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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.

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... In this sense, the contribution of the recycled OC to the autochthonous carbonate (%DIC ORG ) is important to both the OC cycle and the IC cycle in lake systems. The %DIC ORG is a function of OC oxidation, photochemical degradation and biodegradation (Kufel & Kufel, 1997;Leng & Marshall, 2004;Meyers, 1997;Mopper et al., 2015). Although these processes have been recognized, the proportion of carbonate deposition that are derived from the recycled OC is still poorly constrained. ...
... The variation in %DIC ORG during 1955-1980 might have been mainly controlled by both hydrological and ecological conditions. First, the reduced lake area and water depth during 1960-1980 (Figure 2a) facilitated the O 2 dissolution and the light penetration, leading to enhanced oxidation and photochemical degradation of OC (Mopper et al., 2015). Second, macrophyte blooming, as evidenced by high P aq and δ 13 C org values, further transferred O 2 into deeper layers, facilitating the oxidative degradation of OC. ...
... water column (Kufel & Kufel, 1997;Leng & Marshall, 2004;Mopper et al., 2015). Small and deep lakes also show relatively high %DIC ORG values, but with larger variability (22-44%; Figure 3). ...
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Plain Language Summary The mechanisms that control the deposition, on the bottom of lakes, of carbonate matter that forms within the lakes themselves, are important for the carbon cycles at regional and global scales. Here we present data of carbonate content and carbon isotope composition from the past ~150 years recorded at Lake Wuliangsu, where the historical hydrological and ecological conditions have been well studied. We first investigate the factors that control the deposition of carbonate matter and calculate how much of it derives from recycled organic carbon using a carbon isotope mass‐balance model. Furthermore, we compile published data from lake systems across the globe and we incorporate them into our framework, so as to seek a better understanding of organic carbon recycling in a global perspective. Finally, we find that the size of the lake (area and depth) and the lake stratification play a key role in determining the contribution of recycled organic carbon to the overall carbonate deposition.
... Photochemical processes convert biologically recalcitrant organic molecules into low-molecular-weight compounds and inorganic species, enhancing their bioavailability and thus their mobility across environmental compartments. 1 Focus on the role of photochemistry in the biogeochemical cycles has been increasing over the last few years, especially for those environments in which biological processes are inherently slow due to low temperatures, lack of water, or both. 2 For example, photodegradation of dissolved organic carbon (DOC) was reported to be responsible for 75−90% of the total carbon turnover in arctic lakes and rivers, 3 while in dryland systems, incorporation of photochemical processes led to substantial improvements of C-cycling models. 4 In addition, photochemistry is considered an important degradation pathway for terrestrial DOC in coastal areas, with 3−40% of riverine DOC input expected to be photochemically mineralized within a few years. ...
... 4 In addition, photochemistry is considered an important degradation pathway for terrestrial DOC in coastal areas, with 3−40% of riverine DOC input expected to be photochemically mineralized within a few years. 1,2 So far, the organic matter photomineralization studies have focused primarily on carbon and secondarily on nitrogen and phosphorus (see ref 1). However, we can anticipate similar processes for other elements that are part of the organic matter pool, such as sulfur. ...
... Photochemical degradation of DOC into CO 2 , CO and a suite of low-molecular-weight carbonyls and carboxylates has been extensively documented in freshwater and marine environments. 1,5 Similarly, dissolved organic nitrogen (DON) is photochemically degraded to NH 4 + , primary amines, amino acids, and urea, 1,6,7 while dissolved organic phosphorous (DOP) was shown to release PO 3 4− upon photolysis. 1,8 On the other hand, less is known about the photochemical fate of dissolved organic sulfur (DOS). ...
Article
Photodegradation processes play an important role in releasing elements tied up in biologically refractory forms in the environment, and are increasingly recognized as important contributors to biogeochemical cycles. While complete photooxidation of dissolved organic carbon (to CO2), and dissolved organic phosphorous (to PO4³⁻) has been documented, the analogous photoproduction of sulfate from dissolved organic sulfur (DOS) has not yet been reported. Recent high-resolution mass spectrometry studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter, which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds (i.e., CS2 and COS). We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work suggests that photochemistry may play a more significant role in the aquatic and atmospheric fate of DOS than currently believed.
... Temperature can vary multiple degrees Celsius per day in many lakes (Woolway et al. 2016), and drives CR at sub-daily (Forget et al. 2009) and longer timescales (Pace and Prairie 2005;Yvon-Durocher et al. 2012). Simultaneously, daytime photo-mineralization of complex OM yields bioavailable inorganic nutrients and biolabile compounds that can stimulate microbial metabolism (Bushaw et al. 1996;Mopper et al. 2014). Taken together, it is likely that shorter, more frequent incubations that minimize watercontainment artifacts and capture the turnover of highly biolabile OM will help define how the numerous and intricate environmental drivers interactively structure the short-term rates and patterns of planktonic metabolism (Taylor and Doherty 1990). ...
... It could also reflect greater rates of DO consumption due to greater rates of phytoplankton respiration and daytime oxygen consumption linked to processes such as the Mehler reaction, which involves the photoreduction of O 2 (Weger et al. 1989;Lewitus and Kana 1995;Badger et al. 2000;Luz et al. 2002). Elevated daytime CR may also reflect other physical changes in the environment, in particular photomineralization of complex OM (Cory et al. 2014), which can liberate inorganic nutrients, as well as impact the biolability of OM, and thus stimulate microbial growth (Bushaw et al. 1996;Mopper et al. 2014). Although distinguishing the Linking water temperature and DOC patterns to short-term variability in planktonic CR. ...
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Planktonic community respiration (CR) is a major component of aquatic biogeochemical cycling and food web energetics. Accurate, direct characterizations of short-term patterns and drivers of plankton CR are needed to understand aquatic biogeochemical processes and food web functioning. Recent work indicates CR may be commonly underestimated, and may undergo considerable diel changes that are missed using standard methodological approaches. To explore these possibilities, we applied an immediate, in situ, dark incubation approach at ~ 3 h intervals over 2.5 diel cycles in a shallow, productive, sub-arctic lake in interior Alaska, USA. Rates of CR varied 17-fold, strongly coupled to diel oscillations in water temperature. A weak inverse relationship to ~ 3 mg L⁻¹ diel changes in dissolved organic carbon concentrations suggests CR partially modulated the standing stock of organic matter over short timescales. Average rates of CR were ~ 6 to 100-fold greater than published, conventional CR measurements, but comparable to existing free-water estimates of ecosystem respiration for nearby Alaskan lakes. Overall, this study places new weight on the importance of CR in whole-ecosystem biogeochemical transformations by supporting recent suggestions that planktonic CR may be commonly underestimated.
... Carbonyl sulfide (COS) and carbon disulfide (CS 2 ) are important atmospheric gases. COS is important because it serves as greenhouse gas [1] but also because it holds a long (> 1 year) tropospheric lifetime [2], which enables it to reach the stratosphere. This long atmospheric lifetime also enables COS to form sulfate aerosols, which are known to counteract global warming [2] and impact ozone chemistry [3]. ...
... More unexpectedly, the DMS concentrations did not decay considerably after 15 min and essentially remained at a plateau for up to 4 h (Fig. 1). A clear rationale for why DMS decay slowed down over this later time period and appeared to deviate from pseudo-first order kinetics cannot be well established at this point; however, it likely stems from the high DMS concentrations applied, where zero-kinetics have been observed and other complex interactions between DMS and DOM may play a role [2]. Another important and unique finding from these results established that COS formation was relatively unaffected by DOM type when solutions containing DMS were exposed to sunlight ([COS] ranged from ~0.76-1.15 ...
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Carbonyl sulfide (COS) and carbon disulfide (CS2) are important atmospheric gases photochemically generated from organic sulfur precursors in sunlit natural waters. This study examined these processes by evaluating COS and CS2 photo-production from dimethyl sulfide (DMS) in the presence of dissolved organic matter (DOM). DOM was added since it photochemically produces various reactive intermediates (³CDOM*, •OH, ¹O2, and H2O2) potentially involved in these reaction pathways. DMS-amended synthetic waters at pH 8 were varied in terms of their DOM type and concentration, spiked with the ³CDOM* quenching agent, phenol, in certain cases, and subsequently irradiated over varying exposure times. Results indicated that various DOM types ranging from freshwater to open ocean DOM increased COS but did not alter CS2, which remained at non-detect levels. DOM type influenced COS only at higher concentrations (20 mg/L), while increasing DOM concentrations proportionally increased COS concentrations for all DOM types. Phenol addition lowered COS formation for reasons that remained unclear since phenol likely quenched ³CDOM* and DMS-derived sulfur-based radicals. Further comparisons to DMS-spiked natural waters and cysteine (CYS)-spiked synthetic and natural waters assessed previously indicated that COS formation from both precursors in natural waters was always greater than in waters containing DOM alone.
... In contrast, UV FDOM shows elevated concentrations (>1.8 ppb over shelf, 1.6-1.7 ppb off shelf) from the subsurface down to 100-200 m (Figure 3), displaying similar spatial and temporal distributions to cold, fresh surface waters. There is a reduced FDOM concentration in the near surface, likely due to photo-degradation (Mopper et al., 2015), which acts as a major sink of CDOM in the ocean (Figure 3). ...
... The bio-optical properties of the water show significant relationships with the presence of meltwater, as calculated from Equations 1 and 2. The strongest correlation is observed for FDOM, in particular between 50 and 200 m water depth (Table 1). The relationship between meltwater percentage and FDOM breaks down at the very surface, with a switch from a positive to a negative correlation at meltwater concentrations greater than approximately 3.5%, as a result of FDOM breakdown by photoreactions (Mopper et al., 2015). The relationship between FDOM and meltwater shows that there is a second-order dependence on latitude that is consistent between the two gliders, potentially revealing an along-shelf gradient with a higher FDOM concentration in the more northerly freshwater sources (Figure 4) significant correlations with meltwater (Table 1), which likely reflects the multiple transported and in situ sources of chlorophyll, and biological and abiological particles detected by backscatter. ...
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The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio‐optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall colored dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM‐enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind‐driven Ekman transport. Ocean gliders housing bio‐optical sensors can provide the high‐resolution observations of both dissolved and particulate glacially derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change.
... Messenger et al. (2009) suggested that a mechanism for CH 4 production could be the photochemical generation of reactive oxygen species (ROS) in leaves, including the hydroxyl radical (OH). In seawater, sunlight-driven formation of OH and other reactive radicals (e.g., superoxide (O 2 − ), hydrogen peroxide (H 2 O 2 ) and singlet oxygen ( 1 O 2 )) has also been shown to occur, resulting in the direct and indirect photochemical oxidation of DOM to generate low molecular weight carbonyl molecules, CO and CO 2 (Mopper & Kieber, 2005;Mopper et al., 2015). ...
... Photochemical production rates are ideally related to photon absorption and temperature using spectrally dependent apparent quantum yields, which are essential for accurate estimation of global production of trace gases (Mopper et al., 2015). Consequently, our study illustrates the potential of UV to drive gaseous emissions from phytoplankton cells, and appropriate quantification of ambient UV effects requires detailed dose-response studies at appropriate wavelengths. ...
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Marine phytoplankton contribute about one half of global primary production and play a key role in global biogeochemical cycles. High cell densities in extensive phytoplankton blooms are expected to be modified by global changes in ocean circulation and stratification, acidification and carbonation, solar radiation, temperature, and eutrophication. Although photochemical gas production from chromophoric dissolved organic matter (CDOM) has been widely studied, ultraviolet (UV) effects on emissions from phytoplankton cells themselves have not been fully explored. We therefore investigated UV‐driven emissions of carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethene (C2H4), ethane (C2H6), and nitrous oxide (N2O) from cell suspensions of 16 phytoplankton species and their filtrates under controlled experimental conditions. These gases make direct or indirect contributions to radiative forcing of the atmosphere or contribute to atmospheric chemistry including stratospheric ozone (O3) depletion. We observed production of CH4, CO, CO2, C2H4, and N2O from cell suspensions and CO, CO2, and N2O after 0.45 μm‐filtration to remove phytoplankton cells. CH4 production was only observed with cells present, whereas N2O was still produced after filtration. Production of CO from filtrates was 30%–90% of that from cell suspensions in all but two species with a CO2:CO mole ratio from filtrates always below one. Our results clearly demonstrate a need to quantify the production potentials of these climate‐relevant gases in situ under natural sunlight using key phytoplankton species, especially those forming blooms which are predicted to change in prevalence and distribution with future global change scenarios.
... The main processes that remove DOC from the ocean water column (Figure 1) are: (1) Thermal degradation in e.g., submarine hydrothermal systems (Lang et al., 2006), (2) bubble coagulation and abiotic flocculation into microparticles (Kerner et al., 2003) or sorption to particles (Chin et al., 1998); (3) abiotic degradation via photochemical reactions (Moran and Zepp, 1997;Mopper et al., 2015); and (4) biotic degradation by marine heterotrophic prokaryotes (Lønborg and Álvarez-Salgado, 2012). It is suggested that the combined effects of photochemical and microbial degradation represent the major sinks of DOC (Carlson and Hansell, 2015). ...
... However, as the impact of UV damage and ability to repair is extremely variable, there is no consensus on how UV-light changes might impact overall plankton communities (Jeffrey et al., 1996;Rhode et al., 2001). The CDOM absorption of light initiates a complex range of photochemical processes, which can impact nutrient, trace metal and DOC chemical composition, and promote DOC degradation (Mopper et al., 2015). Photodegradation involves the transformation of CDOM into smaller and less colored molecules (e.g., organic acids), or into inorganic carbon (CO, CO 2 ), and nutrient salts (NH + 4 , HPO 2− 4 ) (Miller and Zepp, 1995;Moran and Zepp, 1997;Moran et al., 2000). ...
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The marine dissolved organic carbon (DOC) pool is an important player in the functioning of marine ecosystems. DOC is at the interface between the chemical and the biological worlds, it fuels marine food webs, and is a major component of the Earth’s carbon system. Here, we review the research showing impacts of global change stressors on the DOC cycling, specifically: ocean warming and stratification, acidification, deoxygenation, glacial and sea ice melting, changed inflow from rivers, changing ocean circulation and upwelling, and wet/dry deposition. A unified outcome of the future impacts of these stressors on the global ocean DOC production and degradation is not possible, due to regional differences and differences in stressors impacts, but general patterns for each stressor are presented.
... For instance, the aromatic chromophores that give natural DOC its color are the main absorbers of ultraviolet light in natural waters (Kitidis et al., 2006). As they absorb sunlight these dissolved aromatic compounds are rapidly and preferentially photodegraded in the surface ocean Mopper et al., 2015). Of the plastics studied, only EPS contains chromophoric aromatic functional groups (Gewert et al., 2015). ...
... oxidized functional groups) may also have caused the differences in photoreactivity of the PE and PE std samples. Photoreaction rates in natural waters usually decrease exponentially as the most reactive reactants are removed with increasing irradiation time Mopper et al., 2015). However, DOC photoleaching from PE std , PP, and EPS accelerated over the course of the 54day irradiations (Fig. 4B-D). ...
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Trillions of plastic fragments are afloat at sea, yet they represent only 1-2% of the plastics entering the ocean annually. The fate of the missing plastic and its impact on marine life remains largely unknown. To address these unknowns, we irradiated post-consumer microplastics (polyethylene, PE; polypropylene, PP; and expanded polystyrene, EPS), standard PE, and plastic-fragments collected from the surface waters of the North Pacific Gyre under a solar simulator. We report that simulated sunlight can remove plastics from the sea surface. Simulated sunlight also fragmented, oxidized, and altered the color of the irradiated polymers. Dissolved organic carbon (DOC) is identified as a major byproduct of sunlight-driven plastic photodegradation. Rates of removal depended upon polymer chemistry with EPS degrading more rapidly than PP, and PE being the most photo-resistant polymer studied. The DOC released as most plastics photodegraded was readily utilized by marine bacteria. However, one sample of PE microplastics released organics or co-leachates that inhibited microbial growth. Thus, although sunlight may remove plastics from the ocean's surface, leachates formed during plastic photodegradation may have mixed impacts on ocean microbes and the food webs they support.
... Exposure to light resulted in significantly higher levels of ammonium in all but one soil (soil H); the abiotic nature of this ammonification is supported by the general lack of a difference in microbial activity between exposed and nonexposed samples, and for most soils, the particular lack of a difference in arginine ammonification (an indicator of biological mineralization potential; Table S2). Like the photochemical decomposition of organic matter, photochemical production of ammonium is well recognized in aquatic environments (see studies cited in Mopper et al., 2015;Tarr et al., 2001). In contrast, only a few studies with soil have been reported (e.g., Mayer et al., 2012;Rao & Varadanam, 1938). ...
... Photochemical and biochemical processes, being mechanistically distinct, may not only operate independently but also act on different substances. For example, organic compounds resistant to microbial action are often readily decomposed by light (Bushaw et al., 1996;Francko & Heath, 1979;Mopper et al., 2015). This divergence is particularly significant in the study of substances considered to be more microbially recalcitrant, such as lignin (Austin et al., 2016;Cogulet et al., 2016) or hematite (Lovley & Phillips, 1986;Siffert & Sulzberger, 1991), both of which readily undergo photochemical reactions. ...
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The significance of photochemically active solar radiation is well substantiated in Earth's air and water environments, but no broad connection has yet been ascertained between sunlight and the chemical properties of land surfaces. Since it is obvious that sunlight strikes Earth every day, an inquiry into this connection was made in a simple experiment with a diverse group of 20 soils. Light-induced alteration of most of the major elements in soil was evident, including significant changes in dissolved organic carbon, inorganic nitrogen, pH, phosphorus availability and sorption capacity, and soluble and mineral forms of iron, manganese, aluminum, silicon, and calcium. In many instances the extent of these changes could be predicted from initial values. This broad response to light attests to the vivid photochemistry of soils and has both pedological and edaphological implications that reach beyond the surface. The phenomena reported here affirm the opportunity for novel inquiries into the chemistry of soils and land surfaces in general.
... Marine dissolved organic carbon (DOC) is one of the largest carbon reservoirs on Earth, comparable in size to the reservoir of atmospheric CO 2 (Hansell, 2013). Marine DOC is coupled to the global carbon budget mainly through autotrophic and heterotrophic processes (Benner, 1998) and secondarily through processes that include photochemistry (Mopper et al., 2015), hydrothermal vents (Hawkes et al., 2015;Walter et al., 2018), dissolved and particulate organic carbon interactions and conversions (Druffel & Williams, 1990;Hansell et al., 2009), and primary marine aerosol generation (Beaupré et al., 2019;Kieber et al., 2016) that affect the cycling of DOC in the oceans and the transfer of carbon between the oceans and atmosphere (Siegenthaler & Sarmiento, 1993). It has been shown that sunlight-mediated DOC photochemistry is an important source of low-molecular-weight (LMW) compounds in the surface oceans (Kieber et al., 1989;Miller et al., 2002;Moran & Zepp, 1997). ...
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A photochemical model was used to quantify the global contribution of carbonyl photoproduction in the photodegradation of marine dissolved organic carbon (DOC). As model input, wavelength‐ and temperature‐dependent apparent quantum yields (AQYs) for the photochemical production of carbonyl compounds were determined in seawater collected from the Northwest Atlantic Ocean. These AQY data and published AQY data from the North Pacific were used with remotely sensed seawater optical properties and solar irradiance data in a global model to calculate depth‐resolved, mixed‐layer photochemical fluxes of acetaldehyde and glyoxal in seawater. Based on this model, the annual global surface mixed‐layer photochemical production is 89.7 ± 36 Tg year⁻¹ for acetaldehyde and 20.0 ± 8.0 Tg year⁻¹ for glyoxal. This work significantly improves our understanding of the impact of photochemistry on the cycling of DOC in the surface oceans. Low‐molecular‐weight carbonyl compounds represent the second largest carbon flux among all known carbon products that are produced during the photolysis of DOC. The annual photoproduction of carbonyl‐compound carbon is ~110 ± 23 Tg C year⁻¹, comprising approximately 9.6% of the total carbon and 22% of the biologically labile carbon that are produced globally from the photolysis of marine DOC.
... Within this plethora of molecules are compounds of diverse source, chemistry, and reactivity. Aromatic compounds absorb light, making them the main component of the colored DOM (CDOM; Weishaar et al. 2003) that is the main initiator of photoreactions in natural waters ( Mopper et al. 2015). In terrestrial systems, these aromatics are derived primarily from the structural compounds within vascular plants ( Hedges 2002). ...
Chapter
Dissolved organic matter (DOM) is a master variable that modulates the form and function of many ecosystems. Approximately, half of the mass of DOM is carbon. Fluxes of DOM transfer carbon and other vital elements between ecosystems and between organisms (e.g., trees to bacteria) and components (e.g., vegetation to soil) within ecosystems. The DOM flux out of trees and understory plants to the forest floor is a poorly studied component of the carbon and nutrient budgets of forest ecosystems. In freshwater systems, studies of DOM transport through terrestrial systems usually start at the stream. However, the interception of rainwater by vegetation marks the beginning of the terrestrial hydrological cycle making plant canopies the crowning headwaters of terrestrial aquatic carbon cycling. Rainwater interacts with canopies picking up DOM, which is then exported from the plant in stemflow and throughfall, where stemflow denotes water flowing down the plant stem and throughfall is the water that drips from and through the leaves, branches, and epiphytes of the canopy. As nearly all studies of vegetation-derived DOM to date report DOM derived from tree canopies (tree-DOM), in this chapter we discuss the quality, potential sources, and potential fates of tree-DOM. We then describe and discuss the drivers of variation of quantitative fluxes of tree-DOM and place these quantitative fluxes in biogeochemical and ecological contexts at scales ranging from the individual tree, forest, and watershed to global trends.
... Therefore, an evaluation of PCN congener profiles is important to perform both fingerprints and risk assessments (Fernandes et al., 2011). Solar irradiation is known to have an impact on persistent organic pollutants (POPs) dispersed in the atmosphere and photic zone of surface waters (Berrojalbiz et al., 2011;Mopper et al., 2015). Thus, sunlight is important force shaping compositional profile of POPs in the environment, including highly persistent or POPs considered as "non-degradable". ...
Article
Solutions of technical polychlorinated naphthalene (PCN) Halowax formulations (Halowax 1014 and Halowax 1051) diluted with Milli-Q water and sealed in the Pyrex glass tubes and quartz tubes were subjected to artificial solar and natural solar irradiation under different time intervals and field conditions. In particular, the results of several field irradiation experiments have shown increased PCN photodegradation as altitude increases above sea level. Irradiation in artificial solar conditions caused a substantial change in the PCN congener profiles of Halowax 1014 and Halowax 1051 test solutions. Interestingly, in long-term experiments, the relative abundance of congeners that contribute to dioxin-like activity, i.e. the compounds such as 1,2,3,5,7- and 1,2,4,6,7-PentaCN (PeCNs #52/60), 1,2,3,4,6,7- and 1,2,3,5,6,7-HexaCN (HxCNs #66/67), and 1,2,3,4,5,6,7-HeptaCN (HpCN #73), temporally increased substantially. In the field photodegradation experiments, the PCNs #52/60 and #66/67 were formed, while a relative persistence of PCN #73 was evident. Highest chlorinated octachloronaphthalene (OcCN #75), exposed to strong UV radiation at high altitude, was much less stable than lower molecular mass PCNs. Photodegradation of the technical PCN formulations produced also an unidentified aromatic compound. We conclude, that photodegradation of PCNs, which are considered as a widespread anthropogenic pollutants, is not restricted to any specific environmental condition. It can also be observed at low altitudes.
... The biogeochemical impact of photochemical processing of DOM is a topic of debate: irradiation partially or completely remineralizes DOM, targeting the complex molecules that microbes struggle to degrade. Photoinduced transformations or breakdown of molecules can render the DOM pool more recalcitrant or more labile to microbial uptake, or have no influence at all (reviewed in Mopper et al., 2015), with most studies reporting a positive effect on bioavailability of photo-processed DOM. On the other hand, high solar irradiation can affect bacterial growth and even in high-light environments such as the South Pacific or the Mediterranean Sea, the majority of bacterial isolates showed some response to UV-B irradiation (Matallana-Surget et al., 2012). ...
Article
The subtropical South Pacific Gyre (SPG) encompasses the largest oligotrophic region of the global ocean. In these remote waters dissolved organic matter (DOM) accumulates in the surface waters, though constituting a potential source of nutrients and energy to sustain microbial life. On a zonal transect across the SPG, we quantified bulk dissolved organic carbon (DOC) and assessed the DOM composition via ultrahigh resolution mass spectrometry (UHR-MS) of solid-phase extracted DOC (SPE-DOC) to elucidate the molecular-level reasons behind the apparent recalcitrance of the DOM prevailing in the SPG. We included a comparison between two individual formula assignment approaches to UHR-MS data in absorption and magnitude mode which yielded consistent results. DOC concentrations exceeding 100 μmol C L⁻¹ in the warm and saline waters of the central gyre were higher than in the surface waters of the adjacent western South Pacific. Along the transect, concentrations of SPE-DOC were less variable than bulk DOC. Nevertheless, molecular-level investigation revealed that the composition of the DOM accumulated in the central SPG generally conformed to characteristics of surface ocean DOM, but all assessed properties were more pronounced. We found high abundances of potentially labile unsaturated aliphatic molecular formulas and a low calculated degradation index for the DOM of likely marine microbial origin. Markedly decreased molar N/C ratios in the central gyre indicated preferential microbial utilization of nitrogen-containing DOM. A distinct imprint of extensive photochemical reworking was manifested in the low aromaticity of the DOM in the photic layer. Over the whole water column, ageing of DOM was evident through the small, but significant contribution of SPE-DOC to apparent oxygen utilization as well as on molecular level. Our findings demonstrate that SPE-DOC captures carbon fractions relevant on timescales of seasons to timescales covering ocean circulation and biogeochemical processes in stable gyre systems are imprinted in the DOM molecular composition.
... Photochemical transformations of CDOM play an important role in marine biogeochemical cycles (Mopper et al., 2015;Powers et al., 2015) and produce various atmospherically reactive and/or climatically active trace gases (Mopper & Kieber, 2002). Carbon monoxide (CO) is arguably the most precisely determined CDOM photoproduct whose global open-ocean photoproduction rate is best known (mean: 40-50 TgC year −1 , Fichot & Miller, 2010;Stubbins et al., 2006;Zafiriou et al., 2003). ...
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Although methanogenesis is considered a strictly anaerobic process, oxygen‐replete surface open‐ocean waters are usually supersaturated with methane (CH4 ), a phenomenon termed the oceanic methane paradox. Here, we report that abiotic methane photoproduction from chromophoric dissolved organic matter (CDOM) significantly contributes to this paradox. Methane photoproduction was observed during solar‐simulated irradiations of various waters collected along the land‐ocean continuum. Methane photoproduction rates decreased seaward, whereas its relative production efficiency and the methane‐tocarbon‐monoxide (CO) photoproduction ratio (ΔCH4 /ΔCO) both followed a reversed trend. Remote‐sensing modeling incorporating a ΔCH4 /ΔCO–CDOM absorption relationship yielded an annual methane photoproduction of 118 Gg for the global open ocean, accounting for 20–60% of the open‐ocean methane efflux and being of comparable magnitude to the upper‐ocean methane microbial‐oxidation sink. The photodegradation of CDOM thus plays an important role in maintaining supersaturated methane concentrations in the oxygenated upper ocean and in sustaining oceanic methane emissions to the atmosphere.
... Two phyla of Archaea, Euryarchaeota and Thaumarchaeota, exhibited an increase in abundance of 7-fold and 28-fold (from 2.42 × 10 6 cells L −1 to 1.72 × 10 7 cells L −1 , and from 1.60 × 10 6 cells L −1 to 4.54 × 10 7 cells L −1 , respectively), over 4 days of incubation in lignin-amended treatments. Additionally, an increase of 11-fold and 13-fold (from 2.93 × 10 6 cells L −1 to 3.30 × 10 7 cells L −1 , and from 3.26 × 10 6 cells L −1 to 4.28 × 10 7 cells L −1 , respectively), was observed in the abundance of these phyla in treatments containing lignin with added nitrogen and phosphorus, thus raising questions regarding primary and/or secondary responses to INTRODUCTION Dissolved organic matter (DOM) is an important component of many ocean biogeochemical cycles (Hedges et al., 1997;Fasham et al., 2001;Mopper et al., 2015) and can serve as a substrate for heterotrophic archaea and bacteria (Cottrell and Kirchman, 2000;Carlson et al., 2002). The majority of DOM in the open ocean is of marine origin (Koch et al., 2005;Carlson and Hansell, 2015); however, terrigenous inputs of compounds such as lignin may be important to the oceanic carbon cycle. ...
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The pelagic ocean receives terrigenous inputs of a range of organic compounds; however, the role that this terrigenous material plays in the ocean carbon cycle and biological pump is not entirely understood, and questions remain as to how oceanic cycles of terrigenous and autochthonous carbon interact. A significant portion of organic carbon that cannot be utilized by marine microbes in the epipelagic ocean escapes microbial remineralization to be sequestered in the deep ocean as refractory dissolved organic matter (DOM). Lignin, a “model” terrigenous compound, is thought to be refractory in the open ocean unless chemically altered. However, in this study, incubation experiments performed using lignin-amended oligotrophic seawater from the Sargasso Sea exhibited bacteria and archaea growth that doubled compared to unamended control treatments. The increase in bacteria and archaea cell abundance in lignin-amended treatments coincided with a 21–25% decrease in absorbance (250–400 nm) of chromophoric dissolved organic matter (CDOM), suggesting that certain microbes may be capable of altering fractions of this ostensibly recalcitrant organic matter. Furthermore, the microbial response to the lignin-amended treatments appears to be taxon-specific. Two phyla of Archaea, Euryarchaeota and Thaumarchaeota, exhibited an increase in abundance of 7-fold and 28-fold (from 2.42 × 106 cells L–1 to 1.72 × 107 cells L–1, and from 1.60 × 106 cells L–1 to 4.54 × 107 cells L–1, respectively), over 4 days of incubation in lignin-amended treatments. Additionally, an increase of 11-fold and 13-fold (from 2.93 × 106 cells L–1 to 3.30 × 107 cells L–1, and from 3.26 × 106 cells L–1 to 4.28 × 107 cells L–1, respectively), was observed in the abundance of these phyla in treatments containing lignin with added nitrogen and phosphorus, thus raising questions regarding primary and/or secondary responses to lignin degradation. Our findings indicate that marine bacteria and archaea play a role in the transformation of the optical properties of lignin in the open ocean and that they may serve as a potential sink for a portion of the lignin macromolecule.
... Transformation from labile to RDOM by the bacteria has attracted attention as one of the key processes of accumulating DOM pool as microbial carbon pump (Jiao et al., 2010;Zhang et al., 2018). Concomitant with the microbial activity, DOM undergoes photochemical reaction under sunlight irradiation (Mopper et al., 2015). However, the quantitative and qualitative changes taking place in the DOM in the surface layer due to microbial activity and photochemical reaction are difficult to understand because DOM is a mixture of many unidentified organic compounds (Dittmar and Paeng, 2009). ...
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Humic-like fluorescent dissolved organic matter (FDOM) has been widely used as tracers for bio-refractory dissolved organic matter (RDOM) to understand its dynamics in the oceans. Vertical distributions of humic-like FDOM are controlled by microbial production in the ocean interiors and photobleaching in surface layers. Although humic-like FDOM is expected to be actively produced in surface layers with high bacterial activity, its production in surface seawater is not well understood. To examine the diurnal variations in humic-like FDOM due to microbial production and photobleaching in surface seawater, we conducted seven experiments from night to day using surface seawater in the subtropical Pacific and coastal regions. Parallel factor analysis (PARAFAC) determined that FDOM in the incubated seawater was composed of three components: two types of humic-like FDOM and a protein-like FDOM. The fluorescence intensity of humic-like FDOM increased to 104.0 ± 2.5% of the initial intensity during the night and decreased to 101.2 ± 2.5% under sunlight exposure during the day. Conversely, its intensity significantly increased to 114.0 ± 2.7% under dark conditions during the day. The turnover rates of humic-like FDOM by the increase and decrease in its intensity were estimated to be 0.14 and 0.11 day–1, respectively. These comparable turnover rates indicated that the production and photobleaching of humic-like FDOM were almost in equilibrium in the surface layer, with a low level of humic-like FDOM. Linear correlations between the intensity of humic-like FDOM and concentrations of dissolved oxygen (DO) in all experiments under dark conditions indicated that humic-like FDOM were produced as the by-products of microbial respiration processes in the surface seawater. Using global bacterial respiration rates, the net production rate of humic-like FDOM in the global photic layer was estimated as 4.2–5.5 × 1017 R.U. year–1, contributing to 75% of its production in the entire ocean.
... Chromophoric dissolved organic matter (CDOM) represents a sub-fraction of the total pool of dissolved organic matter (DOM) that absorbs light over a broad range of the sunlight spectrum, particularly in the UV and blue region [1]. CDOM plays key roles in biogeochemical processes [2,3], determining the underwater light availability [4,5], controlling and mitigating the ultraviolet (UV) light penetration [6], and providing physiological protection to UV radiation (UVR)-sensitive organisms from damage (e.g., phytoplankton, coral reefs, macroalgae). Furthermore, CDOM can be a tracer of ecosystem processes in coastal environments [2,7,8]. ...
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Posidonia oceanica is a well-recognized source of dissolved organic matter (DOM) derived from exudation and leaching of seagrass leaves, but little is known about its impact on the chromophoric fraction of DOM (CDOM). In this study, we monitored for two years the optical properties of CDOM in two contrasting sites in the Mallorca Coast (Balearic Islands). One site was a rocky shore free of seagrass meadows, and the second site was characterized by the accumulation of non-living seagrass material in the form of banquettes. On average, the integrated color over the 250-600 nm range was almost 6-fold higher in the beach compared with the rocky shore. Furthermore, the shapes of the CDOM spectra in the two sites were also different. A short incubation experiment suggested that the spectral differences were due to leaching from P. oceanica leaf decomposition. Furthermore, occasionally the spectra of P. oceanica was distorted by a marked absorption increase at wavelength < 265 nm, presumably related to the release of hydrogen sulfide (HS −) associated with the anaerobic decomposition of seagrass leaves within the banquettes. Our results provide the first evidence that P. oceanica is a source of CDOM to the surrounding waters.
... The majority of studies involving PR-DOM conclude that this material enhances microbial activity in the ocean (Mopper and Kieber, 2002;Mopper et al., 2015;Cory and Kling, 2018). In suspended sediments, knowledge of the extent of bioavailability of PR-DOM is inconclusive and the corresponding studies lack detailed chemical analyses of DOM transformations to parse out these inconsistencies (Mayer et al., 2011;Schiebel et al., 2015). ...
Article
Photochemically-released dissolved organic matter (PR-DOM) from resuspended sediments is an understudied flux of dissolved organic carbon (DOC) and nutrients that has the potential to influence estuarine microbial food webs. There is currently limited knowledge on the composition, lability, and biological alterations of this material once released into the water column. This study addresses the composition and fate of PR-DOM from resuspended sediments of the Cape Fear River estuary (CFRE) in southeastern North Carolina. Six-hour irradiation released 22-44% more DOC, and PR-DOM was of a different composition and enhanced lability relative to dark controls. Irradiation led to release of humic-like DOM, indicated by increased chromophoric and fluorescent dissolved organic matter, substantial increases in the humification index, and production of oxidized higher molecular weight compounds with higher aromaticity relative to dark controls. However, DOM of lower molecular weight and reduced aromaticity was produced as well – indicated by increased spectral slope (S275-295) and decreased specific ultraviolet absorbance at 254 nm (SUVA254) relative to dark controls. This latter pool of DOM may be linked to nitrogen-(N-) and sulfur-(S-)containing compounds that were photodegraded or biologically altered during irradiation experiments. In subsequent lability incubation experiments, degradation of PR-DOM was more rapid than DOM released from dark controls, especially for marine humic-like fluorophores. Incubation of PR-DOM led to an 8-fold increase in molecular formulas that were unique to light-exposed DOM relative to unexposed DOM, the majority of which were N- and S-containing compounds. Given that coastal sediments are typically enriched in these nutrients, N- and S-containing compounds appear to influence the lability of PR-DOM from estuarine resuspended sediments. Estimated lability of photoreleased DOC from resuspension events in the CFRE is comparable to previous bioavailable DOC estimates and suggest that episodic photochemical interactions with sediments may act as a previously unrecognized source of labile DOC to bacteria and plankton in these coastal waters.
... Dissolved organic carbon (DOC) is a master variable in aquatic systems where it controls the light field, initiates photochemical reactions (Spencer et al. 2009;Mopper et al. 2015), and provides sustenance to microbes at the base of aquatic food webs (Moran et al. 2016). Additionally, with a DOC pool similar in size to that of atmospheric CO 2 (Hansell and Carlson 1998), DOC is an important component of the global carbon cycle (Dittmar and Stubbins 2014). ...
Article
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Dissolved organic carbon (DOC) is a master variable in aquatic systems. Resolving DOC dynamics requires high‐temporal resolution data. However, DOC concentration cannot be directly measured in situ, and discrete sample collection and analysis becomes expensive as temporal resolution increases. To surmount this problem, an option is to predict site‐specific DOC concentration with linear modeling and optical data predictors collected from high‐cost, high‐maintenance in situ spectrophotometers. This study sought to improve upon the accuracy and field costs of linear predictive DOC methods by using machine learning modeling coupled to low‐to‐zero cost predictors. To do this, we collected 16 months of in situ data (e.g., spectrophotometer attenuation, salinity, temperature), assembled freely available predictors (e.g., point in year, rainfall), and collected samples for DOC analysis, all in a salt marsh creek. At seasonal timescales, machine learning (coefficient of determination [R2] = 0.90) modestly improved upon the accuracy of linear methods (R2 = 0.80) but offered substantial instrumentation cost reductions (~ 90%) by requiring only cost‐free predictors (online data) or cost‐free predictors paired with low‐cost in situ predictors (temperature, salinity, depth). At intertidal timescales, linear methods proved ill‐equipped to predict DOC concentration compared to machine learning, and again, machine learning offered a substantial instrumentation cost reduction (~ 90%). Although our models were developed for and applicable to a single site, the use of machine learning with low‐to‐zero cost predictors provides a blueprint for others trying to model DOC dynamics and other analytes in any complex aquatic system.
... The photodegradation processes play an important role in releasing the elements bound by biologically refractory forms in the environment, and are gradually considered to be an important factor in the biogeochemical cycle. Degraded organic molecules are converted into low-molecularweight compounds and inorganics, which increases their bioavailability and thus their mobility across environmental compartments [6]. Increasing attention has been paid to the role of the photodegradation in the biogeochemical cycle, especially for those environments that are inherently slow due to low temperatures, lack of water, or biological processes that occur in them [7]. ...
... 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). ...
Article
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.
... 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). ...
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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.
... In this context, the term "apparent" indicates that the actual sensitizer(s) responsible for 1 O 2 production is unknown, a consequence of the complex molecular nature of CDOM. 27 The apparent 1 O 2 quantum yield is an intensive property of CDOM, as it accounts for variations in sensitizers' absorption spectra and concentration, as well as for variations in light intensity. This feature makes Φ Δ a useful parameter in environmental chemistry studies, as it can be used to predict variations in 1 O 2 steady-state concentrations as a function of light intensity (thus, water depth, presence of other water constituents, DOM concentration, seasonal light intensity fluctuations, etc.). 2 Indeed, apparent quantum yields of PPRIs, including 1 O 2 , are needed as the input parameters in predictive models of steady-state concentrations and micropollutants' lifetimes. ...
Article
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Singlet oxygen (¹O2) is a reactive oxygen species produced in sunlit waters via energy transfer from the triplet states of natural sensitizers. There has been an increasing interest in measuring apparent ¹O2 quantum yields (ΦΔ) of aquatic and atmospheric organic matter samples, driven in part by the fact that this parameter can be used for environmental fate modeling of organic contaminants and to advance our understanding of dissolved organic matter photophysics. However, the lack of reproducibility across research groups and publications remains a challenge that significantly limits the usability of literature data. In the first part of this review, we critically evaluate the experimental techniques that have been used to determine ΦΔ values of natural organic matter, we identify and quantify sources of errors that potentially explain the large variability in the literature, and we provide general experimental recommendations for future studies. In the second part, we provide a qualitative overview of known ΦΔ trends as a function of organic matter type, isolation and extraction procedures, bulk water chemistry parameters, molecular and spectroscopic organic matter features, chemical treatments, wavelength, season, and location. This review is supplemented with a comprehensive database of ΦΔ values of environmental samples.
... Reaction rates are determined by the availability of light and DOC, and the chemical composition of the DOC. Photooxidation is dependent on the intensity and duration of incoming solar irradiation, mainly in the UV-A, UV-B, and visible light ranges of the spectra (approximately 280-600 nm) (Bowen, Kaplan, et al., 2020;Mopper et al., 2015;Vähätalo et al., 2000). In forested environments, incoming solar irradiation can be physically blocked from entering the water column by the tree canopy cover, which can vary substantially by season in temperate rivers due to the prominence of deciduous tree species (Detenbeck et al., 2016;Julian et al., 2008) (Figure 1). ...
Article
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Sunlight can oxidize dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in freshwaters. The importance of complete photooxidation, or photomineralization, as a sink for DOC remains unclear in temperate rivers, as most estimates are restricted to lakes, high latitude rivers, and coastal river plumes. In this study, we construct a model representing over 75,000 river reaches in the Connecticut River Watershed (CRW), USA, to calculate spectrally resolved photomineralization. We test the hypothesis that photomineralization is a negligible DOC sink across all reaches and flow conditions relative to DOC fluxes. Our model quantifies reaction rates and transport drivers within the river reaches for the ranges of flow conditions, incoming solar irradiance, and canopy cover shading observed throughout the year. Our model predicts average daily areal photomineralization rates ranging from 1.16 mg‐C m⁻² day⁻¹ in low flow river reaches in the winter, to 18.33 mg‐C m⁻² day⁻¹ in high flow river reaches during the summer. Even for high photomineralization fluxes, corresponding photomineralization uptake velocities are typically at least an order of magnitude smaller than those reported for other instream processes. We calculate DOC elimination by photomineralization relative to DOC fluxes through individual stream reaches as well as the entire riverine portion of the CRW. We find that relative photomineralization fluxes are highest in summer drought conditions in low order streams. In median flows and mean light intensities, for an average watershed travel distance, 3%–5% of the DOC fluxes are eliminated, indicating that photomineralization is a minor DOC sink in temperate rivers.
... 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. ...
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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, ...
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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.
... Fig. S5). CDOM refers to the fraction of dissolved organic matter that absorbs UV-infrared electromagnetic radiation, and its absorption coefficient spectrum always decreases exponentially from the UV-B to the red region (Mopper et al., 2015;Stedmon and Nelson, 2015). The exponential shape observed for all K d (λ) spectra and the large range of K d (λ) values in the UV and blue region are consistent with CDOM being a dominant driver of optical variability in this system (Fichot et al., 2008;Stedmon and Nelson, 2015). ...
Article
Seagrass meadows worldwide provide valuable ecosystem services but have experienced sharp declines in recent decades. This rapid loss has prompted numerous restoration efforts with variable levels of success, often depending on the suitability of the restoration sites. The selection of sites can be guided by simple habitat suitability models driven with environmental variables deemed critical to the successful growth of new transplants. Habitat suitability models typically consider the influence of bathymetry, sediment type, salinity, wave exposure, and water quality. However, they typically do not explicitly include benthic exposure to ultraviolet (UV) and commonly use depth as a coarse proxy for photosynthetically active radiation (PAR). Benthic exposure to UV and PAR are both key parameters for habitat suitability but can be challenging to determine, especially in coastal environments influenced by rivers and tides where they are extremely variable. Here, we demonstrate the development of a simple but effective model of spectrally-resolved benthic solar irradiance for a dynamic marsh-influenced mesotidal estuary in Massachusetts. In-situ measurements were used to develop and validate an empirical model predicting the UV–visible vertical diffuse attenuation coefficient spectra of downwelling irradiance, Kd(λ), from simple physical parameters about tides, river discharge and location. Spectral benthic solar irradiances (280–700 nm) were calculated hourly for 3 years (2017–2019) using modeled and validated cloud-corrected surface downwelling irradiances, estimates of water depth, and the modeled Kd(λ) spectra. The mapped irradiances were used to provide improved seagrass habitat suitability maps that will guide future restoration efforts in the estuary. We expect the approach presented here can be adapted to other dynamic coastal environments influenced by tides and rivers and/or applied to other light-dependent organisms and biogeochemical processes.
... Marine dissolved organic matter (DOM), one of the largest reservoirs of organic carbon in the global carbon cycle, has a wide variety of sources (e.g., biological, microbial, and terrestrial sources) (Hansell and Carlson, 2014). The intricate interplay between DOM and physical and biological processes solidifies its important role in aquatic ecosystems (Mopper et al., 2015;Repeta, 2015). ...
Article
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Comprising one of the major carbon pools on Earth, marine dissolved organic matter (DOM) plays an essential role in global carbon dynamics. The objective of this study was to better characterize DOM in the eastern Indian Ocean. To better understand the underlying mechanisms, seawater samples were collected in October and November of 2020 from sampling stations in three subregions: the mouth of the Bay of Bengal, Southern Sri Lanka, and Western Sumatra. We calculated and evaluated different hydrological parameters and organic carbon concentrations. In addition, we used excitation emission matrix (EEM) spectroscopy combined with parallel factor analysis (PARAFAC) to analyze the natural water samples directly. Parameters associated with chromophoric DOM did not behave conservatively in the study areas as a result of biogeochemical processes. We further evaluated the sources and processing of DOM in the eastern Indian Ocean by determining four fluorescence indices (the fluorescence index, the biological index, the humification index, and the freshness index β/α). Based on EEM-PARAFAC, we identified six components (five fluorophores) using the peak picking technique. Commonly occurring fluorophores were present within the sample set: peak A (humic-like), peak B (protein-like), peak C (humic-like), and peak T (tryptophan-like). The fluorescence intensity levels of the protein-like components (peaks B and T) were highest in the surface ocean and decreased with depth. In contrast, the ratio of the two humic-like components (peaks A and C) remained in a relatively narrow range in the bathypelagic layer compared to the surface layer, which indicates a relatively constant composition of humic-like fluorophores in the deep layer.
... 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, ...
... Breakage of algal cells through autolysis, zooplankton grazing, or virus attack releases particulate DMSP into the dissolved phase which can be degraded by bacteria to produce DMS (Sim o 2004;Stefels et al. 2007). Another contributor to DMS production is the diffusive release of DMS from algal cells, which proceeds almost instantaneously after intracellular DMSP cleavage by DMSP lyases or photo-chemically generated radicals (Lavoie et al. 2015;Mopper et al. 2015). The available dissolved DMSP and DMS represent abundant sources of energy, carbon, and sulfur for marine bacterial communities. ...
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Oceanic production and occurrence of dimethyl sulfide (DMS) and its subsequent ventilation to the atmosphere significantly contribute to the global sulfur cycle and impact the climate regulation. Spatial distributions of DMS, dimethylsulfoniopropionate (DMSP, precursor of DMS), and dimethyl sulfoxide (DMSO, oxidation product of DMS), production and removal processes of DMS (including biological production, microbial consumption, photo‐degradation, and sea‐to‐air exchange), and biogenic contributions to the atmospheric sulfate burden were simultaneously studied in the western Pacific Ocean during winter. Sea surface DMS, DMSP, and DMSO were strongly correlated and had similar distribution patterns. The DMS photo‐degradation efficiency ratio (normalized using incident photon flux density) for ultraviolet B radiation (UVB): ultraviolet A radiation (UVA): photosynthetically active radiation (PAR) was 391: 36: 1. However, considering the solar spectral composition, the actual contributions of UVB, UVA, and PAR to DMS photo‐degradation in surface waters were 40.6% ± 10.7%, 41.2% ± 15.6%, and 18.2% ± 7.2%, respectively. When integrated across the entire mixed layer, UVA and PAR became the dominant drivers, accounting for 45.2% ± 18.0% and 38.0% ± 17.3% of DMS photo‐degradation, respectively, as UVB was significantly attenuated in seawater. The DMS budget of the entire mixed layer indicated that microbial consumption, photo‐degradation, and ventilation accounted for about 74.3% ± 11.9%, 19.3% ± 9.3%, and 6.5% ± 4.0% of total DMS removal, respectively. Even if ventilation was a minor DMS removal pathway, DMS emissions still contributed approximately 45.2% ± 25.6% of the atmospheric non‐sea‐salt sulfate burden over the western Pacific Ocean.
... Moreover, the term rDOC is strongly dependent on ecosystem properties, with compounds being "refractory" in one and degraded in other environments or conditions (Ward et al., 2013;Bianchi et al., 2018; Figure 1). For example: (i) ultraviolet radiation can transform DOC compounds from recalcitrant to bioavailable and vice versa (Benner and Biddanda, 1998;Obernosterer et al., 1999;Mopper et al., 2015;Shen and Benner, 2018;Sun et al., 2021); (ii) "Priming, " with the addition of highly bioavailable compounds, may result in enhanced degradation of more refractory compounds (Bianchi, 2011;Shen and Benner, 2018), although the process is controversial outside of soil science (Catalán et al., 2015;Bengtsson et al., 2018); (iii) rDOC that is resistant to degradation by the microbial community of a given ecosystem can be utilized by microbes of another ecosystem (Carlson et al., 2004;Shen and Benner, 2018); (iv) hydrostatic pressure can deform enzymes, making them less effective in cleaving substrates (Penniston, 1971); (v) sorption and aggregation with "(in)organic particles" might hamper degradation in specific settings while stimulating degradation in others similar to soil (Keil and Mayer, 2014). Labile compounds might become less reactive if adsorbed to clay or held under anaerobic conditions, such as with ancient DNA (Lützow et al., 2006). ...
Article
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About 20% of the organic carbon produced in the sunlit surface ocean is transported into the ocean’s interior as dissolved, suspended and sinking particles to be mineralized and sequestered as dissolved inorganic carbon (DIC), sedimentary particulate organic carbon (POC) or “refractory” dissolved organic carbon (rDOC). Recently, the physical and biological mechanisms associated with the particle pumps have been revisited, suggesting that accepted fluxes might be severely underestimated (Boyd et al., 2019; Buesseler et al., 2020). Perhaps even more poorly understood are the mechanisms driving rDOC production and its potential accumulation in the ocean. On the basis of recent conflicting evidence about the relevance of DOC degradation in the deep ocean, we revisit the concept of rDOC in terms of its “refractory” nature in order to understand its role in the global carbon cycle. Here, we address the problem of various definitions and approaches used to characterize rDOC (such as turnover time in relation to the ocean transit time, molecule abundance, chemical composition and structure). We propose that rDOC should be operationally defined. However, we recognize there are multiple ways to operationally define rDOC; thus the main focus for unifying future studies should be to explicitly state how rDOC is being defined and the analytical window used for measuring rDOC, rather than adhering to a single operational definition. We also conclude, based on recent evidence, that the persistence of rDOC is fundamentally dependent on both intrinsic (chemical composition and structure, e.g., molecular properties), and extrinsic properties (amount or external factors, e.g., molecular concentrations, ecosystem properties). Finally, we suggest specific research questions aimed at stimulating research on the nature, dynamics, and role of rDOC in Carbon sequestration now and in future scenarios of climate change.
... As the most far-reaching human modifications of the flows of rivers, reservoirs take on various kinds of functions including flood control, power generation, navigation, drinking water supply. However, the construction and operation of reservoirs lead to signif-tention ( Mopper et al., 2014 ). The significant influence of hydrology (e.g., discharge changes, rainfall alterations) on the quantity and quality of DOM has been assessed in natural aquatic ecosystems ( Inamdar et al., 2011 ;Bao et al., 2019 ), however, the effect of reservoir hydrological management-induced variations in DOM cycling was less constrained Ma and Li, 2020 ;Maavara et al., 2020 ;Wang et al., 2021 ). ...
Article
With the linkage between dissolved organic matter (DOM) and the characteristics of natural ecosystem assessed extensively, the properties of DOM in reservoirs, the typical human interrupted ecosystems, have been focused on in recent years, which is critical for the understanding of human impacts on watershed ecosystems and carbon cycling. This study aims to analyze the effect of hydrological management on the DOM chemistry and organic carbon burial in Daning River tributary of the world’s largest Three Gorges Reservoir (TGR). Based on the application of a combined approach including bulk geochemical analyses, optical spectroscopy, and ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry, various sources of DOM (terrestrial, anthropogenic, and autochthonous sources) were revealed. An increasing trend of terrestrial and recalcitrant DOM was observed along the upstream to downstream transect of Daning River tributary, which was mainly caused by the water intrusion with a higher terrestrial and recalcitrant signature from mainstream to tributary resulted from hydrological management of TGR. Integrated with the analysis of sedimentary organic matter in Daning River tributary in the past decade (after the construction of TGR), our work suggests that organic carbon burial in the reservoir could be enhanced by hydrological management-induced variation in DOM chemistry. Further studies are needed to better constrain the effects of damming reservoirs on carbon cycling considering their booming all over the world.
Article
Plastic contamination of the environment is a global problem whose magnitude justifies the consideration of plastics as emergent geomaterials with chemistries not previously seen in Earth’s history. At the elemental level, plastics are predominantly carbon. The comparison of plastic stocks and fluxes to those of carbon reveals that the quantities of plastics present in some ecosystems rival the quantity of natural organic carbon and suggests that geochemists should now consider plastics in their analyses. Acknowledging plastics as geomaterials and adopting geochemical insights and methods can expedite our understanding of plastics in the Earth system. Plastics also can be used as global-scale tracers to advance Earth system science.
Article
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.
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The sea surface microlayer (SML) at the air–sea interface is <1 mm thick, but it is physically, chemically, and biologically distinct from the underlying water and the atmosphere above. Wind-driven turbulence and solar radiation are important drivers of SML physical and biogeochemical properties. Given that the SML is involved in all air–sea exchanges of mass and energy, its response to solar radiation, especially in relation to how it regulates the air–sea exchange of climate-relevant gases and aerosols, is surprisingly poorly characterized. MILAN (Sea Surface Microlayer at Night) was an international, multidisciplinary campaign designed to specifically address this issue. In spring 2017, we deployed diverse sampling platforms (research vessels, radio-controlled catamaran, free-drifting buoy) to study full diel cycles in the coastal North Sea SML and in underlying water, and installed a land-based aerosol sampler. We also carried out concurrent ex situ experiments using several microsensors, a laboratory gas exchange tank, a solar simulator, and a sea spray simulation chamber. In this paper we outline the diversity of approaches employed and some initial results obtained during MILAN. Our observations of diel SML variability show, for example, an influence of (i) changing solar radiation on the quantity and quality of organic material and (ii) diel changes in wind intensity primarily forcing air–sea CO2 exchange. Thus, MILAN underlines the value and the need of multidiciplinary campaigns for integrating SML complexity into the context of air–sea interaction.
Article
We investigated the bio- and photo-lability of dissolved organic matter (DOM) from the head, mixing zone, and mouth of the Pearl River estuary. At all three sites, bio- and photo-refractory dissolved organic carbon (DOC) and biorefractory chromophoric DOM (CDOM) dominated over the corresponding bio- and photo-labile constituents, while photolabile CDOM dominated over photo-refractory CDOM. Relative to the mixing-zone and mouth waters, the headwater was enriched with bio- and photo-labile DOC and photolabile CDOM and depleted with biolabile CDOM. Biolabile DOC was richer than photolabile DOC in the headwater, while photolabile CDOM was richer than biolabile CDOM at all three sites. Pre-biotransformation inhibited, stimulated, or had little impact on DOM photodegradation, depending on site. Ultra-violet absorption coefficients are indicators of bio- and photo-refractory DOC. The relative proportions of transparent and chromophoric DOM control the turnover of biolabile DOC and the effect of pre-biotransformation on DOM photodegradation.
Article
Salt marshes are highly productive ecosystems that efficiently accumulate carbon. However, it remains unclear how much carbon is exported from salt marshes (i.e., outwelled) through creeks as dissolved organic carbon (DOC). Addressing this uncertainty is critical for quantifying net carbon accumulation and determining the role marshes play in broader coastal carbon cycles. Consequently, this study sought to quantify net DOC fluxes through Groves Creek, the tidally-driven main creek channel of Groves Creek salt marsh on the Georgia coast (USA). To do this, near-continuous records of water flux, salt flux, and DOC flux were determined in Groves Creek's primary creek channel at 10-minute resolution over 16 months. At our study site, we found that generally more water flowed onto the marsh through Groves Creek's primary creek channel during flood tide than left this route during ebb tide. The remaining unsampled water balance was assumed to leave the marsh via secondary marsh channels and over the marsh edge. Due to this limitation, the conservative nature of salt was utilized alongside flow and DOC to determine net DOC trends in Grove Creek's primary creek channel. Cumulative net DOC-salt relationships are linear for most of the study, indicating DOC largely behaved conservatively in the creek channel and wider marsh system. However, a period of non-conservative behavior in which the cumulative net DOC:salt ratio decreased indicated net DOC outwelling (4.1–30.1 g DOC m⁻² y⁻¹) during summer 2014. Results indicate DOC outwelling events, as observed here, may occur in pulses during highly productive summer months. Resolving these “hot” moments of DOC export at high-temporal resolution across larger salt marsh ecosystems is required to assess the true extent and quantitative significance of DOC outwelling to coastal carbon cycles and ecology.
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Systematic surveys to examine seasonal variation, degradation, and bioavailability of dissolved organic matter (DOM) in the Changjiang Estuary and the adjacent East China Sea were conducted in July 2016 and February 2017. Concentrations of dissolved organic carbon (DOC) and total hydrolyzable amino acids (THAA) were higher in July than in February. THAA and chlorophyll a (Chl‐a) were positively correlated in July, but not in February. The carbon normalized yields of THAA (THAA‐C%) in surface waters in July and February were not significantly different. However, degradation index (DI) values in the surface water were higher in February than in July. Compared with outer estuary, the inner estuary had lower THAA‐C% and DI values in both surveys. Solar radiation experiments showed that THAA and THAA‐C% values increased with time at station A6‐11 in the oceanic water but declined at C3 in the freshwater, possibly due to the different origins, chemical compositions, or initial degradation states of the DOM at the time of collection. Microbial incubation experiments showed that accumulated DOC and DON in surface waters were bioavailable to the microbial community of the surface layer, but recalcitrant to the microbial fauna from the bottom layer. Leucine (Leu) was selectively consumed, while glycine (Gly), threonine, and alanine appeared to be recalcitrant in summer (July) microbial incubations; and histidine, Gly, and methionine were preferentially consumed, while aspartic acid, serine, phenylalanine, and Leu were recalcitrant in winter (February) incubations.
Article
Long-term patterns in dissolved organic carbon (DOC) concentrations in 49 eastern Canadian lakes from four sites were re-examined with a ~ 35-year (~1980–2015) dataset. The study sites were Dorset (number of lakes, n = 8), Experimental Lakes Area (ELA, n = 4), Kejimkujik (n = 26) and Yarmouth (n = 11). Lake DOC patterns were synchronous within each site. However, comparisons of DOC patterns across sites showed that they were synchronous only between the Kejimkujik and Yarmouth locations. Hence, these two sites were pooled into a single Nova Scotia site (NS). Increases in DOC concentration were evident in Dorset, Ontario from 1988 (r² = 0.78, p < 0.001) and NS from 2000 (r² = 0.43, p = 0.006). DOC at the ELA in northwestern Ontario had a different pattern compared to the other sites, i.e., DOC had increased earlier (1983–2000), and then, unlike Dorset and NS, neither an increase nor decrease was detected between 2001 and 2015 (p = 0.78). Precipitation and sulfur deposition explained the greatest variance in DOC patterns at the Dorset and NS sites (i.e., precipitation: 21–49% and sulfur deposition: 24–54%). Precipitation was the most important driver of DOC at the ELA. Our results indicate that all the sites have gone through a process of increasing DOC, but at different times. The stabilizing pattern at the ELA since 2001 may suggest that DOC concentrations in ELA lakes have reached, or are approaching a new equilibrium, a phenomenon that was not observed at the other sites. Also, the increase in DOC was not always associated with declining sulfur deposition (e.g., ELA). Therefore, we conclude that there was considerable variation in DOC patterns across this large geographic region of Canada and potential drivers of these patterns were not consistent across these diverse sites.
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Dissolved organic carbon (DOC) impacts water quality, the carbon cycle, and the ecology of aquatic systems. Understanding what controls DOC is therefore critical for improving large-scale models and best management practices for aquatic ecosystems. The two main processes of DOC transformation and removal, photochemical and microbial DOC degradation, work in tandem to modify and remineralize DOC within natural waters. Here, we examined both the photo- and microbial remineralization of DOC (photolability and biolability), and the indirect phototransformation of DOC into biolabile DOC (photoprimed biolability) for samples that capture the spatiotemporal and hydrological gradients of the Connecticut River watershed. The majority of DOC exported from this temperate watershed was photolabile and the concentration of photolabile DOC correlated with UV absorbance at 254 nm (r2 = 0.86). Phototransformation of DOC also increased biolability, and the total photolabile DOC (sum of photolabile and photoprimed biolabile DOC) showed a stronger correlation with UV absorbance at 254 nm (r2 = 0.92). We estimate that as much as 49% (SD = 3.3%) and 10% (SD = 1.1%) of annual DOC export from the Connecticut River is directly photolabile and photoprimable, respectively. Thus, 2.82 Gg C year−1 (SD = 0.67 Gg C year−1) or 1.13 Mg C km−2 year−1 (SD = 0.27 km−2 year−1) of total photolabile DOC escapes photochemical degradation within the river network to be exported from the Connecticut River each year.
Article
Chromophoric dissolved organic matter (CDOM) are the precursors to singlet oxygen (1O2) in natural waters while water is the main scavenger. In this study, we showed that 1O2 in coastal seawater can be successfully predicted from CDOM parameters. The 1O2 steady-state concentration [1O2]ss and photoformation rate (R1O2) varied by a factor of six across thirteen sampling stations in the Seto Inland Sea, Japan, ranging from 1.2 - 8.2 × 10-14 M and 3.32 - 22.7 × 10-9 M s-1, respectively. Investigation of CDOM optical properties revealed that CDOM abundance measured as absorption coefficient at 300 nm (a300) had the strongest correlation (r = 0.96, p<0.001) with [1O2]ss while parameters indicative of CDOM quality (e.g. spectral slope) did not influence the [1O2]ss. A linear relationship between [1O2]ss and a300, normalized to a sunlight intensity of 0.91 kW/m2, was derived as [1O2]ss (10-14 M) = 2.12(a300) + 0.48. This was then used to predict [1O2]ss using a300 values from a subsequent, independent sampling exercise conducted two years after the first sampling. There was a good agreement (r =0.93, p<0.001) between the predicted values and the experimentally-determined values based on a 95% prediction interval plot. Kinetic estimations using the [1O2]ss suggest that 1O2 mediates the degradation of tetrabromobisphenol A in surface seawater (t1/2 = 0.63 days) while also contributing to the indirect photolysis of methyl mercury. The findings from this study suggest that large-scale modelling of 1O2 generation in surface seawater from CDOM parameters is possible with useful environmental significance for determining the fate of pollutants.
Article
In this study, dissolved organic carbon (DOC) data and optical properties (absorbance and fluorescence) of DOM, weekly collected in the Arno River for 2 years, are used to investigate the main processes determining DOM temporal dynamics in a small Mediterranean river, with torrential hydrology and medium-high human impact, and to quantify the contribution of this river to Med Sea carbon budget. A clear seasonal cycle of DOM, with DOC values ranging between 170 and 490 μM, was observed. Optical properties indicates that DOM quality in the river is different depending on the season; terrestrial humic-like substances prevail in winter, when discharge and floods are the main drivers of DOM concentration and quality, whereas autochthonous protein-like substances prevail in spring and summer, when biological processes dominate. Our results provide a robust estimate of the DOC flux to the Med Sea (9.6 · 10⁹ g DOC yr⁻¹) and of its range of variability (12.95 · 10⁹–5.12 · 10⁹ g DOC yr⁻¹). The 80% of this flux was generally delivered during autumn/winter with significant amounts ascribed to single flood events (up to 26% in 2014). This study, by providing a rich dataset on water quantity and quality and by quantifying the importance of the hydrological regime on DOC transport, represents an important step toward a quantitative modeling of the Arno River.
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This photochemistry article introduces the mechanistic principles of photochemical reactions and their importance for the transformation of natural and anthropogenic compounds in surface waters. When chromophores absorb solar radiation, the subsequent photochemical reactions can transform compounds directly or produce reactive intermediates, such as reactive oxygen species, which mediate chemical transformations. Photochemical reactions typically break down organic matter and pollutants into biologically labile forms and can cause complete mineralization of organic matter. Photochemical reactions are a major component in the carbon cycling and self-depollution of inland waters.
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Organic matter has an important role in biogeochemistry in aquatic environments. This study investigated impact of suspended particulate organic matter (SPOM) on fluorescence signal of mixtures of three water types (river water RW, sea water SW, effluent wastewater WW) using fluorescence (excitation-emission matrix, EEM) spectroscopy and parallel factor analysis (PARAFAC) and multilinear regression. Four irradiation experiments (Expt-1, Expt-2, Expt-3, and Expt-4) were conducted during different times of the year ( two in autumn, one in winter, and one in spring season). Samples were exposed to natural sunlight on laboratory rooftop in University of Toulon, France, with another set of samples kept in dark as control samples. Three component (C1, C2, C3) model was validated by split-half and Concordia from the whole EEM dataset of all irradiation experiments. No protein-like fluorophores was found. The study revealed the effect of SPOM presence/absence on fluorescence signal of DOM and on resulting parameters of multilinear regression MLR model and kinetic constant of these MLR parameters. Kinetic constant (k) for all MLR coefficients was in order of greatness as Expt-1 (SPOM of WW only in mixtures) > Expt-3 (SPOM of SW only in mixtures) > Expt-2 (SPOM of RW only in mixtures)> Expt-4 (SPOM of RW + SW + WW in mixtures) indicating that SPOM of WW is the most resistant to photodegradation. For dark control samples, only relative standard deviation RSD could be calculated from dataset. RSD values for C3 were the highest indicating its chaotic variations, and the lowest RSD values were found for both C1 and C2 for all experiments. Statistical differences has been found between control and irradiated experiments. These models developed in this study can be used to predict fluorescence signal of anthropogenic effluent DOM during its transport in river systems to coastal zone.
Article
Time-resolved fluorescence spectra of chromophoric dissolved organic matter (CDOM) from different sources were acquired using UV (280 and 375 nm) and visible light (440 and 640 nm) excitation to probe the structural basis of the emission properties of CDOM. Emission decays were faster at the blue and red edge, particularly at the red edge, relative to those acquired from 480 to 550 nm. Based on the lifetime distribution and multi-exponential analysis of the emission decays recorded at different time resolution, current findings demonstrate that the components recovered based on a superposition model have no defined physical meaning. A substantial increase in steady-state fluorescence intensity and only small changes (<30%) of amplitude-weighted average lifetime caused by sodium borohydride reduction suggests that intramolecular fluorescence quenching occurs mainly through formation of ground state charge transfer interactions. Short-lived species (lifetime < 100 ps) dominate the emission decays over wavelengths from 400−800 nm, particularly under excitation at long wavelengths (440 and 640 nm). Compared to locally-excited (LE) states, the contribution of charge-transfer excited states (ECT state) and other short-lived species to the steady-state emission is small because of their very rapid nonradiative relaxation. This study suggests that a careful choice of observation wavelength is needed to distinguish LE state from ECT state.
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Photochemical reactions convert dissolved organic matter (DOM) into inorganic and low-molecular-weight organic products, contributing to its cycling across environmental compartments. However, knowledge on the formation mechanisms of these products is still scarce. In this work, we investigate the triplet-sensitized photodegradation of cysteine sulfinic acid, a (photo)degradation product of cysteine, to sulfate (SO42-). We use kinetic analysis, targeted experiments, and previous literature from several fields of chemistry to explain the elementary steps that lead to the release of sulfate. Our analysis indicates that triplet sensitizers act as 1-electron oxidants on the sulfinate S lone pair. The resulting radical undergoes C-S fragmentation to form SO2, which becomes hydrated to sulfite/bisulfite (S(IV)). S(IV) is further oxidized to SO42- in the presence of triplet sensitizers and oxygen. We point out that the reaction sequence SO2 ⇌ S(IV) → SO42- is valid independently of the chemical structure of the model compound, and might represent a sulfate photoproduction mechanism with general validity for DOS. Our mechanistic investigation revealed that amino acids in general might also be photochemical precursors of CO2, ammonia, acetaldehyde, and H2O2, and that reaction by-products can influence rate and mechanism of S(IV) (photo)oxidation.
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High-resolution spatial distribution (horizontal and vertical) of dissolved organic matter (DOM) in the western boundary current system of the tropical Northwest Pacific (<200 m) in autumn 2017 was characterized for dissolved organic carbon (DOC), chromophoric DOM (CDOM) and fluorescent DOM (FDOM). A strong correlation between DOC and the stratification index indicated that the vertical DOC profile was primarily regulated by physical processes. The association of high aCDOM(254) with the maximum chlorophyll (Chl a) layer infers phytoplankton-sourced DOM. The aCDOM(325) and humic-like FDOM (FDOMH) showed an accumulation in the deeper layer and positive correlations with apparent oxygen utilization and maximum Chl a concentration, suggesting that these components are related to microbial degradation of biogenic materials. Elevated Chl a at the frontal area between the North Equatorial Current (NEC) and cold Mindanao Eddy enhanced DOM production. Input waters from the NEC showed higher DOC, but lower FDOMH, than inflow waters from the New Guinea Coastal Current/Undercurrent (NGC(U)C). A mass balance model estimated a 6-times higher lateral DOC flux from the NEC tropical-gyre branch (12N–7.5N) than that from the subtropical-gyre branch (12N–17N). Based on comparison with long-term (1994–2015) average DOC fluxes for the same season, eddy and upstream processes were revealed to contribute 38, 46 and 40% of lateral DOC fluxes for the NEC tropical-gyre branch, NGC(U)C and export North Equatorial Counter Current, respectively. These results demonstrated that the quasi-permanent Mindanao and Halmahera eddies greatly enhance lateral export of DOM with altered properties throughout this large conjunction area.
Article
The moderate DI13C isotope enrichment (MoDIE) method by Powers et al. (2017) is a promising method to precisely measure the photochemical mineralization of dissolved organic carbon (DOC) in water samples without dramatically altering a sample's pH or organic carbon pool. Here, we evaluated the analytical uncertainties of the MoDIE method and used Monte Carlo simulations to optimize the experimental design for the most precise measurements of dissolved inorganic carbon (DIC) that is produced photochemically (DIChν). Analytically, we recommend calculating yields of DIChv with an exact expression of conservation of mass that intrinsically reduces error and uncertainty. Methodologically, the overall uncertainty and detection limit of the MoDIE method can be significantly reduced by partially stripping away the original DIC pool, enriching the residual DIC with more DI13C, and increasing the yields of DIChv via longer irradiation. Instrumentally, more precise measurements of enriched δ13C values before and after irradiation are needed to further improve the precision of DIChν concentration determinations. Higher precision DIChv measurements via the optimized MoDIE method can improve our understanding of the photochemical mineralization of DOC and thus the budget of marine DOC. The optimizations and detection limits reported here will become more refined as measurements and associated uncertainties from future MoDIE experiments become available.
Article
Arguably, the largest knowledge gap in the aquatic photochemistry discipline is the wavelength dependence of sunlight-driven reaction rates in surface waters. Here, we introduce a new light-emitting diode (LED)-based approach to directly quantify the wavelength dependence of aquatic photochemical reaction rates. The LEDs generate narrow-banded, spatially uniform light at five wavelengths (275, 309, 348, 369, and 406 nm), with irradiances that are stable and easily adjusted to desired levels. Strong agreement was observed between irradiance measurements in each LED reactor using chemical actinometry and spectroradiometry. Apparent quantum yield (AQY) spectra of photochemical oxygen consumption by Suwannee River organic matter were determined four times across a six-month period. The shape and magnitude of the AQY spectra were highly reproducible, as indicated by strong exponential fits (R 2 ≥ 0.98) and low variability in oxidation rates across the four trials (coefficient of variation = ∼10%). This LED-based approach is cost effective, high throughput, and portable, allowing a broader community to study the wavelength dependence of aquatic photochemical processes in more detail than was previously possible. We anticipate that this approach will substantially advance our understanding of the wavelength dependence of photochemical reactions in surface waters and improve the accuracy of kinetic models.
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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 <sup>2</sup> > 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.
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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.
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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 (NH<sub>4</sub><sup>+</sup>). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH<sub>4</sub><sup>+</sup> 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 CO<sub>2</sub> and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO<sub>2</sub>:CO:NH<sub>4</sub><sup>+</sup> molar ratio of 165:11:1 in the estuary, 60:3:1 on the shelf, and 18:2:1 in the Canada Basin. The NH<sub>4</sub><sup>+</sup> 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<sup>−1</sup> d<sup>−1</sup> on the Mackenzie Shelf and 3.7 nmol l<sup>−1</sup> d<sup>−1</sup> further offshore; the depth-integrated rates were correspondingly 8.8 μmol m<sup>−2</sup> d<sup>−1</sup> and 11.3 μmol m<sup>−2</sup> d<sup>−1</sup>. The offshore depth-integrated rate in August (8.0 μmol m<sup>−2</sup> d<sup>−1</sup>) 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 NH<sub>4</sub><sup>+</sup> photoproduction in the entire study area was estimated to be 1.4 × 10<sup>8</sup> 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 NH<sub>4</sub><sup>+</sup> 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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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 (R² = 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. Abstract 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 (R² = 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.
Article
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.
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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.
Chapter
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).
Chapter
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).
Chapter
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.
Article
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.
Article
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.