cycle (Fig. 1). Throughout the aquatic continuum, DOC is produced, transformed, and mineralized to CO 2 by a combination of biotic and abiotic processes, and transported laterally and vertically by physical processes (Carlson and Hansell, 2015;Mopper et al., 2015;Najjar et al., 2018). Together, these processes sustain large fluxes of DOC that biogeochemically connect terrestrial landscapes to the deep ocean. ...
... The vertical export of DOC from the surface ocean to the interior and its isolation from atmospheric exchange contributes significantly to the biological carbon pump (5-10% of the vertical flux below 500 m) and the sequestration of carbon in the deep ocean on timescales of hundreds to thousands of years (Hansell et al., 2009). In the sunlit ocean, the strong absorption of high-energy UV radiation by CDOM makes it a precursor for photochemical reactions leading to the net oxidation and degradation of DOC, the generation of nutrients, and the production of dissolved inorganic carbon (DIC) and other climaterelevant volatile species (e.g., COS, CO, CH 4 ), among other processes (Mopper et al., 2015). Photochemical processing also increases the bioavailability of terrigenous DOC and enhances its mineralization (Grunert et al., 2021;Logozzo et al., 2021;Moran and Zepp, 1997;Obernosterer and Benner, 2004). ...
... The absorption of solar radiation by CDOM in the surface ocean triggers a multitude of photochemical reactions that alters the composition, structure, and reactivity of DOC, and produces a "soup" of shortlived radicals and stable photoproducts (Mopper et al., 2015). Overall, these reactions lead to a direct loss of DOC through transformations into dissolved inorganic carbon (DIC: CO 2 , HCO 3 − , CO 3 2− ), a suite of inorganic trace gases (e.g., carbon monoxide, carbonyl sulfide, carbon disulfide), or volatile organic compounds (e.g., methyl iodide, methane, isoprene, acetaldehyde, glyoxal) that can vent to the atmosphere. ...
... Under the framework of the MCP, and due to the apparent resistance of this OM pool to microbial remineralization , the production of RDOM constitutes a relevant C-storage mechanism that limits the return of DIC from the ocean to the atmosphere . However, the photoexposure in the surface results in direct photomineralization and in enhanced microbial degradation of this otherwise recalcitrant DOM pool Mopper et al., 2015). Thus, due to its photolability, the C-sequestration capacity of RDOM depends on the depth at which is produced. ...
... These two fractions of fluorescent organic matter have been extensively studied and their sources and sinks in marine systems are relatively well identified (Nelson and Gauglitz, 2016). In the open ocean, away from terrestrial inputs, surface humic-like FOM composition is mainly controlled by solar radiation, which supresses its visible fluorescence in a process known as photobleaching (Dainard et al., 2015;Mopper et al., 2015). Marine organisms also modify visible FOM composition through the production of humic-like substances by phytoplankton (Romera-Castillo et al., 2010;Fukuzaki et al., 2014) and the production and consumption of a variety of humic-like substances by heterotrophic prokaryotes (Romera-Castillo et al., 2011). ...
... can be easily integrated in autonomous monitoring devices such as ARGO floats and gliders(Moore et al., 2009; Carstea et al., 2020).Two main categories of fluorescent organic matter (FOM) substances can be distinguished: humic-like FOM, composed of highly condensed, C-rich aromatic substances that fluoresce in the visible spectrum, and protein-like FOM, with fluorescent signals in the ultraviolet spectrum similar to those of the aromatic amino acids tryptophan and tyrosineCoble, 2007; Martínez-Pérez et al., 2017).These two fractions of fluorescent organic matter have been extensively studied and their sources and sinks in marine systems are relatively well identified(Nelson and Gauglitz, 2016). In the open ocean, away from terrestrial inputs, surface humic-like FOM composition is mainly controlled by solar radiation, which supresses its visible fluorescence in a process known as photobleaching(Dainard et al., 2015;Mopper et al., 2015). Marine organisms also modify visible FOM composition through the production of humic-like substances by phytoplankton(Romera-Castillo et al., 2010; Fukuzaki et al., 2014) and the production and consumption of a variety of humic-like substances by heterotrophic prokaryotes(Romera-Castillo et al., 2011). ...
Marine phytoplankton are responsible for approximately half of the photosynthetic production of organic matter (OM) and oxygen in Earth. The composition and reactivity of phytoplankton- derived OM influences two of the main C-sequestration mechanisms of the ocean: the biological carbon pump and the microbial carbon pump. Phytoplankton-derived OM can be classified as particulate (POM) or dissolved (DOM) and these size-fractions are subject to diverse production, consumption and transport processes involving biotic and abiotic interactions. Understanding how these processes influence OM composition and reactivity is essential to accurately describe the role of phytoplankton ecology in the marine Carbon cycle and ultimately in the regulation of Earth climate. This thesis aims, precisely, to better understand the controls over these processes. To do so, we combined fluorescence spectroscopy and elemental analysis of POM and DOM with multiple biotic and abiotic parameters during the development and decay of phytoplankton proliferations in micro- and mesocosm experiments and under natural conditions. The microcosm degradation experiment revealed that POM derived from diatom-dominated proliferations is degraded at a much slower rate than that of POM produced by a mixed phytoplankton community. In addition, accumulation of DOM of apparent recalcitrant nature was observed during the processing of diatom-derived POM. The analysis of four phytoplankton proliferations in Antarctic waters revealed that protein-like fluorescent OM was contributed by dissolved and particulate materials. The abundance and composition of phytoplankton and their interactions with viruses and grazers were identified as the main controls over the quantity and fractionation of protein-like fluorescent OM. By contrast, humic-like substances were mostly in the dissolved fraction, and their composition was related to photochemical degradation and microbial transformation. The mesocosm experiment showed that the balance between production and degradation of protein-like fluorescent DOM was controlled by the nitrogen availability of the planktonic community. Whereas the humic-like fluorescent DOM composition was influenced by photochemical processes and production of specific humic-like substances by autotrophic and heterotrophic prokaryotes, the taxonomic composition of eukaryotic phytoplankton did not have a profound influence over the fluorescent DOM composition. Overall, this thesis shows that the composition of plankton assemblages, and the interactions between organisms and between organisms and environmental conditions influence the composition and reactivity of phytoplankton-derived OM, ultimately determining its fate and role in the marine Carbon cycle.
... These estimates are restricted to the open ocean, thereby excluding potential CO hotspots that may increase the relative importance of processes driven by UV radiation. Specifically, coastal and freshwaters have more terrestrial DOM than the open ocean and produce CO more rapidly than marine DOM ( [120] and references therein). High latitude watersheds in spring may also represent an overlooked source of this gas, as CO is always produced alongside CO 2 during DOM photodegradation (CO 2 /CO ratio ranges from 4 to 73 [121]). ...
... The average value is comparable to previous estimates for boreal lakes [140]. Substrate limitations [139] and variations in DOM chemistry across seasons [141] can justify contrasting literature results in DON photomineralisation-as ammonia photoproduction has been inconsistently observed across studies (see [120] and references therein). ...
... In addition to nitrogen species, UV-irradiation of dissolved and particulate organic matter in aquatic ecosystems releases phosphate [120,142], which may fuel algal blooms. This process is particularly prevalent in shallow eutrophic lakes, where sediments have high phosphorous loads, are resuspended frequently, and are thus susceptible to photochemical processing [142,143]. ...
Variations in stratospheric ozone and changes in the aquatic environment by climate change and human activity are modifying the exposure of aquatic ecosystems to UV radiation. These shifts in exposure have consequences for the distributions of species, biogeochemical cycles, and services provided by aquatic ecosystems. This Quadrennial Assessment presents the latest knowledge on the multi-faceted interactions between the effects of UV irradiation and climate change, and other anthropogenic activities, and how these conditions are changing aquatic ecosystems. Climate change results in variations in the depth of mixing, the thickness of ice cover, the duration of ice-free conditions and inputs of dissolved organic matter, all of which can either increase or decrease exposure to UV radiation. Anthropogenic activities release oil, UV filters in sunscreens, and microplastics into the aquatic environment that are then modified by UV radiation, frequently amplifying adverse effects on aquatic organisms and their environments. The impacts of these changes in combination with factors such as warming and ocean acidification are considered for aquatic micro-organisms, macroalgae, plants, and animals (floating, swimming, and attached). Minimising the disruptive consequences of these effects on critical services provided by the world’s rivers, lakes and oceans (freshwater supply, recreation, transport, and food security) will not only require continued adherence to the Montreal Protocol but also a wider inclusion of solar UV radiation and its effects in studies and/or models of aquatic ecosystems under conditions of the future global climate.
Graphical abstract
... The complex and highly diverse nature of DOM intrinsically relates the cycles of key macronutrients: C, N, P, and S. While multiple studies have focused on the photoproduction of low molecular weight (MW) compounds and the photo-mineralization of dissolved organic carbon (DOC), 19 DON, 19,20 and dissolved organic phosphorus (DOP), 20 less attention has been given the photo-transformation of DOS constituents of DOM (e.g., S-containing amino acids). S is often not limiting in freshwaters; 21,22 however, recent estimates suggest that average cellular requirements are similar to that of phosphorus (C:N:P:S = 124:16:1:1.3). ...
... The complex and highly diverse nature of DOM intrinsically relates the cycles of key macronutrients: C, N, P, and S. While multiple studies have focused on the photoproduction of low molecular weight (MW) compounds and the photo-mineralization of dissolved organic carbon (DOC), 19 DON, 19,20 and dissolved organic phosphorus (DOP), 20 less attention has been given the photo-transformation of DOS constituents of DOM (e.g., S-containing amino acids). S is often not limiting in freshwaters; 21,22 however, recent estimates suggest that average cellular requirements are similar to that of phosphorus (C:N:P:S = 124:16:1:1.3). ...
... 23,24 Mass spectrometry studies on the photochemical lability of DOS in various environments demonstrated more selective and rapid degradation of 5 CHOS-containing formulas relative to CHO, with conversion of CHOS into CHOcontaining formulas. [25][26][27][28] More recent studies have investigated the photo-production of low MW DOS compounds (e.g., dimethyl sulfide, dimethyl sulfoxide, methanesulfonic acid) 13,19,29,30 and inorganic S (e.g., carbonyl sulfide, carbon disulfide, sulfate) 13,19,29 in aquatic systems. Furthermore, a comparison of the mass spectra of a reference NOM sample with EfOM showed significantly more S-containing molecular formulas (CHOS) unique to EfOM. 4 While environmental DOS concentrations are relatively rare compared to inorganic S in the literature, existing data suggests DOS cannot be ignored in the study of S-cycling. ...
Sulfur (S)-containing amino acids are key sources of carbon, nitrogen, and sulfur involved in protein synthesis, protein function, and providing energy for microbial growth. Dissolved free and combined methionine is one of two S-containing amino acids incorporated into proteins and has been attributed to their stability and function. The oxidation of methionine has received considerable attention given its ubiquitous presence in most biological systems and has been associated with losses in protein function and pathological disorders. In natural waters, methionine is rapidly and selectively taken up by microorganisms to achieve cellular requirements of carbon, nitrogen, and sulfur. The abiotic transformation of methionine is ultimately a sink of key macronutrients and attributed to cycling across environmental compartments. In particular, the photochemical transformation of methionine in the presence of dissolved organic matter (DOM) is an important component of cycling in sunlit surface waters globally, yet knowledge is lacking on the fate and transformation of methionine in the environment. In this study, we investigated the photo-transformation products involved in the photochemical fate of dissolved free methionine in the presence of surrogate and standard isolate dissolved organic matter (DOM). Temperature-dependent, bench-top photolysis experiments under simulated sunlight at 10, 20, and 30 oC were conducted and a wide array of analytical analyses were employed to elucidate transformation products and provide insights into reaction mechanisms. Two surrogate DOM compounds structurally unique and relevant to complex mixtures of DOM were employed, including 1,4-naphthoquinone and 2-naphthaldehyde. The two surrogate DOM have common base structures and critical functional groups known to be important photosensitizers in the natural environment generating photochemically-produced reactive intermediates including excited triplet-state chromophoric DOM, singlet oxygen, and hydroxyl radicals. The quinone and carbonyl functionalities in 1,4-naphthoquinone and aldehyde and naphthalene functionalities in 2-naphthaldehyde generated unique transformation pathways for methionine and novel photo-transformation products were identified, providing key insights into the mechanisms of transformation. Photolysis experiments were expanded to two unique standard isolate DOM (Suwannee River Humic Acid, Elliott Soil Humic Acid) and previously identified transformation products were quantified to validate results in environmentally-relevant solutions of DOM. Mass balance analyses were performed to assess the transformation of key macronutrients including carbon, nitrogen, and sulfur present in methionine.
... Albeit contributing only a minimal fraction of the global CO budget, 1 photochemical production from DOM photolysis is relevant in remote ocean regions or in environments characterized by significant inputs of terrestrial DOM. 1,5 CO is also involved in the production of carbonyl sulfide (OCS) from DOM photolysis. 6 Photochemical processes are significant players in the global OCS budget, but their contributions are not yet well constrained. ...
... 8,11 In addition, CO has been used as a proxy for the photoproduction of CO 2 , CH 4 , and biolabile organic carbon 5,12−16 (even though this approach proved to be inaccurate for CO 2 ) 5,17,18 and for developing models of mixed layer processes. 5,19,20 Despite the interest in CO biogeochemistry, little is currently known about its photochemical formation mechanism. Redden 21 hypothesized that acetone, acetaldehyde, and other carbonyl-containing DOM photoproducts might undergo Norrish type I fragmentation and release CO. ...
... In the presence of OH • or other reactive species, methanol can further oxidize to CO, as it has been observed in several systems. 27,44−47 Our hypothesis that lignin is an important source of CO also agrees with the following observations: Apparent quantum yields for CO production are higher in freshwater compared to seawater (Mopper et al. 5 and refs therein); both CO yields and lignin DOM content generally decrease along a salinity gradient; 5,13,32,48,49 CO photoproduction rates 28 and quantum yields 49 are correlated with DOM aromaticity and the specific ultraviolet absorbance at 254 nm, respectively (even though this correlation is weak for large marine-based data sets); 12,18 and, photodegradation of dead plant leaves can release CO. 50,51 Finally, several authors reported loss of methoxy groups during photodegradation of lignin from different sources (Paulsson and Parkås 52 and refs therein). ...
Carbon monoxide (CO) is the second most abundant identified product of dissolved organic matter (DOM) photodegradation after CO2, but its formation mechanism remains unknown. Previous work showed that aqueous photodegradation of methoxy-substituted aromatics (ArOCH3) produces CO considerably more efficiently than aromatic carbonyls. Following on this precedent, we propose that the methoxy aromatic groups of lignin act as the C source for the photochemical formation of CO from terrestrial DOM via a two-step pathway: formal hydrolytic demethylation to methanol and methanol oxidation to CO. To test the reasonableness of this mechanism, we investigated the photochemistry of eight lignin model compounds. We first observed that initial CO production rates are positively correlated with initial substrate degradation rates only for models containing at least one ArOCH3 group, regardless of other structural features. We then confirmed that all ArOCH3-containing substrates undergo formal hydrolytic demethylation by detecting methanol and the corresponding phenolic transformation products. Finally, we showed that hydroxyl radicals, likely oxidants to initiate methanol oxidation to CO, form during irradiation of all models. This work proposes an explicit mechanism linking ubiquitous, abundant, and easily quantifiable DOM functionalities to CO photoproduction. Our results further hint that methanol may be an abundant (yet overlooked) DOM photoproduct and a likely precursor of formaldehyde, formic acid, and CO2 and that lignin photodegradation may represent a source of hydroxyl radicals.
... Bio-and photo-mineralisation are the main paths for DOP to be the important source of eutrophication. Photochemical formation of phosphate was first reported by Heath (1982, 1979), who observed that sunlight exposure of DOM (dissolved organic matter)-rich, acidic lake water caused the release of phosphate complexed to high molecular weight DOM, and the rate of release of bound phosphate was highly correlated to Fe(III) reduction to Fe(II) (Mopper et al. 2015). Studies also have shown that when the suspended particles in the ocean and lakes are exposed to sunlight, the contents of soluble nutrients such as ammonium ion (NH 4 + ) and phosphate ion (PO 4 3− ) increase obviously, while there is no significant change of them in the dark. ...
... When Heath (1982, 1979) first reported on the photochemical formation of phosphate, they found that the release rate of bound phosphate was highly correlated with the reduction of Fe(III) to Fe(II). (Mopper et al. 2015;Jiang et al. 2016 In the SW medium, a similar result was obtained. The inorganic phosphorus production in the presence of Fe 3+ was higher compared with the result of in the medium without Fe 3+ added, while the inorganic phosphorus derived from AMP degradation in both media were generally low. ...
Environmental context
Organophosphorus (OP) is bioavailable to phytoplankton with photolysis can play an important role in the process. The photolysis behaviour of an OP (adenosine 5′-monophosphate, AMP) in seawater was investigated, and AMP can release inorganic phosphate under environmentally relevant light conditions, indicating OP photodegradation might be important in the phosphorus biogeochemical cycle. The results are helpful to further understand the bioavailability and cycle of OP in marine environment.
Rationale
Organic phosphorus (OP) is a potential source of bioavailable phosphorus for phytoplankton through photolysis and other degradation processes. Therefore, OP photodegradation plays an important role in phosphorus biogeochemical cycle.
Methodology
Taking adenosine 5′-monophosphate (AMP) as a model OP, we investigated the photolysis behaviour in seawater and discussed the mechanism. The photolysis dynamics were studied based on the inorganic phosphorus production at appropriate time intervals, which was analysed by spectrophotometric molybdenum blue method. The effects of medium, light and radicals were investigated.
Results
It was found that AMP can release inorganic phosphate under photosynthetically active radiation and ultraviolet (UV) with UVB being the most reactive band. The degradation of AMP in seawater was lower than that in deionised water under the same conditions, and the fresh seawater was more beneficial than aged seawater. The kinetics could be described by a pseudo-first order equation. Fe³⁺ can promote the photolysis due to the generation of ·OH radicals, while within the range of this study, changes of Fe³⁺ content have no substantial effect on the promotion. The influence of ethanol and tetrahydrofuran as radical inhibitor showed evident inhabitation to the degradation, indicating that ·OH and ¹O2 played an important role in the process, and ·OH seemed more important than ¹O2.
Discussion
OP photodegradation is of importance in the phosphorus biogeochemical cycle. Varying properties of the medium and light can affect the OP transformation in seawater. The results are helpful to further understand the bioavailability and cycle of OP in the marine environment.
... Chromophoric dissolved organic matter (CDOM), the optically active component of dissolved organic matter (DOM), is a major regulator of solar radiation exposure in surface waters ranging from wetlands and rivers to the open ocean. It is also the main precursor for a multitude of ubiquitous photochemical reactions of biogeochemical significance and relevance to climate (Mopper et al., 2015). Its chemical makeup includes a suite of dissolved organic molecules containing conjugated systems, aromatic functional groups, metal complexes, and other components involved in intramolecular charge-transfer (CT) interactions (McKay, 2020;Sharpless and Blough, 2014;Stedmon and Nelson, 2015). ...
... Water temperature mostly affected the overall magnitude of the AQY-M, with little influence on the spectral characteristics, consistent with an earlier report that increasing water temperature enhances photobleaching (Song et al., 2017). This dependence on temperature and its reasonable description using an Arrhenius equation suggest that photobleaching is, at least partially, driven by secondary reactions involving the photochemical production of intermediary radicals (Mopper et al., 2015). The dependence of the AQY-M on the extent/duration of solar exposure is more spectrally complex, although it generally leads to a decrease in photobleaching efficiency and a preferential loss of the triangular pattern as exposure is prolonged. ...
... Because direct measurement of environmental photochemical rates in natural waters is challenging, rates are usually determined in more controlled settings and then extrapolated to the field (e.g., Mopper et al., 2015). To reduce the number of variables, but still maintain the functionality of key drivers, photochemical microcosm and mesocosm studies are employed, with irradiation times varying from hours to years. ...
... The photodissolution of plastics is assessed in quartzware, as quartz allows environmentally relevant wavelengths of ultraviolet (UV) light (>280 nm) to enter. This wavelength range is responsible for most environmental photochemical reactions (Loiselle et al., 2009a;Mopper et al., 2015). ...
Plastics have accumulated in the environment to become a globally significant pool of organic carbon (Stubbins et al., 2021) and a contaminant of ecological concern (McLeod et al., 2021). Most studies still report plastics in terms of counts (i.e., pieces of plastics) though some report masses of plastics, and even fewer report plastic carbon. In a recent review, we assumed plastics to be 83% carbon by mass based on data for oceanic microplastics (Zhu et al., 2020; Stubbins et al., 2021). This overly simplistic conversion allows plastics to be placed in a carbon cycle context, a context critical to plastic-derived dissolved organic carbon (DOC; Figure 1). However, this conversion should be improved to account for variations in the carbon content of different polymers. To make these improvements, studies should report data for sizes, masses, and types of base polymer of plastics collected. These data would allow comparison among studies and facilitate improved accounting for plastics and their fates in the environment. Reporting masses of specific polymers would make conversion from plastics to carbon more accurate and provide a common chemical unit for comparison among plastic polymer types (i.e., we could use carbon as we do biomass).
... This necessarily implies the existence of other light-mediated processes. Part of the unaccounted DOC loss could have been transformed into carbon monoxide (not measured in the present study), another common photoproduct that depends on the presence of certain DOM aromatic groups (Mopper et al., 2015;, although this production would be low (a 1:15 ratio to DIC) according to Moran and Zepp (1997). Most importantly, the natural formation of flocs of organic matter (flocculation) could explain this gap. ...
... Most importantly, the natural formation of flocs of organic matter (flocculation) could explain this gap. Several studies showed that sunlight is a triggering factor for the formation of particles and colloids, particularly in CDOM and iron-rich waters (Gao and Zepp, 1998;Helms et al., 2013;Mopper et al., 2015;Oleinikova et al., 2017;. For example, von reported that the synthesis of particulate organic matter from DOC of a sphagnum-dominated mire water almost reached 0.3 mgC L −1 d −1 . ...
Dissolved organic matter (DOM) leaching from thawing permafrost may promote a positive feedback on the climate if it is efficiently mineralized into greenhouse gases. However, many uncertainties remain on the extent of this mineralization, which depends on DOM lability that is seemingly quite variable across landscapes. Thermokarst peatlands are organic-rich systems where some of the largest greenhouse gas (GHG) emission rates have been measured. At spring turnover, anoxic waters release the GHG accumulated in winter, and the DOM pool is exposed to sunlight. Here, we present an experiment where DOM photoreactivity and bioreactivity were investigated in water collected from a thermokarst lake in a subarctic peatland during late winter (after 6 months of darkness). We applied treatment with or without light exposure, and manipulated the bacterial abundance with the aim to quantify the unique and combined effects of light and bacteria on DOM reactivity at ice-off in spring. We demonstrate that sunlight was clearly driving the transformation of the DOM pool, part of which went through a complete mineralization into CO2. Up to 18 % of the initial dissolved organic carbon (DOC, a loss of 3.9 mgC L-1) was lost over 18 d of sunlight exposure in a treatment where bacterial abundance was initially reduced by 95 %. However, sunlight considerably stimulated bacterial growth when grazers were eliminated, leading to the recovery of the original bacterial abundance in about 8 d, which may have contributed to the DOC loss. Indeed, the highest DOC loss was observed for the treatment with the full bacterial community exposed to sunlight (5.0 mgC L-1), indicating an indirect effect of light through the bacterial consumption of photoproducts. Dark incubations led to very limited changes in DOC, regardless of the bacterial abundance and activity. The results also show that only half of the light-associated DOC losses were converted into CO2, and we suggest that the rest potentially turned into particles through photoflocculation. Sunlight should therefore play a major role in DOM processing, CO2 production and carbon burial in peatland lakes during spring, likely lasting for the rest of the open season in mixing surface layers.
... Whether DOC is labile or recalcitrant can be a function of its molecular structure (Barber 1968;Benner and Amon 2015;Shen and Benner 2020), concentration (Arrieta et al. 2015), nutrient availability (Letscher et al. 2015), or microbial community composition (Carlson et al. 2004). Additionally, a fraction of DOC is photoreactive and can be partially or fully oxidized when exposed to sunlight (Helms et al. 2008;White et al. 2008;Bittar et al. 2015;Mopper et al. 2015;Ward and Cory 2016). ...
Macroalgae are important primary producers in the coastal ocean, and they release a large fraction of their net primary production as dissolved organic carbon (DOC). It is assumed that much of this DOC is recalcitrant and results in the sequestration of large amounts of carbon. We lack sufficient knowledge about the bioavailability of this material and the role of other sinks such as photooxidation. We conducted dark remineralization and photooxidation experiments on DOC derived from an abundant brown macroalga and quantified the fraction of amended DOC that was remineralized by both processes. The bioavailability of the amended DOC ranged from 14% to 99% and was significantly negatively correlated with its phenolic content. Upon exposure to light, the biologically recalcitrant compounds were quickly oxidized to CO2, indicating that photooxidation is an important sink for recalcitrant brown macroalgal DOC. These results are especially important as macroalgae cultivation is being considered to sequester atmospheric CO2.
... This contrasts with ~ 70% of the DOM formulae photodegraded in bacterial-dominated clean snow 38 . Our samples were exposed to light radiations high enough to breakdown photo-reactive DOM 44 , although the experimental bottles filtered out the highest energy of UV-B (see Methods), and thus the magnitude of the photochemical effects are more conservative. Our results suggest that our microalgae-DOM was less photosensitive (Fig. S10) likely due to the algae producing fresher, less aromatic DOM 30 . ...
Surface melting supports the development of pigmented algal blooms on the Greenland Ice Sheet, decreasing albedo and further accelerating melting. The interplay between carbon-fixing algae and carbon-respiring heterotrophic microorganisms ultimately controls the amount and composition of organic matter (OM) and thus the ice and snow color. Yet, the dynamics of microbially-derived OM on the Greenland Ice Sheet remain unclear. To address this knowledge gap, we incubated in situ algae-dominated snow and ice samples under light and dark conditions and characterized the changes in dissolved and particulate OM (DOM and POM) with the help of ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. We show that glacier ice-algae habitats are dominated by highly unsaturated and aromatic compounds resistant to bio- and photo-degradation. In contrary, snow-algae habitats are enriched in bioavailable and more photosensitive unsaturated aliphatics and sulfur- and phosphorus-containing compounds. In both habitats, light exposure increased water-soluble DOM compounds derived from POM, which accounted for ~ 50–70% of the initial DOM composition. Of the initial DOM, 35–50% were heterotrophically degraded in the dark, while light alone photodegraded 6–16%. The significant accumulation of light-absorbing aromatics from POM and DOM at the end of the ice-algae experiments, underscore the greater impact of glacier ice-algae habitats on altering glacier color and accelerating melting.
... For example, photo-produced molecules partially contributed to both bio-labile (n = 5) and bio-refractory (n = 8), indicating that photodegradation of DOM can be persistent or serve as substrates for further biodegradation. This result further supports that photochemical processes can initiate cross-linking of organic compounds into labile biomolecules (Gonsior et al., 2014;Kieber et al., 1989) or recalcitrant substances (Mopper et al., 2015;Obernosterer et al., 1999). ...
Microbial activities drive the cycling of dissolved organic matter (DOM) from labile to refractory states, thus contributing to the long‐term carbon sequestration in the ocean. However, due to the intricate molecular composition of DOM, identifying indicators of microbially related DOM remains a challenge. In this study, we propose molecular candidates for bio‐labile (n = 537) and bio‐refractory (n = 1,025) formulas, which were discerned through incubation experiments using ultrahigh‐resolution mass spectrometry. Bio‐labile formulas exhibited greater hydrogenation, whereas bio‐refractory formulas comprised oxidized, unsaturated and aromatic molecules with higher molecular weight. Bio‐candidates, in contrast to photo‐ or terrestrial‐related counterparts, dominated molecular composition by higher relative intensity. The application of these molecular candidates facilitated the tracing of molecular distribution and transformation patterns across large‐scale aquatic environmental gradients. This molecular identification framework offers insights into resolving microbially mediated molecules and advancing our understanding of biologically related DOM at the molecular level.
... The oxidation of CS2 accounts for 30-75% of the global carbonyl sulfide (COS) 35 budget (Chin and Davis, 1993;Khalil and Rasmussen, 1984;Toon et al., 1987;Whelan et al., 36 2018). COS is the most abundant sulfur compound present in the atmosphere with a lifetime 37 more than one year (Mopper et al., 2015), which is eventually transported to the upper 38 atmosphere and then converted to stratospheric sulfur aerosol (SSA) via sulfur dioxide (SO2) 39 oxidation (Crutzen, 1976;Weisenstein et al., 1997). SSA shields solar infrared radiation and 40 when produce in large amounts by episodic stratospheric volcanic eruptions significantly 41 affects global temperatures. ...
A 1D model of the CS2 reaction network with the addition of the photo-oxidation pathway has been developed and quantitatively studied. The reaction pathway analysis focusing on the sulfur element was applied to determine the importance of the photo-oxidation pathway in the atmospheric CS2 sink resulting in a 15.8% of sulfur in the CS2 reaction network passes through the photo-oxidation pathway under a global average solar radiation conditions and ranging from 8.1% to 18% depending on the irradiance intensity. The concentration of COS and SO2, the main products of CS2 atmospheric oxidation, changed slightly from the sulfur cycle developed with the updated CS2 reaction network. 7.4% of the COS comes from the new pathway and a total of 40.9% of COS comes from the conversion of CS2. A sulfur budget for the main species in the sulfur cycle was constructed, and the CS2 lifetime was estimated to be 2-3 days. The newly added photo-oxidation pathway plays an moderate role in the CS2 reaction network and has a high variability under specific geochemical conditions. The results of this report should be taken as an incentive for 3D climate-chemistry models to account for local COS sources.
... Hu et al. (2020) participation of NO 3 − (Mostofa et al., 2012), which was also supported by the negative correlation between DIN and DOC ( Figure 5B). Although some photoreleased DOM could be further remineralized into CO 2 (Mopper et al., 2015), the increase in DOC concentration ( Figure 2A) and the content of related compounds ( Figures 4A, B) in light cultures display the continuous enrichment of DOM, underlining the important contribution of photochemical transformations of soils to the aquatic DOM pool. ...
Eroded soils sustain a substantial part of organic matter in tidal rivers adjacent to estuaries, and photochemical transformations of soils in tidal rivers would influence estuarine elemental cycles. However, complex aquatic environments and diverse soil sources complicate the enrichment of dissolved organic matter (DOM) photoreleased from soils. Here, we conducted a 7-day irradiation experiment for seven kinds of soils from the lower basin of Dagu River (DGR) in the laboratory to study the influence of salinity and soil properties on DOM chemistry by characterizing the content and optical properties of DOM. Results showed that light cultures had higher amount of DOM and humic-like components than dark cultures. Principal component analysis (PCA) and Mantel’s analysis found that salinity and soil properties significantly influence the production of photoreleased DOM, especially humic-like components. Salinity could inhibit the photodissolution of soils, and aged soils with low δ¹³CSOM released more DOM and humic-like components. Although the DGR is impacted by intruded seawater, high content of photoreleased DOM in seawater cultures still pointed out the important contribution of soil photodissolution to the DOM reservoir of tidal rivers. Considering high proportion of humic-like components in photoreleased DOM, photochemical transformations of soils in tidal rivers would promote the export flux of carbon from estuaries to open seas. This study emphasizes the importance of soil photodissolution of tidal rivers in the carbon transfer from lands to oceans.
... Chromophoric dissolved organic matter (CDOM) is the primary component responsible for light absorption in the ocean. High-energy UV light absorbed by CDOM is sufficient to break down its chemical bonds, leading to alterations of its composition or the generation of new compounds, such as small nitrogen-containing molecules (Santos et al., 2014;Mopper et al., 2015). The photochemical degradation of CDOM can also induce the production of a series of free radicals, leading to further oxidation or degradation of other DOM, including colorless DOM and CDOM. ...
Cyanate is a nitrogen and energy source for diverse marine microorganisms, playing important roles in the nitrogen cycle. Despite the extensive research on cyanate utilization, the sources of this nitrogen compound remain largely enigmatic. To unravel the sources of cyanate, distributions and production of cyanate during photochemical degradation of natural dissolved organic matter (DOM) were investigated across various environments, including freshwater, estuarine, coastal areas in Florida, and the continental and slope regions of the North American mid-Atlantic Ocean (NATL). Cyanate production was also examined during the photochemical degradation of exudates from a typical strain of Synechococcus, an important phytoplankton component. To deepen our understanding of the sources and production mechanisms of cyanate, its production was assessed during the photochemical degradation of a natural seawater DOM supplemented with five nitrogen–containing compounds with distinguishing structures and functional groups. Generally, cyanate exhibited higher concentrations in the Florida coastal, estuarine, and freshwater environments than the NATL. However, cyanate distribution did not consistently align with its production rates. Despite significantly low concentrations in the NATL, DOM from this region exhibited cyanate production rates comparable to estuarine and Florida coastal environments. Although relatively high cyanate concentrations were observed in the freshwaters, DOM in this environment exhibited very low cyanate production rates. A highly significant correlation was observed between cyanate and chlorophyll a (Chl a) concentrations in these areas. Moreover, in most estuarine and NATL stations, cyanate concentration and production rate in the Chl a maximum layer were significantly higher than in other layers. Cyanate was produced during the photochemical degradation of the Synechococcus exudates. The cyanate production was significantly enhanced when the natural seawater DOM was supplemented with GlycylGlycine, 4-(methylamino) benzoic acid, 4-[ethyl(methyl)amino] benzaldehyde or methyl 2-aminobenzoate. Our study implies that photochemical degradation of marine DOM, especially phytoplankton-derived DOM, is a substantial source of cyanate in the ocean. Additionally, cyanate may form during the degradation of peptides and small aromatic compounds in DOM, providing novel insights into the nitrogen cycle.
... Climate and environmental changes such as sea level rise, storm surge, and flooding exert significant effects on the transport and mixing of reactants within coastal wetlands by shifting the hydrologic regimes (Erwin, 2009;Paerl et al., 2019;Robinson et al., 2018). For example, rising seawater carries more oxygen-rich water onto coastal TAIs during high tides or storm surges, while groundwater brings more terrestrial-derived organic matter and nutrients to the coastal TAIs during heavy precipitation events (Mattone & Sheaves, 2017;Moore, 2010;Mopper et al., 2015;Regier et al., 2021;Slomp & Van Cappellen, 2004). Transport of organic matter and nutrients below and at the surface is then influenced by heterogeneous soil properties and surface morphology, while microbial activity also plays an important role in mediating carbon and nutrient transformations in the coastal TAIs (Ganju et al., 2019;Lee et al., 2006;Waska et al., 2019). ...
Plain Language Summary
The hydrological environment of vegetated coastal ecosystems is directly influenced by precipitation and seawater flooding, which mediates biogeochemical processes within these areas. However, the specific effects of dynamic precipitation and flooding on oxidation‐reduction conditions in these complex terrestrial‐aquatic interfaces (TAIs) are poorly understood, especially when considering the ecological processes of above‐ground plants. To address this gap, this study used integrated process‐based models, the Advanced Terrestrial Simulator (ATS) and PFLOTRAN, to examine the effects of hydrological and ecological controls on biogeochemical reactions and exchange fluxes across a TAIs transect spanning from a coastal upland forest and salt marsh to the open seawater. Our numerical experiments showed that the mixing of different waters within the TAIs significantly influenced the spatial and temporal variability in exchange fluxes across this interface along with the spatial extent of oxic subsurface zones. The interface between the oxic and anoxic zones shifts in response to periodic fluctuations in tidal elevations as higher tides drive more oxygenated water toward the TAIs. Meanwhile, vegetation evapotranspiration removes more water from the subsurface during warm summer months, leading to larger exchange fluxes across the TAIs. Reaction rate parameters that depend on the interactions between the soil and microbes have a large effect on carbon and oxygen consumption represented in our models. A higher aerobic respiration rate results in larger hypoxic and anoxic zones because the dissolved oxygen is consumed more quickly. Our modeling‐based study provided insights into the mechanisms that control the exchange fluxes and cycling of carbon and nitrogen at coastal TAIs, which can be used to inform potential management strategies for mitigating the impacts of climate change on these ecosystems.
... Absorption of light by CDOM reduces the penetration of solar radiation into the water column and mitigates the harmfulness of ultraviolet (UV) radiation to marine organisms (Häder et al., 2011). Moreover, CDOM undergoes photochemical transformation, decreasing its absorbance (i.e., photobleaching, e.g., Osburn et al., 2009), producing biologically labile substrates (Mopper et al., 2015) and climate-active gases such as carbon dioxide (CO 2 ), carbon monoxide (CO), and methane (CH 4 ) (i.e., photomineralization, e.g., White et al., 2010;Powers and Miller, 2015;Zhang and Xie, 2015). ...
The epipelagic macroalgae of Ulva prolifera and Sargassum are the primary contributors to widespread seaweed tides globally. Both ocean plants release large amounts of chromophoric dissolved organic matter (CDOM) into the surrounding seawater. The photochemical reactivity of this CDOM, however, has not been adequately addressed. In this study, we extracted CDOM from Ulva prolifera and Sargassum, examined their ultraviolet (UV)-visible absorption characteristics, and quantified their broadband apparent quantum yields (AQY) of absorbance photobleaching and photomineralization (in terms of CO2, CO, and CH4 photoproduction). On a per-unit-weight basis, Sargassum leached 3.5 times more CDOM than did Ulva prolifera in terms of the absorption coefficient averaged over 254–500 nm. Both Ulva prolifera and Sargassum CDOM were characterized by quasi-exponential decay absorption spectra, with Sargassum CDOM exhibiting a distinct shoulder over 310–350 nm suggestive of mycosporine amino acids. The Sargassum CDOM had a higher photobleaching AQY but lower photomineralization AQYs compared to Ulva prolifera CDOM. The photobleaching and photomineralization AQYs of both macroalgal CDOM are, however, orders of magnitude higher than those of CDOM in various natural waters. Potential photoproduction rates of CO2 and CO from the Ulva prolifera CDOM and Sargassum CDOM during the bloom periods are several times to orders of magnitude higher than the air-sea fluxes of these gases in the absence of the macroalgae. This study demonstrates that CDOM released by Ulva prolifera and Sargassum is extremely prone to photobleaching and photomineralization, rendering floating mats of these plants in oceans as potential “hotspots” of greenhouse gas emissions to the atmosphere. This photochemical feedback should be considered when assessing ocean afforestation as a CO2 removal approach to mitigate climate warming.
... The light-absorbing (both ultraviolet and visible wavelengths) fraction of DOM is referred to as chromophoric dissolved organic matter (CDOM) (McKay et al., 2017;Mostofa et al., 2013;Zhang et al., 2009). Since CDOM is the dominant light absorbing component in surface waters, it plays an important role in photophysical and photochemical processes in the aquatic environment (Mopper et al., 2015). CDOM photochemistry produces a variety of reactive intermediates (RIs) (Vione et al., 2014;Vione and Scozzaro, 2019). ...
... Throughout the aquatic land-ocean continuum, organic carbon is produced, transformed, and mineralized to CO 2 by a combination of biotic and abiotic processes, and transported laterally and vertically by physical processes. [43][44][45] On land, terrigenous OC is mobilized from soils and transported through streams, rivers, wetlands, and lakes before reaching the ocean. These inland-water systems are major reactors that extensively process and mineralize terrigenous OC and contribute globally significant fluxes of CO 2 to the atmosphere. ...
... In this case, turnover rates in the absence of light might be more comparable to refractory dissolved organic matter, leading to estimates of inert Cu lifetimes of centuries or longer. Interestingly, photochemical oxidation is thought to be a major sink for dissolved organic matter as well (Mopper et al., 2015). The conversion of labile Cu into inert Cu within the ocean is presumably a slow process. ...
The chemistry of copper (Cu) in seawater is well known to be dominated by complexation with organic ligands. The prevailing paradigm is that Cu forms strong but labile complexes. Recently, a novel procedure revealed that only a small fraction of dissolved Cu exists as labile complexes. The majority is present as a fraction that is relatively inert on timescales of weeks or more and probably does not participate in coordination exchange reactions, including biologically mediated processes. Samples collected from the 2018 GEOTRACES GP15 cruise show that throughout the interior of the Pacific Ocean, this inert fraction comprises about 90% of the dissolved Cu. Labile Cu accumulates in surface waters, probably arising from photochemical decomposition of the inert fraction. There is also a modest accumulation of labile Cu near deep sea sediments and along the Alaskan shelf and slope. The results have important implications for Cu transport and biological availability. Inert Cu may influence Cu transport throughout the water column and contribute to the linear increase in Cu with depth, a distribution which is hard to explain for a biologically active trace metal. The origins of inert Cu are unknown. It may be produced slowly within the water column on the timescale of meridional overturning circulation. In the Columbia River, between 92% and 98% of the dissolved Cu is in the inert fraction, suggesting a possible terrestrial source of inert Cu to the ocean.
... The absorption of solar UV and visible radiation by CDOM profoundly impacts the optics, microbial ecology, and primary production in the upper ocean (Arrigo and Brown 1996;Morel et al. 2010). It also leads to multifaceted photoreactions having far-reaching biogeochemical implications (Mopper et al. 2015). The resulting loss of chromophores (photobleaching) is the main sink of CDOM and FDOM H in the sunlit surface oceans (Coble et al. 1998;Stedmon and Markager 2005). ...
The absorption spectral slope, S 275–295 , is an optical metric frequently employed for characterizing chromophoric dissolved organic matter (CDOM). We collected CDOM absorption ( a CDOM ) and fluorescence spectra from the oligotrophic offshore South China Sea to identify the major determinant of S 275–295 and to explore the potential of S 275–295 as physical and biogeochemical tracers. S 275–295 linearly decreased with increasing humic‐like fluorescent DOM, revealing the latter as the primary control on S 275–295 . The variability of S 275–295 consistently declined from the surface to deep water, making S 275–295 a potential indicator of CDOM molecular heterogeneity. The S 275–295 vs. a CDOM plot was found to be a useful tool for characterizing water masses. The ¹⁴ C content of dissolved organic carbon (Δ ¹⁴ C‐DOC) shows a strong correlation to S 275–295 , demonstrating S 275–295 to be a proxy of the ¹⁴ C‐age of DOM. A provisional equation, ln(−Δ ¹⁴ C‐DOC) = (−0.85 ± 0.06) × ln( S 275–295 ) + (8.78 ± 0.19), was proposed to estimate Δ ¹⁴ C‐DOC from S 275–295 for global oligotrophic oceans.
... Marine DOC is lost due to heterotrophic consumption (Hansell & Carlson, 196 1998b;Carlson & Hansell, 2015), which results in progressive decreases in DOC concentration 197 with depth and along circulation pathways (Hansell & Carlson, 1998b). Additionally, DOC can be 198 lost due to photolysis (Mopper et al., 2015) or hydrothermal circulation . Our rates of NPP had R 2 = 0.41, p <0.0000001, R 2 = 0.28, p <0.0000001, and R 2 = 0.09, p <0.0000001, 354 respectively, evaluated using Type II regression model (reduced major axis regressions) ( Figure 355 4). ...
Surface ocean marine dissolved organic matter (DOM) serves as an important reservoir of carbon (C), nitrogen (N), and phosphorus (P) in the global ocean, and is produced and consumed by both autotrophic and heterotrophic communities. While prior work has described distributions of dissolved organic carbon (DOC) and nitrogen (DON) concentrations, our understanding of DOC:DON:DOP stoichiometry in the global surface ocean has been limited by the availability of DOP concentration measurements. Here we estimate mean surface ocean bulk and labile DOC:DON:DOP stoichiometry in biogeochemically and geographically defined regions, using newly available marine DOM concentration databases. Global mean surface ocean bulk (C:N:P = 387:26:1) and labile (C:N:P = 179:20:1) DOM stoichiometries are higher than Redfield stoichiometry, with labile DOM stoichiometry similar to that of global mean surface ocean particulate organic matter (C:N:P = 160:21:1) reported in a recent compilation. DOM stoichiometry varies across ocean basins, ranging from 251:17:1 to 638:43:1 for bulk and 83:15:1 to 414:49:1 for labile DOM C:N:P, respectively. Surface ocean DOP exhibits larger relative changes than DOC and DON, driving surface ocean gradients in DOC:DON:DOP stoichiometry. Inferred autotrophic consumption of DOP helps explain intra- and inter-basin patterns of marine DOM C:N:P stoichiometry, with regional patterns of water column denitrification and iron supply influencing the biogeochemical conditions favoring DOP use as an organic nutrient. Specifically, surface ocean marine DOM exhibits increasingly P-depleted stoichiometries from east to west in the Pacific and from south to north in the Atlantic consistent with patterns of increasing P stress and alleviated iron stress, respectively.
... In marine surface waters, photochemical reactions participate in redox chemistry by creating a diverse blend of oxidized and reduced chemical products (Andrews et al., 2000;Sharpless and Blough, 2014). The absorption of ultraviolet radiation (UVR; 290-400 nm) by chromophoric dissolved organic matter (CDOM) initiates photochemical reactions in the ocean (Mopper et al., 2015). Many of these photochemical reactions involve molecular oxygen, leading to the formation of an array of ROS, including those mentioned above (O 2 − , H 2 O 2 , and OH • ) and singlet oxygen (Kieber et al., 2003;White et al., 2003;Rose and Waite, 2006;Kieber et al., 2014;Powers and Miller, 2015a;Ossola et al., 2021). ...
Hydrogen peroxide (H 2 O 2) is an important reactive oxygen species (ROS) in natural waters, affecting water quality via participation in metal redox reactions and causing oxidative stress for marine ecosystems. While attempts have been made to better understand H 2 O 2 dynamics in the global ocean, the relative importance of various H 2 O 2 sources and losses remains uncertain. Our model improves previous estimates of photochemical H 2 O 2 production rates by using remotely sensed ocean color to characterize the ultraviolet (UV) radiation field in surface water along with quantitative chemical data for the photochemical efficiency of H 2 O 2 formation. Wavelength-and temperature-dependent efficiency (i.e., apparent quantum yield, AQY) spectra previously reported for a variety of seawater sources, including coastal and oligotrophic stations in Antarctica, the Pacific Ocean at Station ALOHA, the Gulf of Mexico, and several sites along the eastern coast of the United States were compiled to obtain a "marine-average" AQY spectrum. To evaluate our predictions of H 2 O 2 photoproduction in surface waters using this single AQY spectrum, we compared modeled rates to new measured rates from Gulf Stream, coastal, and nearshore river-outflow stations in the South Atlantic Bight, GA, United States; obtaining comparative differences of 33% or less. In our global model, the "marine-average" AQY spectrum was used with modeled solar irradiance, together with satellite-derived surface seawater temperature and UV optical properties, including diffuse attenuation coefficients and dissolved organic matter absorption coefficients estimated with remote sensing-based algorithms. The final product of the model, a monthly climatology of depth-resolved H 2 O 2 photoproduction rates in the surface mixed layer, is reported for the first time and provides an integrated global estimate of~21.1 Tmol yr −1 for photochemical H 2 O 2 production. This work has important implications for photo-redox reactions in seawater and improves our understanding of the role of solar irradiation on ROS cycling and the overall oxidation state in the oceans. CITATION Zhu Y, Powers LC, Kieber DJ and Miller WL (2022), Depth-resolved photochemical production of hydrogen peroxide in the global ocean using remotely sensed ocean color.
... Finally, photodissolution of MPs may also produce chromophoric dissolved organic matter (CDOM), which has rarely been quantified in previous studies. CDOM is a principal control on ocean optics (Stedmon and Nelson, 2015) and a primary driver for marine photochemistry that affects major marine biogeochemical cycles (Mopper et al., 2015). ...
Photodissolution has the potential to efficiently remove microplastics from the surface ocean. Here, we examined the effects of temperature and incident sunlight composition on the photodissolution of submillimeter-sized microplastics of polypropylene (PP), polystyrene (PS), and thermoplastic polyurethane (TPU) in seawater. The photoproduction of dissolved organic carbon (DOC), chromophoric dissolved organic matter, and dissolved nitrogen (TPU only) was observed to increase exponentially within 7 days of full-spectrum irradiation. The temperature dependence of photodissolution increased with irradiation time for PP and PS but remained relatively constant for TPU. A 20 °C increase in temperature enhanced DOC photoproduction by 10 times for PP, three times for PS, and four times for TPU at 7-d irradiation, giving activation energies of 59.4–84.8 kJ mol⁻¹. Photodissolution of all three polymers was exclusively driven by ultraviolet-B (UVB) radiation. PS-derived DOC was photomineralizable, while PP- and TPU-derived DOC appeared photo-resistant. Extrapolating the lab-based DOC photoproduction rates to warm surface oceans yields lifetimes of 6.5 years for PP, 3.6 years for PS, and 3.7 years for TPU. This study demonstrates that photodissolution of the tested microplastics is restricted to the thin UVB-penetrable surface ocean and that water temperature plays a critical role in controlling the photodissolution of these microplastics.
... These two fractions of fluorescent organic matter have been extensively studied and their sources and sinks in marine systems are relatively well identified (Nelson and Gauglitz, 2016). In the open ocean, away from terrestrial inputs, surface humic-like FOM composition is mainly controlled by solar radiation, which supresses its visible fluorescence in a process known as photobleaching (Dainard et al., 2015;Mopper et al., 2015). Marine organisms also modify visible FOM composition through the production of humic-like substances by phytoplankton Fukuzaki et al., 2014) and the production and consumption of a variety of humic-like substances by heterotrophic prokaryotes (Romera-Castillo et al., 2011). ...
Phytoplankton-derived organic matter sustains heterotrophic marine life in regions away from terrestrial inputs such as the Southern Ocean. Fluorescence spectroscopy has long been used to characterize the fluorescent organic matter (FOM) pool. However, most studies focus only in the dissolved FOM fraction (FDOM) disregarding the contribution of particles. In order to assess the dynamics and drivers of the dissolved and particulate fractions of FOM, we used a Lagrangian approach to follow the time evolution of phytoplankton proliferations at four different sites in the Southern Ocean and compared the FOM in filtered and unfiltered seawater aliquots. We found that filtration had little effects on FOM visible spectrum fluorescence intensities, implying that most of this signal was due to dissolved fluorophores. On the other hand, protein-like fluorescence was strongly supressed by filtration, with fluorescence of particles accounting for up to 90 % of the total protein-like FOM. Photobleaching was identified as the main driver of visible FDOM composition, which was better described by indices of phytoplankton photoacclimation than by measurements of the incident solar radiation dose. In contrast, protein-like FOM intensity and fractionation were primarily related to abundance, composition and physiological state of phytoplankton proliferations. The chlorophyll a concentration from non-diatom phytoplankton explained 91 % of the particulate protein-like FOM variability. The proportion of protein-like fluorescence found in the dissolved phase was predicted by the combination of potential viral and grazing pressures, which accounted for 51 and 29 % of its variability, respectively. Our results show that comparing FOM measurements from filtered and unfiltered seawater provides relevant information on the taxonomic composition and cell integrity of phytoplankton communities. A better understanding of the commonly overlooked FOM fractionation process is essential for the implementation of in situ fluorescence sensors and will also help us better understand the processes that govern OM cycling in marine systems.
... The majority of DOC is modified into recalcitrant DOC and exported to the deep sea (Hansell et al., 2012). Chromophoric dissolved organic matter (CDOM) is the optically active fraction of DOM and is an essential part of the microbial process, controlling the attenuation of light and photochemical reactions, and influencing primary productivity (Blough and Siegel, 2002;Mopper et al., 2015). In comparison, lysate organic matter (LOM) from phytoplankton forms a pool of OM that is created extracellularly through metabolic excretion and adsorption (McIntyre and Guéguen, 2013). ...
Bacterial transformation and processing of phytoplankton-derived organic matter are extremely important for the formation of ubiquitous organic matter (OM) in aquatic ecosystems. Heterotrophic bacteria convert OM into biomass and recycle inorganic components, contributing to the production of microbial food webs. While phytoplankton-derived organic matter is commonly studied, the transformation and processing of dissolved OM (DOM) and lysate OM (LOM) by culturable epiphytic bacteria remains poorly understood. In this study, cultivable epiphytic bacteria from the marine diatom, Skeletonema dohrnii, were isolated, purified, and identified. Three bacteria, Roseobacteria sp., Marinobacter sp., and Bacillus sp., were selected to study the transformation and processing of S. dohrnii-derived DOM and LOM using excitation-emission matrix (EEM) fluorescence methods, and bacterial abundance, dissolved organic carbon (DOC) concentration, and transparent exopolymer particle (TEP) content were measured. Meanwhile, the bacterial transformation of DOM and LOM was further evaluated by the fluorescence index, biological index, β/α, and humification index. The primary fluorophores, peak A (humic-like), peak C (humic-like), peak M (humic-like), peak B (protein-like), and peak T (tryptophan-like), were present in the sample. The fluorescence of DOM and LOM was dominated by protein-like signal that became increasingly humic-like over time, suggesting that more complex molecules (e.g., recalcitrant OM) are being produced. The fluorescence of DOM and LOM was dominated by a protein-like signal that became increasingly humic-like over time, suggesting that epiphytic bacteria produced more complex molecules. Results showed that the bacteria utilized LOM more rapidly than DOM. While the three bacteria transformed OM to different degrees, all were able to facilitate microbial reprocessing of OM into refractory OM.
... About 20-70% of oceanic dissolved organic matter (DOM) absorbs light over the ultraviolet (UV) and visible wavelengths due to chromophores present in DOM (Coble, 2007). The light-absorbing property of chromophoric DOM (CDOM) affects ocean optics (Blough and Del Vecchio, 2002) and leads to an array of photoreactions involved in marine biogeochemical cycles of organic carbon, nutrients, and trace gases (Mopper et al., 2015). CDOM in the ocean is mainly sourced from terrestrial input, particularly in coastal waters, and in situ biological production implicating various organisms such as phytoplankton, bacteria, and virus (Coble, 2007;Hansell and Carlson, 2015). ...
Phytoplankton blooms can be an important source of autochthonous chromophoric dissolved organic matter (CDOM) in surface oceans. Here we report the first detection and optical characterization of CDOM produced during a Dictyocha fibula bloom occurring in the western central Bohai Sea in mid-summer 2019. The mean CDOM absorption coefficient at 330 nm (aCDOM(330)) in the surface water of the bloom area (3.16 ± 0.48 m⁻¹) was 2.5 times higher than that of the adjacent bloom-absent area (1.26 ± 0.34 m⁻¹). The aCDOM(330) increased exponentially to a maximum with increasing chlorophyll a and cell abundance of D. fibula, indicating a steady state was reached between production and consumption of CDOM. The biological index (BIX) of the fluorescent DOM (FDOM) showed little variation between bloom and background areas. The combination of these observations points to an extensive production of autochthonous CDOM by D. fibula in the bloom area. Mass budgeting indicates that the net production of CDOM by D. fibula (aCDOM(330): 1.98 ± 0.48 m⁻¹) accounted for 63% of the total net accumulation of CDOM in the bloom area (aCDOM(330): 3.16 ± 0.48 m⁻¹) since the onset of the bloom event. The spectral slope coefficients over 275–295 nm (S275–295) in the bloom area (range: 0.0228–0.0241 nm⁻¹; mean: 0.0235 nm⁻¹) were substantially lower than those in the bloom-absent area (range: 0.0240–0.0311 nm⁻¹; mean: 0.0271 nm⁻¹), suggesting a high-molecular-weight nature of the D. fibula-derived CDOM. The freshly produced CDOM contained two humic-like FDOM components and one protein-like FDOM component, and the three FDOM components were depleted in fresh CDOM relative to their counterparts in CDOM within the background area. Owing to the episodic nature of algal bloom-driven CDOM production, future attention should be paid to accelerated biogeochemical cycles (e.g., spiked oceanic trace gas emission) associated with CDOM photochemistry during algal blooms which can be easily missed for pre-scheduled field surveys.
... During the past year, there has been an increasing interest in understanding the role of solar UV radiation in triggering the release of phosphate. Not only DOM [130], but also suspended sediments [125] and river margins are subject to periodic flooding [131], releasing phosphate upon UV irradiation. This effect of UV radiation on release of organically bound nitrogen and phosphorus can have positive or negative consequences for the environment in terms of increased bioavailability or eutrophication of these forms of nutrients. ...
The Environmental Effects Assessment Panel of the Montreal Protocol under the United Nations Environment Programme evaluates effects on the environment and human health that arise from changes in the stratospheric ozone layer and concomitant variations in ultraviolet (UV) radiation at the Earth’s surface. The current update is based on scientific advances that have accumulated since our last assessment (Photochem and Photobiol Sci 20(1):1–67, 2021). We also discuss how climate change affects stratospheric ozone depletion and ultraviolet radiation, and how stratospheric ozone depletion affects climate change. The resulting interlinking effects of stratospheric ozone depletion, UV radiation, and climate change are assessed in terms of air quality, carbon sinks, ecosystems, human health, and natural and synthetic materials. We further highlight potential impacts on the biosphere from extreme climate events that are occurring with increasing frequency as a consequence of climate change. These and other interactive effects are examined with respect to the benefits that the Montreal Protocol and its Amendments are providing to life on Earth by controlling the production of various substances that contribute to both stratospheric ozone depletion and climate change.
... CDOM photodegradation (SML and SSW) coincided with SA photoproduction across the salinity range sampled (0.3-32.0). CDOM is an important seawater surfactant component (e.g., Tilstone et al., 2010) whose photodegradation in coastal and oceanic waters is widely documented (Mopper et al., 2014). Eight of 12 irradiations where CDOM was quantified showed significant positive correlations between SA and S 275−295 (τ b (10-15) = 0.529-0.740, ...
Plain Language Summary
Surface‐active substances (surfactants) are ubiquitous in seawater and freshwater. They accumulate in the uppermost <1,000 μm (surface microlayer), where they slow the rate of gas exchange between water and air. Improved knowledge of surfactant distributions and behavior will improve global gas flux estimates (e.g., for CO2) used to inform climate models. While increased temperature is known to enhance the microlayer accumulation of surfactants, further slowing gas exchange, our knowledge of other potentially important processes (e.g., surfactant photo‐reactivity) is lacking. In the laboratory, we simulated the natural solar irradiation of estuarine waters (Tyne, UK), and found surfactant enhancement additional to that from increased temperature, presumably reflecting photo‐degradation of larger organic molecules. We argue that sunlight induced changes in other coastal waters, in the open ocean, and in freshwater will likely reflect differences in their organic compositions, prompting a need for wider investigation of this process.
The marine cyanobacterium Prochlorococcus numerically dominates the phytoplankton communities in all lower latitude, open ocean environments. Having lost the catalase gene, Prochlorococcus is highly susceptible to exogenous hydrogen peroxide (H 2 O 2 ) produced at the ocean’s surface. Protection by H 2 O 2 -scavenging heterotrophic “helper” bacteria has been demonstrated in laboratory cultures and implicated as an important mechanism of Prochlorococcus survival in the ocean. Importantly, some other phytoplankton can also scavenge H 2 O 2 , suggesting these competing microbes may inadvertently protect Prochlorococcus . In this study, we assessed the ability of co-occurring phytoplankton, the cyanobacterium Synechococcus and picoeukaryotes Micromonas and Ostreococcus , to protect Prochlorococcus from H 2 O 2 exposure when cocultured at ecologically relevant abundances. All three genera could significantly degrade H 2 O 2 and diminish Prochlorococcus mortality during H 2 O 2 exposures simulating photochemical production and rainfall events. We suggest that these phytoplankton groups contribute significantly to the H 2 O 2 microbial sink of the open ocean, thus complicating their relationships with and perhaps contributing to the evolutionary history of Prochlorococcus .
IMPORTANCE
The marine cyanobacterium Prochlorococcus is the most abundant photosynthetic organism on the planet and is crucially involved in microbial community dynamics and biogeochemical cycling in most tropical and subtropical ocean waters. This success is due, in part, to the detoxification of the reactive oxygen species hydrogen peroxide (H 2 O 2 ) performed by “helper” organisms. Earlier work identified heterotrophic bacteria as helpers, and here, we demonstrate that rival cyanobacteria and picoeukaryotic phytoplankton can also contribute to the survival of Prochlorococcus during exposure to H 2 O 2 . Whereas heterotrophic bacteria helper organisms can benefit directly from promoting the survival of carbon-fixing Prochlorococcus cells, phytoplankton helpers may suffer a twofold injury: production of H 2 O 2 degrading enzymes constrains already limited resources in oligotrophic environments, and the activity of these enzymes bolsters the abundance of their numerically dominant competitor. These findings build toward a better understanding of the intricate dynamics and interactions that shape microbial community structure in the open ocean.
Abiotic photochemistry and microbial degradation are the two main removal processes of marine dissolved organic matter (DOM). However, the combined and repeated effects of irradiation and biodegradation on DOM remains poorly resolved due to their complex interactions. To disentangle the effects of abiotic photochemistry from photobiology, we alternately exposed coastal DOM to repeated exposures to simulated solar radiation and then to microbial communities in darkness. Our results demonstrated selective impacts on the DOM pool by photochemical and microbial degradation. Photodegradation resulted in the loss of fluorescent (both protein- and humic-like), the enrichment of aliphatic and nitrogen-containing compounds, and an increase in microbial diversity. However, biodegradation drove changes in key molecules (significantly altered) and enhanced the contribution of alicyclic compounds and aromatic compounds containing carboxyl/ester functional groups. Network analysis implicated the irradiation adapted (i.e., Methylophagaceae) microbes in DOM transformations involving the gain and loss of methyl groups, while the non-irradiation adapted (i.e., Alteromonadaceae) microbes appeared to alter DOM composition by the gain and loss of oxygen atoms. Our findings distinguish the selective contributions of irradiation and biodegradation processes and point to the complex interactions between photochemical and biological processes that jointly shape the DOM pool.
Propagation of anticyclonic eddies (ACEs) facilitates the lateral transport of dissolved organic matter (DOM). However, the transformation of DOM within ACEs and its regulation of lateral fluxes of dissolved organic carbon (DOC) remain unclear. High‐resolution sectional distribution of DOC and humic‐like fluorescent DOM (FDOMH) were investigated across a Kuroshio‐derived ACE (K‐ACE) on the slope of northern South China Sea (SCS). A two‐endmember isopycnal mixing model revealed higher Kuroshio water fraction (0.59 ± 0.13) in K‐ACE than non‐eddy region. DOC removal (maximum 6%) was identified in the upper 200 m of K‐ACE. The shallower mixed layer (<50 m) also revealed signals of FDOMH photobleaching (8%–34%). However, the substantial production of FDOMH (36%–43%) happened below the mixed layer (50–250 m). The regression slope between net FDOMH and apparent oxygen utilization in this layer was steeper in K‐ACE than in non‐eddy region and dark oceans, suggesting K‐ACEs had higher microbial‐mediated in situ production efficiency and rate of refractory DOM. This enhancement could be attributed to increased biogenic particle export, more Kuroshio‐derived DOM transport, and higher oxygen utilization rate at warmer condition within K‐ACE. Analysis of 27‐year (1993–2020) daily eddy data revealed that K‐ACE‐induced lateral transport DOC flux to the northern SCS slope (1.8 ± 0.2 Tg C yr⁻¹) constitutes half of the external DOC flux via cross‐shelf transport, highlighting the importance of K‐ACEs in connecting carbon cycles between west Pacific and the northern SCS slope and even shelf areas.
Organic UV filters like oxybenzone (BP3) in sunscreens are seawater pollutants suspected to transfer to the atmosphere via sea spray aerosol (SSA). This study examines the photoinitiated degradation of BP3 in artificial and real seawater compared to SSA mimics containing NaCl and 4-benzoylbenzoic acid (4-BBA). We investigated pure, binary, and ternary mixtures of BP3, NaCl, and 4-BBA using solar-simulated light to isolate the effects of salt and photosensitization on BP3 degradation. Results showed significantly faster degradation in the aerosol phase (Jeff,env ≈ 10–3–10–2 s–1 or t1/2 < 10 min) compared to bulk solutions (Jeff,env ≈ 10–6 s–1 or t1/2 > 1 day). The photosensitizer enhanced BP3 photodegradation in both phases more than when mixed with salt or all three components in solutions. BP3 photodegradation was most enhanced by salt in the aerosol phase. High-resolution molecular analysis via Orbitrap LC-MS/MS revealed more acutely toxic compounds (benzophenone, benzoic acid, and benzaldehyde) in irradiated aerosols than in solution, supported by electronic structure and toxicity modeling. These findings highlight that seawater may serve as a reservoir for BP3 and other organic UV filters and that upon transfer into SSA, BP3 rapidly transforms, increasing aerosol toxicity.
Carbonyls have previously been dismissed as significant precursors for carbon monoxide (CO) photoproduction from natural chromophoric dissolved organic matter (CDOM). Here, we used hydrogen cyanide (HCN), which reacts with carbonyls to form photochemically inert cyanohydrins, as a probe to reexamine the role of carbonyls in CO photoproduction. Adding HCN to low-absorbance euphotic zone seawater decreased CO photoproduction. Modeling [HCN] (∼5 to 364 μM) vs the percent decrease in CO photoproduction (%CO↓) yielded carbonyl-cyanohydrin dissociation equilibrium constants, K D , and maximum %CO↓, %CO↓ max values. Four Atlantic and Pacific seawater K D s (66.7 ± 19.6 μM) overlap aqueous aliphatic but not aromatic aldehyde K D s. Phenylacetaldehyde (PA) and other β,γ-unsaturated aldehydes are proposed as prototypical CO precursors. Direct photolysis of ∼10 nM PA can supply the measured daily production of HCN-sensitive CO at an open-ocean site near Bermuda. HCN's %CO↓ max was 31 ± 2.5% in North Atlantic seawater vs the 13 ± 2.5% inhibition of CO photoproduction by borohydride, a dilemma since only borohydride affects most ketones. Borohydride also decreased CDOM absorption much more than did HCN. This puzzle probably reflects differing steric and solvation requirements in HCN-and borohydride-CDOM reactions. This study demonstrates cyanophilic aldehydes to be a significant source of open-ocean CO and reveals new clues regarding CDOM photochemistry mechanisms.
Plain Language Summary
Our study explores the relationship between anthropogenic activity and breakdown of dissolved organic matter (DOM) from rivers, which represents a vital link between land and ocean ecosystems. The composition of DOM in rivers is linked to the characteristics of the surrounding land. Urbanization and agricultural land‐use change the age and chemical composition of the riverine DOM. Consequently, these alterations induced from human activity would be expected to impact the bioavailability of riverine DOM to microorganisms in coastal environments. We compared DOM sourced from a natural river system (Suwannee River) to DOM from a river impacted by anthropogenic activity (Upper Mississippi River Basin) to understand how the availability of DOM from these distinct rivers varies to marine microorganisms. We carried out laboratory experiments with a model marine bacterium and measured the respiration of carbon dioxide and associated isotopic signatures during the breakdown of riverine DOM. Surprisingly, we discovered striking similarities in the breakdown patterns of DOM from both rivers, despite their differing origins. This suggests that the impact of human activities on downstream transformation of DOM may not be as straightforward as previously assumed and underscore the need for a nuanced understanding of how microorganisms process DOM in coastal environments.
Over the past decade, extensive research has delved into the methane (CH4) paradox, which involves aerobic CH4 production. We present noteworthy observations of CH4 oversaturation within the surface layer of the central Chile upwelling zone (36° S, 73° W) over two consecutive seasonal cycles (2018–2021). Complementing these observations, CH4 cycling experiments were conducted, utilizing distinct plankton fractions (encompassing the natural planktonic community, fractions < 150, < 3 and < 0.2 µm), in different productivity periods of phytoplanktonic production and composition throughout the year. Our findings underscore the pivotal role of picoplankton (< 3 µm) in CH4 production on the ocean surface, contrasting with the limited contribution of larger microorganisms (< 150 µm). Notably, incubations with methylated substrates, such as methylphosphonic acid (MPn) and trimethylamine (TMA), induce heightened CH4 production within the picoplanktonic fraction. This phenomenon is consistently observed during both upwelling (austral spring–summer) and non-upwelling (winter) seasons, with significance in the latter period, when Synechococcus sp. exhibits notably high relative abundance. Long-term microcosm experiments highlight the crucial roles played by heterotrophic bacteria and cyanobacteria in methylotrophic methanogenesis. This process enhances CH4 production, facilitated by the recycling of dissolved organic carbon (DOC). Picoplankton emerges as a pivotal factor influencing the recycling of methylated substrates, and it is responsible for maintaining CH4 supersaturation. These findings provide valuable insights into the biogeochemical processes driving CH4 dynamics, particularly in highly productive upwelling areas.
Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at 10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmolCL-1; median: 103 µmolCL-1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmolNL-1; median: 8.0 µmolNL-1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmolPL-1; median: 0.18 µmolPL-1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.
Plastics are accumulating on Earth, including at sea. The photodegradation of microplastics floating in seawater produces dissolved organic matter (DOM), indicating that sunlight can photodissolve microplastics at the sea surface. To characterize the chemistry of DOM produced as microplastics photodissolve, three microplastics that occur in surface waters, polyethylene (PE), polypropylene (PP), and expanded polystyrene (EPS), were incubated floating on seawater in both the light and the dark. We present the molecular signatures of the DOM produced during these incubations, as determined via ultrahigh-resolution mass spectrometry. Zero to 12 products were identified in the dark, whereas 319–705 photoproducts were identified in the light. Photoproduced DOM included oxygen atoms, indicating that soluble, oxygen-containing organics were formed as plastics photodegrade. PP and PE plastics have hydrogen-to-carbon (H/C) ratios of 2 and generated DOM with average H/C values of 1.7 ± 0.1 to 1.8 ± 0.1, whereas EPS, which has an H/C of 1, generated DOM with an average H/C of 0.9 ± 0.2, indicating the stoichiometry of photoproduced DOM was related to the stoichiometry of the photodegrading polymer. The photodissolution of plastics produced hundreds of photoproducts with varying elemental stoichiometries, indicating that a single abiotic process (photochemistry) can generate hundreds of different chemicals from stoichiometrically monotonous polymers.
In the last decade, there have been several research articles on the methane paradox (aerobic CH4 production) first described in the 1960s. In this study, we present observations of CH4 supersaturation in the surface layer in the central Chile upwelling zone (36° S, 73° W) throughout two seasonal cycles (2018–2021). Additionally, CH4 cycling experiments were performed using plankton fractions (natural planktonic community, <150, <3 and <0.2 µm) in a seasonal phytoplankton succession. Our findings highlight the significant role of picoplankton (<3 μm) in CH4 production on the ocean surface, contrasting with the limited involvement of larger organisms (<150 μm). Incubations with methylated substrates such as methylphosphonic acid (MPn) and trimethylamine (TMA) stimulated CH4 production in the picoplankton fraction during both upwelling (austral spring-summer) and non-upwelling (winter) seasons, being particularly relevant in the later period when Synechococcus contributed with high relative abundance. Long-term microcosm experiments underscore the importance of heterotrophic bacteria and cyanobacteria in methylotrophic methanogenesis, enhancing CH4 regeneration, mediated by dissolved organic matter (DOM) recycling. In conclusion, picoplankton emerges as a key factor in both production and metabolization of methylated substrates, being responsible for maintaining CH4 supersaturation. These findings provide valuable insights into the biogeochemical processes driving CH4 dynamics in highly productive upwelling waters.
The measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://figshare.com/s/512289eb43c4f8e8eaef) compiling previously published and unpublished measurements of DOC, DON, and DOP collected in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g., Chlorophyll-a, inorganic nutrients) and the inorganic carbon system (e.g., dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations from all continents however, most data were collected in the Northern Hemisphere, with a clear gap in coastal water DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62339 data points for DOC, 20360 for DON and 13440 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average (standard deviation; SD): 182 (314) µmol C L−1; median: 103 µmol C L−1), is 13-fold greater than the average coastal DON concentrations (average (SD): 13.6 (30.4) µmol N L−1; median: 8.0 µmol N L−1), which was itself 39-fold greater than the average coastal DOP concentrations (average (SD): 0.34 ± 1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful to identify global spatial and temporal patterns in DOM and to facilitate reuse of DOC, DON and DOP data in studies aimed at better characterising local biogeochemical processes, closing nutrient budgets, estimating carbon, nitrogen and phosphorous pools, as well as identifying a baseline for modelling future changes in coastal waters.
This chapter reviews and synthesizes key advances in our understanding of organic carbon (OC) cycling in estuaries over the past decade, combining a discussion of both particulate and dissolved OC (POC and DOC). Estuaries receive OC from terrestrial and oceanic sources as well as internal production, and this OC is highly diverse in chemical composition and biogeochemical reactivity. We review methodological advances as well as key established techniques that are used to quantify OC, distinguish OC sources, and track OC transformations. We also examine the different sources themselves and how their OC fluxes are controlled. Estuaries have high rates of OC transformation, and we examine the effects of flocculation, sorption to minerals, and microbial and photochemical degradation. We discuss how advances in our knowledge are applied in biogeochemical ocean models of estuaries and coastal zones, and examine the challenges in modeling OC across the interface from land to ocean. We conclude by evaluating how our understanding of the carbon budget for the coastal zone has evolved, highlighting key challenges for the future.
Soils and sediments are major reservoirs of organic matter (OM), whose dynamic turnover has a major impact on carbon cycling and global climate. OM in soils and sediments is predominantly associated with minerals, which decelerate OM decomposition and could help store carbon. However, iron (Fe) minerals could also degrade OM and release a fraction of OM to the atmosphere as CO 2 and CH 4 , but the coupling of these processes is only partly understood. In this Review, we describe the mechanisms and importance of coupled iron-carbon (Fe-C) cycles. Oxygenation of structural Fe(II) in minerals generates reactive oxygen species, which either degrades or synthesizes OM. Reactive oxygen species can also either decrease or increase extracellular enzyme activity and microbial activity, thus indirectly transforming OM. In addition, Fe(III) reduction contributes to OM oxidation through anaerobic respiration. By contrast, OM affects the redox properties of Fe minerals by serving as electron donor, acceptor, shuttle, buffer or conductor and by co-precipitation and complexation with Fe minerals. These feedback mechanisms can result in complex interconnected Fe-C cycling processes; hence, future work must focus on attaining the net impact of combined Fe-C cycles. Sections
The dinoflagellate Ostreopsis cf. ovata produces several families of toxic polyketides. Despite only a few field measurements of these phycotoxins in seawater and aerosols, they are believed to be responsible for dermatitis and the toxic inhalations reported during blooms of this species. Therefore, the stability of these compounds in seawater is essential to understanding the causes of these symptoms, however, this has never been assessed. In the current study, the optimization of a solid phase extraction (SPE) procedure was first performed to ensure the most efficient extraction of all phycotoxins known to be produced by this strain, including the recently described liguriatoxins. The SPE cartridge SDBL® under non acidified conditions offered the best option. The stability of the ovatoxins and the liguriatoxins under biotic and abiotic stress was assessed by exposing the spent medium of a culture of Ostreopsis cf. ovata to its bacterial consortium and natural sunlight. A rapid biotic transformation was detected for both families of compounds. When exposed to bacteria, the half-lives of the ovatoxins were reached before 10 h and at 36 h, 97% of these toxins had been transformed. The half-lives of the liguriatoxins were 10 h under these conditions. Photolysis (abiotic degradation) of the ovatoxins (T1/2 < 36 h) was faster than for the liguriatoxins (T1/2 > 62 h). Although none of the catabolites of these phycotoxins were thoroughly identified, an untargeted metabolomics approach combined with molecular networking highlighted the presence of several compounds exhibiting structural similarities with the ovatoxins. Additional work should confirm the preliminary findings on these potential ovatoxins’ catabolites and their biological properties. The rapid transformation of O. cf. ovata’s phycotoxins introduces questions concerning their presence in seawater and their dispersion in the sea spray aerosols. The compounds involved in the toxic inhalations and dermatitis often experienced by beachgoers may stem from the catabolites of these toxins or even unrelated and as yet unidentified compounds.
Marine chromophoric dissolved organic matter (m-CDOM) mediates many vital photochemical processes at the ocean's surface. Isolating m-CDOM within the chemical complexity of marine dissolved organic matter has remained an analytical challenge. The SeaSCAPE campaign, a large-scale mesocosm experiment, provided a unique opportunity to probe the in situ production of m-CDOM across phytoplankton and microbial blooms. Results from mass spectrometry coupled with UV-VIS spectroscopy reveal production of a chemodiverse set of compounds well-correlated with increases in absorbance after a bacterial bloom, indicative of autochthonous m-CDOM production. Notably, many of the absorbing compounds were found to be enriched in nitrogen, which may be essential to chromophore function. From these results, quinoids, porphyrins, flavones, and amide-like compounds were identified via structural analysis and may serve as important photosensitizers in the marine boundary layer. Overall, this study demonstrates a step forward in identifying and characterizing m-CDOM using temporal mesocosm data and integrated UV-VIS spectroscopy and mass spectrometry analyses.
Humic substances, a component of terrestrial dissolved organic matter (tDOM), contribute to dissolved organic matter (DOM) and chromophoric DOM (CDOM) in coastal waters, and have significant impacts on biogeochemistry. There are concerns in recent years over browning effects in surface waters, due to increasing tDOM inputs, and their negative impacts on aquatic ecosystems, but relatively little work has been published on estuaries and coastal waters. Photodegradation could be a significant sink for tDOM in coastal environments, but the rates and efficiencies are poorly constrained. We conducted large-scale DOM photodegradation experiments in mesocosms amended with humic substances and nutrients in the Gulf of Finland to investigate the potential of photochemistry to remove added tDOM and the interactions of DOM photochemistry with eutrophication. The added tDOM was photodegraded rapidly, as CDOM absorption decreased and spectral slopes increased with increasing photons absorbed in laboratory experiments. The in situ DOM optical properties became similar amongst the control, humic-, and humic+nutrients-amended mesocosm samples towards the end of the amendment experiment, indicating degradation of the excess CDOM/DOM through processes including photodegradation. Nutrient additions didn't significantly influence the effects of added humic substances on CDOM optical property changes, but induced changes in DOM removal.
Carbon monoxide (CO) apparent quantum yields (AQYs) are reported for a suite of riverine, estuarine and sea water samples, spanning a range of coloured dissolved organic matter (CDOM) sources, diagenetic histories, and concentrations (absorption coefficients). CO AQYs were highest for high CDOM riverine samples and almost an order of magnitude lower for low CDOM coastal seawater samples. CO AQYs were between 47 and 80% lower at the mouth of the estuary than at its head. Whereas, a conservative mixing model predicted only 8 to 14% decreases in CO AQYs between the head and mouth of the estuary, indicating that a highly photoreactive pool of terrestrial CDOM is lost during estuarine transit. The CDOM absorption coefficient ( a ) at 412 nm was identified as a good proxy for CO AQYs (linear regression r 2 > 0.8; n = 12) at all CO AQY wavelengths studied (285, 295, 305, 325, 345, 365, and 423 nm) and across environments (high CDOM river, low CDOM river, estuary and coastal sea). These regressions are presented as empirical proxies suitable for the remote sensing of CO AQYs in natural waters, including open ocean water, and were used to estimate CO AQY spectra and CO photoproduction in the Tyne estuary based upon annually averaged estuarine CDOM absorption data. A minimum estimate of annual CO production was determined assuming that only light absorbed by CDOM leads to the formation of CO and a maximum limit was estimated assuming that all light entering the water column is absorbed by CO producing photoreactants (i.e. that particles are also photoreactive). In this way, annual CO photoproduction in the Tyne was estimated to be between 0.99 and 3.57 metric tons of carbon per year, or 0.004 to 0.014% of riverine dissolved organic carbon (DOC) inputs to the estuary. Extrapolation of CO photoproduction rates to estimate total DOC photomineralisation indicate that less than 1% of DOC inputs are removed via photochemical processes during transit through the Tyne estuary.
The fate of dissolved organic matter (DOM) in lakes and streams is significantly affected by photochemical transformation of DOM. A series of laboratory photochemical experiments was conducted to describe seasonal changes in photochemical properties of DOM. The stream samples used in this study originated from three different catchments in the southernmost part of the Boreal ecozone near Dorset, Ontario, Canada. A first-order kinetics equation was used to model photochemical degradation of DOM and the kinetic rate constant, K, was used as an indicator of photochemical properties of DOM. Kinetic rate constants from all three catchments showed a sinusoidal pattern during the hydrological year. K increased steadily during autumn and winter and decreased during spring and summer with a more than 3-fold range in each stream. The highest values were observed during spring melt events when DOM was flushed from terrestrial sources by high flows. The minimum rate constants were found in summer when discharge was lowest. K was strongly correlated with pH and iron. DOM molecular weight and specific absorbance at 254 nm also exhibited annual cycles corresponding to the seasonal cycles of terrestrial organic matter, but the relationships between these properties and K differed between seasons and may have been affected by previous exposure to solar radiation during transit from the catchment.
Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen : carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2:CO:NH4+ molar ratio of 165:11:1 in the estuary, 60:3:1 on the shelf, and 18:2:1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85 % of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4 % of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a~DIN source corresponding to 7 % of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities would increase correspondingly to 6 %, 11 %, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.
This book, first published in 2000, provides a comprehensive review of UV radiation effects in the marine environment. A multidisciplinary approach is adopted to discuss all aspects from a physical, chemical and biological perspective. The book begins by describing the attenuation of UV radiation in the atmosphere and sea water, outlining the photochemical reactions involved and highlighting the role that such chemistry can play in influencing the biogeochemical cycling of various elements. The deleterious consequences of such radiation on organisms and strategies adopted to mitigate these harmful repercussions are discussed. The organisms considered range from virus and bacteria through phytoplankton and zooplankton to fish and mammals. The book is aimed at researchers and graduate students in photobiology, photochemistry and environmental science. It will also be useful as a supplementary text for courses in oceanography, climatology and ecology.
We investigated the effects of solar radiation on brevetoxin (PbTx2). Our findings suggest that natural sunlight mediates brevetoxin (PbTx2) degradation and results in brevetoxin by-product formation via photochemical processes.
Spectrally resolved efficiency (i.e. apparent quantum yield, AQY) of carbon monoxide (CO) photoproduction is a useful indicator of substrate photoreactivity and a crucial parameter for modeling CO photoproduction rates in the water column. Recent evidence has suggested that CO photoproduction from particles in marine waters is significant compared to the well-known CO production from chromophoric dissolved organic matter (CDOM) photodegradation. Although CDOM-based CO AQY spectra have been extensively determined, little is known of this information on the particulate phase. Using water samples collected from the Mackenzie estuary, shelf, and Canada Basin in the southeastern Beaufort Sea, the present study for the first time quantified the AQY spectra of particle-based CO photoproduction and compared them with the concomitantly determined CDOM-based CO AQY spectra. CO AQYs of both particles and CDOM decreased with wavelength but the spectral shape of the particulate AQY was flatter in the visible regime. This feature resulted in a disproportionally higher visible light-driven CO production by particles, thereby increasing the ratio of particle- to CDOM-based CO photoproduction with depth in the euphotic zone. In terms of depth-integrated production in the euphotic zone, CO formation from CDOM was dominated by the ultraviolet (UV, 290–400 nm) radiation whereas UV and visible light played roughly equal roles in CO production from particles. Spatially, CO AQY of bulk particulate matter (i.e. the sum of organics and inorganics) augmented from the estuary and shelf to the basin while CO AQY of CDOM trended inversely. Water from the deep chlorophyll maximum layer revealed higher CO AQYs than did surface water for both particles and CDOM. CO AQY of bulk particulate matter exceeded that of CDOM on the shelf and in the basin, but the sequence reversed in the estuary. Without consideration of the potential role of metal oxides (e.g. iron oxides) in particle photochemistry, mineral absorption-corrected CO AQY of particulate organic matter (POM) could, however, surpass its CDOM counterpart in all three sub-regions and displayed magnitudes in the estuary that overtook those in shelf and offshore waters. In terms of CO photoproduction, POM was thus more photoreactive than CDOM, irrespective of the organic matter's origins (i.e. terrigenous or marine). Riverine CDOM exhibited higher photoreactivity than marine CDOM and land-derived POM appeared more photoreactive than marine POM. AQY-based modeling indicates that CO photoproduction in the study area is underestimated by 12–32% if the particulate term is ignored.
This dissertation explores processes affecting the composition of dissolved organic matter (DOM) and how DOM composition changes in sunlit surface waters and in the dark interior ocean. Simulated solar irradiations were used to investigate the impact of photochemistry on terrestrial waters and deep ocean DOM. The photochemically mediated processes observed in Dismal Swamp samples included (i) light induced flocculation of up to 12% of the organic matter and 84% of the dissolved iron originally present; (ii) 74-88% mineralization of dissolved organic carbon (DOC) and 95-99% bleaching of chromophoric DOM (CDOM) during 110 days of irradiation; and (iii) nearly complete loss of the biochemical markers for terrestrial DOM: lignin phenols, CDOM absorption and fluorescence, and aromaticity determined by nuclear magnetic resonance (NMR) spectroscopy. Extensively photo-degraded terrestrial DOM exhibited spectroscopic signatures similar to DOM isolated from ocean water (except that it lacked protein-like fluorescence and appeared to contain excess carboxyl carbon), and photodegraded
deep ocean DOM exhibited optical properties similar to surface ocean DOM.
The heretofore-unexamined DOM removal process of light induced flocculation was further investigated using solid-state 13C NMR and infrared spectroscopy. Photochemical decarboxylation and production of alkyl functionality drives the initial phase of photochemical flocculation, while adsorption to iron flocculates is important during later phases of the process. Carboxyl amides appeared to resist mineralization, but were susceptible to photochemical flocculation. A fraction of the photodegraded DOM is more susceptible to mineral adsorption, which may be an important pathway for DOM export from surface waters to the sediments and subsequent preservation.
Advanced solid-state 13C NMR characterization of DOM isolated by reverse osmosis – electrodialysis (RO/ED) from marine environments with varying biogeochemistries revealed new insights into the biodegradation of carbohydrates as well as preservation of carboxyl groups and condensed aromatic structures in the ocean’s interior. Quaternary anomeric carbons were identified as a potentially important
structural component of the poorly characterized pool of bio-refractory carbohydrates. The present biogeochemical paradigm for ocean DOC cycling, the “three-pool” model, is re-examined along with the “three-pool photoreactivity” classification system. A new conceptual model is proposed, which incorporates both biological and photochemical reactivity of dissolved organic matter.
This chapter describes the dynamics of dissolved organic nitrogen (DON). DON is that subset of the dissolved organic matter (DOM) pool that contains nitrogen (N). From the perspective of a microorganism, this is where the action is—one-stop shopping for N, carbon (C), and energy. Research in DON, however, has lagged far behind that of the larger dissolved organic carbon (DOC) pool. This situation is primarily the result of the substantial analytical challenges inherent in DON research: (1) DON exists in substantially lower concentrations than DOC, (2) multiple chemical analyses are required for a single DON measurement, (3) inorganic N removal is a nightmarish undertaking, and (4) unless one has an easy access to a nuclear reactor manufacturing short-lived 13N, he or she must be content with labor-intensive stable isotopes rather than the quicker and more sensitive radiotracers. Measurements of DON concentrations have become a routine component of many studies. Nonetheless, because of space limitations DON concentrations in lakes, streams, or groundwater—with some exceptions—are not included in studies.
This chapter reviews and synthesizes the current state of knowledge on sources, transformations, cycling, and fluxes of dissolved organic carbon (DOC) in river-estuarine and coastal ocean systems. Because estuarine waters represent a complex interface between terrestrial, marine, seafloor, and even atmospheric environments, significant consideration is given to the diverse sources of organic matter contributing to estuarine and coastal marine DOC pools. Distributions of bulk DOC, its stable (δ13C) and radioactive (Δ14C) isotopes, and its biochemical components are reviewed with representative estuaries globally highlighted as examples of the variable types of DOC mixing and processing dynamics in different systems. The significant microbial, photochemical, and physical transformation processes affecting both the amounts and characteristics of DOC during estuarine transport are also evaluated. The chapter concludes by providing some examples of important and compelling recent studies of land–river estuary–coastal ocean DOC fluxes and transfers and their observed long-term changes, both natural and anthropogenically driven.
Photodegradation of dissolved organic carbon (DOC) can generate labile substrates readily available for microbial consumption, thus increasing DOC removal, especially in fresh water humic ecosystems. While a few studies have evaluated the effects of sunlight on DOC removal and CO2 production in aquatic environments, none have investigated the seasonal variation and interaction of photodegradation and microbial metabolism of DOC in a large tropical black-water river system. We present the results of experiments designed to evaluate the rates of photodegradation and subsequent microbial metabolism of DOC in the Negro River and an associated floodplain lake (Lake Tupe) in the central Brazilian Amazon Basin. Water samples collected in both environments at different phases of the river hydrological cycle were filtered and exposed to natural sunlight to estimate photodegradation; they were then inoculated with natural bacteria and incubated in the dark to evaluate bacterial metabolism. Changes in incident solar radiation and in DOC concentration and quality throughout the hydrological cycle directly affected the DOC photodegradation rates and microbial metabolism. Total DOC mineralization (photodegradation plus bacterial consumption) was more intense in the falling water period. DOC photodegradation generally stimulated further microbial DOC degradation, enhancing total DOC removal in samples exposed to solar radiation in both ecosystems. While direct photodegradation represented only a small part of the total DOC mineralization (6.7 % in the high water period in the Negro River), the combined effect of photodegradation and stimulus of bacterial metabolism could account for a significant part of the CO2 production in Amazonian black water ecosystems.
We assessed the responses of a nitrogen (N)-limited < 10 mu m plankton community from the Baltic Sea to the 12 d photochemical transformation of dissolved organic matter (DOM). The photochemical transformation of DOM increased the biomass and the production of heterotrophic bacteria, flagellates, and ciliates in the following 10 d bioassay. The succession of heterotrophic plankton indicated a 3-level trophic transfer of photoproduced bioavailable DOM through bacteria and flagellates to ciliates. The photochemical transformation of DOM also stimulated the biomass and the production of phytoplankton through the photoproduction of bioavailable N initially incorporated into bacterial biomass. The grazing of bacterioplankton supplied N to phytoplankton directly, presumably due to mixotrophy, and indirectly by releasing dissolved N. The carbon stable isotope signature of plankton biomass was similar to that of allochthonous carbon, indicating that the photochemical transformations concerned primarily terrestrial DOM and therefore represented a microbial link between terrestrial DOM and planktonic production. The bacterial production stimulated by the photochemically produced labile DOM was related to the number of photons absorbed during the photochemical transformation of DOM for the determination of apparent quantum yield. According to the apparent quantum yield, the calculated summertime photoproduction of labile substrates contributes 2 to 5% to total bacterial production in the northern Baltic Sea. According to this study, the photochemical transformation of terrestrial DOM influences not only the initial production of bacterioplankton but can also stimulate higher trophic levels and autotrophic plankton in coastal waters.
Numerous studies published in recent years emphasize the role of solar radiation in degradation of dissolved organic matter (DOM) in lake and marine waters. The photochemical degradation may act in concert with the activity of heterotrophic microorganisms, transforming recalcitrant DOM into labile organic intermediates that are readily utilized by bacteria. We present results illustrating that the organically bound carbon, phosphorus, and nitrogen may also become increasingly available to bacteria upon exposure to UV radiation. In addition, we summarize recent evidence for an opposite effect of photochemical reactions on freshly produced DOM components, turning them into more recalcitrant forms. In a survey of a large number of lakes, we found both positive and negative effects of photochemical alteration of DOM on bacterial growth potential. The effect on bacterial growth was predominantly positive in oligotrophic softwater lakes, but negative in alkaline lakes with a large indigenous production of DOM, as indicated by high concentrations of algal chlorophyll a.
In the Baltic Sea ice, the spectral absorption coefficients for particulate matter (PM) were about two times higher at ultraviolet wavelengths than at photosynthetically available radiation (PAR) wavelengths. PM absorption spectra included significant absorption by mycosporine‐like amino acids (MAAs) between 320 and 345 nm. In the surface ice layer, the concentration of MAAs (1.37 µg L ⁻¹ ) was similar to that of chlorophyll a , resulting in a MAAs‐to‐chlorophyll a ratio as high as 0.65. Ultraviolet radiation (UVR) intensity and the ratio of UVR to PAR had a strong relationship with MAAs concentration ( R ² = 0.97, n = 3) in the ice. In the surface ice layer, PM and especially MAAs dominated the absorption (absorption coefficient at 325 nm: 0.73 m ⁻¹ ). In the columnar ice layers, colored dissolved organic matter was the most significant absorber in the UVR (< 380 nm) (absorption coefficient at 325 nm: 1.5 m ⁻¹ ). Our measurements and modeling of UVR and PAR in Baltic Sea ice show that organic matter, both particulate and dissolved, influences the optical properties of sea ice and strongly modifies the UVR exposure of biological communities in and under snow‐free sea ice.
Siderophores, high-affinity Fe(III) ligands produced by microorganisms to facilitate iron acquisition, might contribute significantly to dissolved Fe(III) complexation in ocean surface waters. In previous work, we demonstrated the photoreactivity of the ferric ion complexes of several alpha-hydroxy carboxylic acid-containing siderophores produced by heterotrophic marine bacteria. Here, we expand on our earlier studies and detail the photoreactivity of additional siderophores produced by both heterotrophic marine bacteria and marine cyanobacteria, making comparisons to synthetic and terrestrial siderophores that lack the alpha-hydroxy carboxylate group. Our results suggest that, in addition to secondary photochemical reaction pathways involving reactive oxygen species, direct photolysis of Fe(III)-siderophore complexes might be a significant source of Fe(II) and reactive Fe(III) in ocean surface waters. Our findings further indicate that the photoreactivity of siderophores is primarily determined by the chemical structure of the Fe(III) binding groups that they possess-hydroxamate, catecholate, or alpha-hydroxy carboxylate moieties. Hydroxamate groups are photochemically resistant regardless of Fe(III) complexation. Catecholates, in contrast, are susceptible to photooxidation in the uncomplexed form but stabilized against photooxidation when ferrated. alpha-Hydroxy carboxylate groups are stable as the uncomplexed acid, but when coordinated to Fe(III), these moieties undergo light-induced ligand oxidation and reduction of Fe(III) to Fe(II). These photochemical properties appear to determine the reactivity and fate of Fe(III)-binding siderophores in ocean surface waters, which in turn might significantly influence the biogeochemical cycling of iron.
Dissolved organic carbon (DOC) is one of the major reservoirs of active organic carbon on Earth. Although the bulk of the marine DOC pool is largely composed of small refractory polymeric material, new evidence suggests that similar to10% of the DOC pool (10(16) g C) can enter the microbial loop by forming microscopic gels that can eventually be colonized and degraded by bacteria. Marine microgels result from a spontaneous and reversible assembly/dispersion equilibrium of DOC polymers forming hydrated Ca-bonded tangled polymer networks. Here we test the hypothesis that ultraviolet (UV) photocleavage should strongly inhibit the formation of microgels, because the stability of tangled networks decreases exponentially with polymer length. Because of the loss of ozone shielding, the UV-B spectral component of solar radiation (lambda = 280-320 nm) has undergone a dramatic increase in the past few decades, particularly in the polar regions. We used dynamic laser-scattering spectroscopy and flow cytometry to investigate UV-induced DOC polymer cracking and the effect of UV on DOC assembly/dispersion equilibrium in 0.2 mum filtered seawater. Results indicate that exposure of seawater to UV-B fluxes equivalent to those found in Antarctica during summer solstice can cleave DOC polymers, inhibit their spontaneous assembly, and/or disperse assembled microgels. Our results agree with previous observations that indicated that fragmentation produced by UV photolysis increases exponentially with exposure time and suggested that UV could limit the supply of microbial substrate by hindering microgel formation. UV cleavage yields short-chain polymers that do not assemble and could eventually account for the old refractory DOC pool found in seawater.
Most of the solar radiation that reaches land or water is converted into thermal energy, but a significant part, especially that in the ultraviolet and visible region, is diverted into photochemical and photobiological processes that affect the global carbon cycle. The most prominent photobiological process on the earth’s surface is biological photosynthesis. Terrestrial vegetation and marine algae use the solar energy to convert annually approximately 100 Gt (gigatons) of carbon in the form of atmospheric carbon dioxide (CO2) into organic matter (Zepp 1994). When plants and algae die, the resulting non-living matter is transformed by various biological and chemical processes that either convert it back to CO2 (and other trace carbon gases) and water or to biologically refractory organic substances. The refractory organic matter is a mixture of substances, including litter and more refractory compounds, a large portion of which consists of humic substances (Thurman 1985). The term “humic substances” is usually used to refer to the organic matter that has been isolated from natural waters or from soils using well-defined techniques (Frimmel and Christman 1988; Huber and Frimmel 1994). Humic substances make up the largest single class of dissolved organic matter (DOM), accounting for 30 to 60% of the DOM in most natural waters (Thurman 1985). [The term “dissolved organic matter, DOM” is here used as synonym of “dissolved organic carbon, DOC.”] The term “colored dissolved organic matter (CDOM)” is used for the fraction of DOM that is colored (Blough and Green 1995) and includes humic substances. Based on Orinoco River data, Blough et al. (1993) estimated that only about 65% of the total DOM absorbs solar radiation and is subject to direct photochemical reactions (see Table 3.1).
Marine phytoplankters are the primary source of dimethylsulfoniopropionate (DMSP). Only certain groups of phytoplankton, notably the prymnesiophytes and the dinoflagellates, produce significant amounts of these compounds on a per cell basis. However, even within these groups, there is considerable variability in concentration, and the function of DMSP within the cells is not fully resolved. In macroalgae, there is a clear relationship between DMSP and osmotic adaptation. Most planktonic species, however, do not experience significant salinity variation; other factors must be responsible for any observed shifts in intracellular DMSP content. We have examined the effects of light intensity and nitrogen availability on the production of DMSP by several isolates of marine phytoplankton. An inverse relationship appears to exist between intracellular DMSP levels and nitrogen availability. The algae examined in this study produced more DMSP per cell under nitrogen-deplete conditions. There was no common response to light variations. It is important to understand the factors that control the production of DMSP in the oceans, as DMSP is the major precursor of the atmospherically important gas, dimethyl sulfide (DMS).
This book offers extensive coverage of the most important aspects of UVR effects on all aquatic (not just freshwater and marine) ecosystems, encompassing UV physics, chemistry, biology and ecology. Comprehensive and up-to-date, UV Effects in Aquatic Organisms and Ecosystems aims to bridge the gap between environmental studies of UVR effects and the broader, traditional fields of ecology, oceanography and limnology. Adopting a synthetic approach, the different sections cover: the physical factors controlling UVR intensity in the atmosphere; the penetration and distribution of solar radiation in natural waters; the main photochemical process affecting natural and anthropogenic substances; and direct and indirect effects on organisms (from viruses, bacteria and algae to invertebrate and vertebrate consumers). Researchers and professionals in environmental chemistry, photochemistry, photobiology and cell and molecular biology will value this book, as will those looking at ozone depletion and global change.
This chapter presents a discussion on photochemistry and the cycling of carbon, sulfur, nitrogen, and Phosphorus. Dissolved organic matter (DOM) plays a dominant role in the absorption of ultraviolet (UV) and visible light in the open ocean. As DOM absorbance is regulated in part by photobleaching processes, light availability for photosynthesis and the penetration of UV radiation within the marine environment are influenced by photochemical transformations. In addition to its control on UV light fields, DOM photochemistry strongly impacts the biogeochemical cycling of biologically important elements in surface seawater. By the conversion of DOM into volatile species such as carbonyl sulfide, DOM photochemistry influences atmospheric chemistry and climate. The chapter illustrates an important light-driven chemical reaction in the photic zone—that is, the reduction of trace metals such as iron, manganese, and copper. The chemistry of these reduced species is quite different from their oxidized counterparts. Photochemical oxidation of DOM also produces a suite of free radicals and other short-lived species including the superoxide anion, carbonate radical, singlet oxygen, hydroxyl radical, di-bromide radical anion, and a number of poorly described organic radicals and excited state triplets. These species are much more reactive than their corresponding diamagnetic and ground state species and are expected to influence biological and chemical processes in sunlit surface waters. Furthermore, DOM photolysis is an important source or sink of a variety of atmospherically important gases that are emitted from the ocean, some of which affect the Earth's radioactive balance. Concentrations—and hence emissions—of carbon monoxide (CO), carbon dioxide (CO2), carbonyl sulfide (OCS), and di-methyl sulfide (DMS) are all partly regulated through photochemical processes involving DOM.
Dimethylsulfide (DMS) is generally thought to be lost from the surface oceans by evasion into the atmosphere as well as consumption by microbe. However, photochemical process might be important in the removal of DMS in the oceanic photic zone. A kinetic investigation into the photochemical oxidation of DMS in seawater was performed. The photo-oxidation rates of DMS were influenced by various factors including the medium, dissolved oxygen, photosensitizers, and heavy metal ions. The photo-oxidation rates of DMS were higher in seawater than in distilled water, presumably due to the effect of salinity existing in seawater. Three usual photosensitizers (humic acid, fulvic acid and anthroquinone), especially in the presence of oxygen, were able to enhance the photo-oxidation rate of DMS, with the fastest rate observed with anthroquinone. Photo-oxidation of DMS followed first order reaction kinetics with the rate constant ranging from 2.5 × 10-5 to 34.3 × 10-5 s-1. Quantitative analysis showed that approximately 32% of the pbotochemically removed DMS was converted to dimethylsulfoxide. One of the important findings was that the presence of Hg2+ could markedly accelerate the photo-oxidation rate of DMS in seawater. The mechanism of mercuric catalysis for DMS photolysis was suggested according to the way of CTTM (charge transfer to metal) of DMS - Hg2+ complex.
The kinetic characterizations of direct photolysis and indirect photoreactions in natural waters are described, including the reactions of multiple species in rapid reversible equilibrium. The photochemical sources, fates, and environmental effects of various photo-reactants are discussed. It is concluded that in the oceans, hydroxyl radical (*OH) and the aquated electron (eaq-) will ultimately oxidize and reduce, respectively, nearly all organic pollutants that are otherwise not degraded before transport to the oceans. In atmospheric, and surface waters (fresh and saline), Superoxide radical ion (•O2-) can be involved in the oxidation of reduced forms of transition metals (e. g. Cu (I), Fe (II)), and for certain transition metals (e. g. Cu(I)) can also be the dominant source of the reduced form of the transition metal. In atmospheric waters and probably also in surface (fresh and saline) waters, hydrogen peroxide (HOOH) is a significant source of hydroxyl radical, through the iron photo-Fenton’s reaction. Hydrogen peroxide is the single most important oxidant for oxidizing sulfur dioxide to sulfuric acid during periods of cloudiness. Excited state triplets and organic peroxyl radicals derived from natural organic chromophores play significant roles for the oxidation of phenols in natural waters.
During the last 10-20 years, an increasing number of toxic or noxious events related to the proliferation of unicellular algae appeared all over the world. Dinoflagellates represent the most important taxonomic group involved in such events but new taxa are now under focus for their toxicity, among which some belong to the Prymnesiophyceae, Raphidophyceae or Diatoms. Some species recently observed on the Atlantic coast in France were probably introduced accidentally as were the dinoflagellate cysts through the ship's ballast water in Australia, with disastrous consequences for commercial shellfish farm operations. The nuisance effects of blooms can be spectacular for example the foamproduction by Phaeocystis colonies or red tides induced by dinoflagellate proliferations. Besides the environmental detrimental effects and a strong influence on biodiversity in bloom formations, toxin production is often responsible for damage to aquaculture and fisheries activities. A review of the main groups involved in toxic or harmful events is presented, including remarks on taxonomy and biodiversity. The species concept is examined in the case of phytoplankton, with permanently asexual organisms. Finally, governemental institutions and programs involved in harmful and toxic algal blooms are cited, particularly for European countries.
Respiration represents the major area of ignorance in our understanding of the global carbon cycle. In spite of its obvious ecological and biogeochemical importance, most oceanographic and limnological textbooks deal with respiration only superficially and as an extension of production and other processes. The objective of this book is to fill this gap and to provide a comprehensive review of respiration in the major aquatic systems of the biosphere. The introductory chapters review the general importance of respiration in aquatic systems, and deal with respiration within four key biological components of aquatic systems: bacteria, algae, heterotrophic protists, and zooplankton. The central chapters of the book review respiration in major aquatic ecosystems: freshwater wetlands, lakes and rivers, estuaries, coastal and open oceans, and pelagic ecosystems, as well as respiration in suboxic environments. For each major ecosystem, the corresponding chapter provides a synthesis of methods used to assess respiration, outlines the existing information and data on respiration, discusses its regulation and links to biotic and abiotic factors, and provides regional and global estimates of the magnitude of respiration. This is followed by a chapter on the modelling of respiration for various components of the plankton. The final chapter provides a general synthesis of the information and data provided throughout the book, and places aquatic respiration within the context of the global carbon budget.
The standing stock of dissolved organic matter (DOM) in surface waters depends on import, washout, indigenous primary production and processes of internal loss, including abiotic mineralization (particularly photooxidation), microbial mineralization and flocculation followed by sedimentation. The DOM in such waters is a complex mixture of different compounds. Some of these, such as free and combined amino acids and carbohydrates, have in many cases been identified and quantified. Although the bulk of the DOM has not been described in detail, a major constituent of it is generally humic matter. The composition of the fraction of the DOM that is utilized and mineralized by bacteria, however, is poorly known. This chapter concerns both the importance of microbial utilization for the dynamics of DOM, and the importance of recalcitrant DOM as a substrate for microbial growth in humic waters. The impact of such factors as flocculation and photochemical processes upon the microbial degradation will also be discussed. The further consequences which the production of bacterial biomass can have on the structure and function of the ecosystem through the consumption of DOM will be considered as well, but is elucidated in greater detail in Chapter 11.
Solar radiation is a universal and regular phenomenon in biosphere that is vital for all life in the Earth’s crust. It maintains all the physical, chemical and biological processes of organic matter and dissolved organic matter (DOM) in natural waters.
Interest in reactive oxygen species (ROS) in the environment was originally stimulated by certain atmospheric problems such as smog formation, stratospheric ozone depletion, and, more recently, acid rain. As a result of the need to understand the causes of these problems better, gasphase reactions of tropospheric oxidants and related primary photochemical processes have been intensely studied, and atmospheric photochemistry has become a highly developed subdiscipline.
To quantify the effects of photochemistry on the biological lability of dissolved organic carbon in a terrestrially influenced system, a quarterly sampling effort was undertaken at three estuarine locations along the coast of Georgia, producing a total of 15 apparent quantum yield (AQY) spectra for biologically labile products (BLPs). Prefiltered samples were irradiated and then inoculated with natural microbial communities isolated from the same sample. Oxygen consumption was used as a proxy for community carbon utilization over 10-12 d dark incubations. Seasonal microbial response to irradiated samples ranged from ‒21% to +155% relative to dark controls. AQY spectral shapes were not consistent over the data set, nor were there any apparent patterns in season or location. Progressive photon dose experiments showed that increasing irradiation length strongly decreased the total biolabile product as assayed with microbial community oxygen consumption, as well as altering the spectral shape of the AQY spectra. A conceptual model is presented to explain this dependence on photon dose, by illustrating the competition between the photochemical production and photochemical destruction of BLPs. The varying dependence on photon dose is fundamentally different from published results for other photochemical products and further complicates attempts to quantify the total effect of photochemistry on organic carbon cycles in natural environments.
Bacterioplankton are the primary consumers of dissolved organic matter in the ocean, thus the quantification of bacterioplankton production (BP) is essential to our understanding of carbon cycling in the largest ecosystems on Earth. We compared BP, measured as the rate of C-14-leucine or H-3-leucine uptake at close to saturating concentration (20 nmol L-1), with ambient uptake measured from dilution bioassays. We hypothesized that saturation with leucine would lead to its respiration as a carbon source, thereby not truly representing ambient BP. Seawater was collected from the photic zone throughout the Atlantic Ocean. Respiration as a proportion of total consumption (uptake + respiration) of close to ambient (0.4 nmol L-1) and close to saturating (20 nmol L-1) C-14-leucine concentrations were compared. Saturating H-3-leucine additions overestimated ambient leucine uptake at low rates (200% +/- 100% ambient) and underestimated uptake at high rates (90% + 20% ambient). The proportion of total leucine uptake that was respired was threefold higher for 20 nmol L-1 C-14-leucine additions than 0.4 nmol L-1 C-14-leucine additions (15% +/- 8% and 5% +/- 4%, respectively). Consequently, microbial efficiency of leucine assimilation-an indicator of bacterioplankton growth efficiency-was significantly higher and more stable at close to ambient C-14-leucine additions than at saturating concentrations (95% +/- 4% and 85% +/- 8%, respectively). Thus, saturation of oligotrophic open Atlantic Ocean bacterioplankton with leucine, or other molecules indicative of microbial metabolism, leads to the measurement of a response to a nutrient addition, rather than an ambient measurement.
We investigated the influence of solar radiation on the transfer of organic matter from the particulate to dissolved phase during resuspension of coastal sediments collected from seven sites across Florida Bay (organic carbon values ranged from 2% to 9% by weight). Sediments were resuspended in oligotrophic seawater for 48 h in 1-liter quartz flasks in the dark and under simulated solar radiation (SunTest XLS+) at wet weight concentrations of 100 mg L(-1) and 1 g L(-1) (dry weights ranged from 27 to 630 mg L(-1)). There were little to no dissolved organic carbon (DOC) increases in dark resuspensions, but substantial DOC increases occurred in irradiated resuspensions. DOC levels increased 4 mg C L(-1) in an irradiated 1 g L(-1) suspension (dry weight 400 mg L(-1)) of an organic-rich (7% organic carbon) sediment. At a particle load commonly found in coastal waters (dry weight 40 mg L(-1)), an irradiated suspension of the same organic-rich sediment produced 1 mg C L(-1). DOC increases in irradiated resuspensions were well-correlated with particulate organic carbon (POC) added. Photodissolution of POC ranged from 6% to 15% at high sediment levels and 10% to 33% at low sediment levels. Parallel factor analysis modeling of excitation-emission matrix fluorescence data (EEM PARAFAC) suggested the dissolved organic matter (DOM) produced during photodissolution included primarily humic-like components and a less important input of protein-like components. Principal component analysis (PCA) of EEM data revealed a marked similarity in the humic character of photodissolved DOM from organic-rich sediments and the humic character of Florida Bay waters.
The UV component in solar radiation increased the availability of DOC for bacterial growth, and led to an increase in bacterial and bacterivore abundance in laboratory plankton cultures. UV radiation may thus stimulate ecosystem productivity by increasing dissolved organic carbon lability and facilitating the transfer of carbon to higher trophic levels via the microbial loop.
Unsaturated fatty acids rapidly photodecomposed in filtered seawater to aliphatic aldehydes and an ω‐oxocarboxylic acid (CHO‐(CH 2 ) n ‐COOH) upon a 5‐h exposure to ambient sunlight. The photoreactivity of fatty acids increased as their degree of unsaturation increased. Palmitic acid displayed no photodegradation while monounsaturated oleic acid photodecomposed to nonanal and 9‐oxononanoic, respectively. Photooxidation of the polyunsaturated linoleic acid produced significantly higher concentrations, primarily hexanal, and a wider variety of aldehydes. The rate of photodegradation was more than 10 times greater for the polyunsaturated fatty acids than for the monounsaturated fatty acids. Photolysis of the triglyceride, trilinolein, gave an aldehyde profile similar to its component fatty acid, indicating each undergoes a similar mechanism of photodegradation. Twenty percent of the initial triglyceride was photodegraded during the 6‐h irradiation. The rapid photoreactivity of unsaturated fatty acids may explain their low in situ levels relative to saturated fatty acids, the latter of which are found in the dissolved phase at relatively higher levels.
Fluorescent hydrophobic dissolved organic matter, operationally defined as humic substances, increased after irradiation of trilinolein‐treated seawater over a 14‐d period. The concentraticn of humic products was enhanced by postphotolysis addition of ammonia. Dissolved ammonia concentrations declined as the dissolved organic nitrogen of the extracted humics increased, indicating the added ammonia was becoming incorporated into the organic structure.
A free-floating drifter was designed to directly determine in situ photochemical production rates, photolysis rates, and light fluxes in seawater. This drifter consisted of six trays that were suspended in series from a single buoy line. The trays were constructed so that the attached quartz vessels, containing filtered seawater, were exposed to both downwelling and upwelling irradiation. The quartz vessels were sealed at both ends with ribbed TFE Teflon stopper that permitted multiple subsampling without introduction of a headspace. The free-floating drifter was used to study photochemical processes in antarctic waters. It performed well during 12-15-h deployments, even in rough seas with sustained winds between 15 and 26 m s-1. Although not tested here, the drifter should perform equally well in the study of photochemical processes in freshwater environments.
Several recent lines of literature point toward strong photoreactivity of phytoplanktonic detritus. We examined effects of irradiation of algal membrane fragments in various stages of decay, with emphasis on transfer of materials from solid to dissolved phase (photodissolution). After simulated solar irradiation for 24 h, up to several tens of percent of particulate organic matter converted to photodissolved organic matter (PDOM). Prior microbial decay enhanced PDOM production. PDOM had initially high C: N ratios, which decreased with irradiation time. Dissolved organic nitrogen dominated nitrogen photodissolution, followed by minor photoammonification and negligible nitrite plus nitrate production. Chromophoric particulate organic matter bleached at visible wavelengths and underwent dissolution, creation, and bleaching at ultraviolet (UV) wavelengths, resulting in net loss of color in particulates and net gain of largely UV-absorbing PDOM that also exhibited humic-type fluorescence. Solid phase proteinaceous material became less accessible to proteases after microbial decay but regained this accessibility upon irradiation. Irradiation under anoxic conditions roughly halved production of PDOM, including chromophores and humic fluorophores. Oxygen enhancement of these reactions, along with production of peroxides, implies a strong role for photosensitization. Pigments, unsaturated lipids, and tryptophan emerged as likely sources of reactive oxygen species. Lipid peroxides appeared as a reactive intermediate product. If these reactions in the ocean scale to pigment loss as found in our experiments, at least 5-15% of particulate organic matter could undergo photodissolution before settling in some planktonic environments. This photodissolution would enhance remineralization by photic zone microbial communities and thus upper ocean elemental recycling. © 2009, by the American Society of Limnology and Oceanography, Inc.
Unsaturated lipids in cell membranes, including phospholipids and cholesterol, are well-known targets of oxidative modification, which can be induced by a variety of stresses, including ultraviolet A (UVA)- and visible light-induced photodynamic stress. Photodynamic lipid peroxidation has been associated with pathological conditions such as skin phototoxicity and carcinogenesis, as well as therapeutic treatments such as antitumor photodynamic therapy (PDT). Lipid hydroperoxides (LOOHs), including cholesterol hydroperoxides (ChOOHs), are important non-radical intermediates of the peroxidative process which can (i) serve as in situ reporters of type I vs. type II chemistry; (ii) undergo one-electron or two-electron reductive turnover which determines whether peroxidative injury is respectively intensified or suppressed; and (iii) mediate signaling cascades which either fortify antioxidant defenses of cells or evoke apoptotic death if oxidative pressure is too great. The purpose of this article is to review current understanding of photodynamic (UVA- or visible light-induced) lipid peroxidation with a special focus on LOOH generation and reactivity. Future goals in this area, many of which depend on continued development of state-of-the-art analytical techniques, will also be discussed.
Photodissolution converts particulate organic matter to dissolved organic matter, which may affect its availability to heterotrophic organisms. We examined the ability of planktonic microbes to utilize photodissolved organic matter (PDOM) obtained from irradiating coastal sediments and a preparation of algal membranes. About half of the organic carbon that was photodissolved from algal particulate detritus was subsequently available to microbes over 2 wk of incubation in the dark, while that from the sediments appeared to be less bioavailable, the extent of utilization being 16 to 29%. Losses in total organic carbon during microbial incubation appeared as dissolved inorganic carbon, substantiating our inference of microbial usage. Accumulation of bacterial biomass was small relative to respiratory loss, which may be due to low growth efficiency by bacteria, or possibly grazing. Photoammonification converted some of the particulate nitrogen into inorganic ammonium; in the subsequent microbial processing of PDOM from sediments, this ammonium was reincorporated into organic matter. Thus, photodissolution shifts particulate organic matter to physical and chemical forms less available to metazoans and more available to osmotrophic microbes. This bioavailability, on a time scale of weeks, indicates that PDOM produced in nearshore resuspension zones can add fuel to planktonic microbial regeneration processes while the water remains on the shelf, but that major fractions of the PDOM are likely to be exported offshore.
Photochemical processes appear to have an important impact on the cycling of dissolved organic matter (DOM) in a wide variety of aquatic environments. Numerous photoproducts of DOM, including dissolved inorganic carbon and low-molecular-weight organic compounds, have been identified, and the types of photoproducts and extent of phototransformation of DOM is dependent upon its chemical composition. Heterotrophic bacteria are the primary consumers of DOM, and photochemical transformations of DOM can affect its bacterial utilization in a variety of ways. Numerous studies spanning fresh and marine waters have demonstrated enhanced bacterial growth following exposure of DOM to solar radiation. These studies indicate that photochemical processes can enhance the microbial utilization of biorefractory DOM. Other studies in fresh and marine waters, however, have demonstrated that exposure of DOM to solar radiation can result in a reduction in bacterial growth rates, suggesting that photochemical and microbial processes "compete" for the same substrates. It appears that some photoproducts of DOM are resistant to microbial degradation, suggesting that photochemical processes could be involved in the production as well as destruction of biorefractory DOM.