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The organic complexation of iron and copper: An intercomparison of competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques
Abstract
Characterization of the speciation of iron and copper is an important objective of the GEOTRACES Science
Plan. To incorporate speciation measurements into such a multinational program, standard practices must be
adopted that allow data from multiple labs to be synthesized. Competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) is the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater. The determination of concentrations and conditional stability constants of metal-binding ligands is particularly challenging, as results can be influenced both by experimental conditions and interpretation of titration data. Here, we report an investigation between four laboratories to study the speciation of iron and copper using CLE-ACSV. Samples were collected on the GEOTRACES II intercomparison cruise in the North Pacific Ocean in May 2009 at 30° N, 140° W. This intercomparison was carried out shipboard and included an assessment of the viability of sample preservation by freezing. Results showed that consensus values could be obtained between different labs, but that some existing practices were problematic and require further attention in future work. A series of recommendations emerged from this study that will be useful in implementing multi-investigator programs like GEOTRACES.
... For dissolved metal speciation (organic ligand) samples, filtered samples (0.2 μm) should be stored in acid-clean bottles, kept at natural pH (without acidification) and either stored in the fridge (+4 C) or frozen (À4 C or À20 C) until voltammetric analysis in the home laboratory or measured 'fresh' directly on-board ship (Bruland et al. 2000;Buck et al. 2012;Pađan et al. 2020;Sander et al. 2005). In all cases, speciation samples must be stored in the dark in order to prevent photodegradation of the prevalent ligands, and it should be verified that the pre-cleaning procedure does not result in leaching of acid into the sample (i.e. a gentle acid cleaning procedure should be used). ...
... The most appropriate storage procedure of metal speciation samples, which avoids changes in speciation parameters pending analyses, is still a topic of discussion in the marine chemistry community (e.g. Buck et al. 2012). ...
... However, published intercomparison studies showed that consensus values could be largely obtained between different laboratories and using different voltametric methods, but that the number of identified ligand classes present and the groups of ligands resolved in a target sample need more attention in future work (e.g. Bruland et al. 2000;Buck et al. 2016Buck et al. , 2012Pižeta et al. 2015). ...
In this chapter an overview of sampling and analytical techniques for the marine trace metals (and their stable isotope ratios) is given, focusing largely on the six bio-essential transition metals (iron, manganese, copper, nickel, zinc and cobalt). The aim of this chapter is to introduce the reader to the breadth of techniques and methods currently available to study the biogeochemical cycles of trace metals and their isotopes in the ocean. We note that we do not cover all existing and historical techniques as some are no longer used, some remain immature for trace metal studies, and some are just emerging or are still being developed. A more detailed focus on the methods used by the authors is also provided. We anticipate the continuing development and refinement of methods; as with any expanding and developing scientific field, novel strategies and techniques continuously come and go. For further background reading on marine trace metal distribution and key biogeochemical processes in the ocean, the reader is referred throughout the chapter to appropriate overviews, articles and textbooks available online, including the freely available GEOTRACES electronic atlas and data products, as well as the GEOTRACES ‘cookbook’.KeywordsMarine trace metal biogeochemistryMarine trace metal samplingMarine trace metal sample handlingMarine trace metal analysisMarine metal stable isotope analysisMarine trace metal speciation analysis
... There are three main ways to use these voltammetric methods to determine information on the stability constant of a given metal ligand complex in the sample. The first approach that is most often used is the titration of the sample with metal [e.g., Fe(III), Cu(II)] followed by a CLE-adCSV measurement on each metal titration solution (e.g., Buck et al., 2012Buck et al., , 2016van den Berg, 2006). The resulting titration curve is linearized to fit the titration data and to obtain the total ligand concentration [L] and the conditional stability constant, K cond M ′ L ′ , of the metal unknown ligand complex, ML unknown Pižeta et al., 2015). ...
... Unfortunately, in natural waters, the ligand is normally unknown because there are several ligands that can bind the metal; thus, the conditional constant from eq. 1 can only be corrected for the free metal ion. A measure of the total ligand concentration can be determined by metal titration and other experiments [e.g., Bruland, 1989;Buck et al., 2012]. ...
... Lewis et al. (1995a) noted that the value of E ½, ML is similar at nM (deposition experiments) and μM concentrations (fast scans with no deposition) of metal and ligand. (b) A strong and inert ML complex (defined as log K therm ≥ 6) acts as a discrete species and does not dissociate at the electrode prior to electron transfer, but its wave (peak) exhibits Nernstian behavior and its potential is independent of the concentration of L. [Strong ML unknown complexes in seawater are considered inert (e.g., Buck et al., 2012;Buck et al., 2016).] A weak ML complex (defined as log K therm ≤ 6) also gives a wave (peak), but can dissociate at the electrode; the peak potential will depend on the concentration of L (DeFord and Hume, 1951;Luther III et al., 2000). ...
Dissolved chemical speciation of metals in natural waters encompasses a wide range of inorganic and organic compounds including metal organic ligand complexes, ML. Because of the different filters used, “dissolved” speciation can range from simple metal-ligand complexes with an average size of about 0.66 nm (mass of <3 k-daltons) to nanoparticles of 1 to 100 nm to colloidal forms that are 10 to 200–400 nm in size. Strong metal-ligand, ML, complexes are normally considered to be in <1 nm size fraction. Over the last 3 decades, competitive ligand exchange – cathodic stripping voltammetry (CLE-CSV) titrations have been the method of choice to study complexation. These titrations primarily give information on the excess ligands in the sample rather than the actual ligand in MLunknown complexes because they require adding metal to the sample. Thus, metal-ligand CLE-CSV titrations do not provide much information on the actual ligand present in MLunknown. However, pseudovoltammetry provides the thermodynamic stability constant, Ktherm, for Zn, Cu, Cd and Pb as the MLunknown complex is destroyed by reduction at the Hg electrode to form metal(Hg). Pseudovoltammetry does not require the addition of any reagents to the sample, but cannot be performed for ions such as Fe(III) [and Mn(III)] because reduction of the ion results in the reduction of the metal ion in the complex without destroying MLunknown. For these ions, kinetic experiments to recover the metal in the ML complex can provide information on the MLunknown dissociation rate constant, kd, and the conditional equilibrium constant, KcondML′. In these kinetic experiments, a competitive ligand (Lcomp) is added to the sample, and over time the MLcomp complex is measured by CSV. If all the metal in MLunknown is recovered, kd of MLunknown can be determined. If only a portion of the metal in MLunknown is recovered, equilibrium is achieved and KcondML′ as well as kd can be determined in a single experiment; kf can then be calculated. We describe how these methods can be used to determine information on the actual MLunknown complex. We show that 7 thermodynamic, kinetic and speciation parameters (Ktherm, KcondML′, KcondM′L′, kf, kd, αM′, αL′) for MLunknown complexes can be derived from a combination of two of these experiments. The approaches described here are useful to determine these parameters for known ML complexes once a ligand has been isolated by advanced separation methods (e.g., LC-MS) and reacted with a metal of interest.
... The range of copper concentrations between its necessity and toxicity is relatively narrow (Amin et al., 2013;Zitoun et al., 2019). However, the formation of strong complexes with organic ligands can reduce the bioavailable Cu fraction and, in most cases, maintains it in the optimal range (Buck et al., 2012;Whitby and van den Berg, 2015;Whitby et al., 2017). The presence of organic ligands is therefore of main significance in assessing the Cu bioavailability, with respect to both toxicity and necessity. ...
... Due to experimental limitations of separation, extraction, and direct measurement of different metal-organic ligand complexes in seawater, an alternative approach based on complexometric titrations using electrochemical techniques (known as the determination of the metal complexing capacity of the sample) is preferentially used (Buck et al., 2012;Pižeta et al., 2015). As the concentration of metals in seawater is very low, various electrochemical techniques with low detection limits are commonly used, mainly stripping techniques: anodic and competitive ligand exchange adsorptive cathodic stripping voltammetry (ASV and CLE-AdCSV, respectively) (Sanchez-Marin et al., 2010;Buck et al., 2012;Omanović et al., 2015;Pižeta et al., 2015;Whitby et al., 2018). ...
... Due to experimental limitations of separation, extraction, and direct measurement of different metal-organic ligand complexes in seawater, an alternative approach based on complexometric titrations using electrochemical techniques (known as the determination of the metal complexing capacity of the sample) is preferentially used (Buck et al., 2012;Pižeta et al., 2015). As the concentration of metals in seawater is very low, various electrochemical techniques with low detection limits are commonly used, mainly stripping techniques: anodic and competitive ligand exchange adsorptive cathodic stripping voltammetry (ASV and CLE-AdCSV, respectively) (Sanchez-Marin et al., 2010;Buck et al., 2012;Omanović et al., 2015;Pižeta et al., 2015;Whitby et al., 2018). Knowledge about sources and chemical identity of detected ligands is scarce (Vraspir and Butler, 2009) and its acquisition is hindered by very complex chemical composition of natural DOM (Repeta, 2015). ...
The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L ⁻¹ . T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L ⁻¹ ) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L 1 and L 2 ) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L 1 and 17 ± 4 to 104 ± 7 nM eq Cu for L 2 , with stability constants log K Cu,1 = 9.6 ± 0.2–10.8 ± 0.6 and log K Cu,2 = 8.2 ± 0.3–9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.
... We used a mix of procedures with SA as competing ligand as described by Buck et al. (2007Buck et al. ( , 2012 and Abualhaija and van den Berg (2014). Specifically, we adapted the pH to pH = 8.4 to acquire sufficient peak separation on our system (Buck et al., 2007 used pH = 8.2;Abualhaija et al., 2015 used pH = 8.18) and adopted the equilibration time according to Buck et al. (2007). ...
... Sample handling like freezing and conservation cannot be the reason for a difference in results. The TAC method was applied both to stored and frozen samples and at sea, and there was no difference in the results obtained, as also observed by (Buck et al., 2012 Laglera & van den Berg, 2009). The detection window is traditionally assumed to be 1 order of magnitude above and below the center of the detection window (Apte et al., 1988;Gerringa et al., 2014;van den Berg et al., 1990), so they overlap considerably and can ...
... hardly hinder in comparing the results of the two methods. We followed the method of Buck et al. (2007Buck et al. ( , 2012 in using a short equilibration time for the SA method (see section 2.2). Although we observed a steady state, we cannot be sure that equilibrium was reached between the natural ligands, SA and DFe (Laglera & Filella, 2015). ...
Samples inside and outside the Arctic Ocean's TransPolar Drift (TPD) have been analyzed for Fe-binding organic ligands (L t ) with Competitive Ligand Exchange Adsorptive Stripping Voltammetry (CLE-AdCSV) using salicylaldoxime (SA). This analysis is compared to prior analyses with CLE-AdCSV using 2-(2-thiazolylazo)-p-cresol (TAC). The TPD's strong terrestrial influence is used to compare the performance of both CLE-AdCSV methods in representing the nature of natural organic ligands. These measurements are compared against direct voltammetric determination of humic substances (HS) and spectral properties of dissolved organic matter. The relationship between the two CLE-AdCSV derived [L t ] versus HS in the TPD has a comparable slope, with a 40% offset toward higher values obtained with SA. Higher [L t ] values inside the TPD, most probably due to HS, explain high dissolved Fe concentrations transported over the Arctic Ocean by the TPD. Outside of the TPD in the surface Arctic Ocean HS occur as well but at lower concentrations. Here changes in HS relate to changes in dissolved Fe concentration and to [L t ] obtained with SA, whereas [L t ] obtained with TAC remain constant. Moreover, with decreasing HS the offset between the methods using TAC and SA decreases. We surmise that in the presence of HS, the TAC method detects HS only either at higher concentrations or of specific composition. On the other hand, the SA method might overestimate [L t ], as an offset with the TAC method that remains constant where HS are not detected. Regardless, HS are the dominant type of Fe-binding organic ligand in the surface of the Arctic Ocean.
... Following equilibration, the concentration of the resulting Me (AL)x complexes are determined with cathodic stripping voltammetry (CSV). In CSV, the Me(AL)x complex is adsorbed to the surface of a working electrode at a set voltage potential and subsequently reduced, as the potential at the working electrode is scanned in the negative direction and the reductive current response is measured (Buck et al., 2012). The metal's reduction potential presents itself as a current peak, which is plotted for each metal addition in the titration. ...
... Copper speciation determined via CLE-ACSV, with salicylaldoxime (SA) as the added competitive ligand, was initially detailed in Campos and van den Berg (1994). SA is widely used in Cu speciation studies, given its high sensitivity for Cu (Campos and van den Berg, 1994;Buck et al., 2012). ...
The Strait of Georgia (SoG) is a semi-enclosed, urban basin with seasonally dependent estuarine water circulation, dominantly influenced by Northeast Pacific waters and the Fraser River. To establish a baseline and understand the fate and potential toxicity of Cu in the SoG, we determined seasonal and spatial depth profiles of dissolved Cu (dCu) speciation, leading to estimates of the free hydrated copper (Cu ²⁺ ) concentrations, as a proxy for Cu toxicity. The concentration of dCu was largely controlled by conservative mixing of the ocean and freshwater endmembers in the SoG. In all samples, ligand concentrations exceeded dCu, by a ratio greater than 1.5, resulting in the complexation of 99.98% of the dCu by strong binding organic ligands. The concentrations of Cu ²⁺ were less than 10 -13.2 M, significantly lower than the well-established Cu toxicity threshold (10 ⁻¹² M Cu ²⁺ ) for microorganisms. Our results indicate that ambient Cu-binding ligands effectively buffer Cu ²⁺ concentrations within the Strait of Georgia, posing no threat to marine life. In almost 90% of the samples, the ligands were best classified as a single ligand class, with a log K C u L , C u 2 + c o n d between 12.5 and 14.1. The concentrations of these single class ligands were greatest in warm, low salinity, nutrient depleted waters, suggesting that either terrestrially sourced ligands dominate dCu speciation in the SoG, or freshwater sources in the SoG establish the conditions that promote the production of Cu binding ligands in its surface waters. The remaining 10% of the samples were from the euphotic zone, where we detected a stronger ligand class, L 1 , of log K C u L 1 , C u 2 + c o n d between 13.5 and 14.3, and a weaker ligand class, L 2 , of log K C u L 2 , C u 2 + c o n d between 11.5 and 12.3. In these surface samples, log K C u L 1 , C u 2 + c o n d and log K C u L 2 , C u 2 + c o n d were positively correlated with temperature, while L 2 concentrations were positively correlated with chromophoric dissolved organic matter of terrestrial origin. This study is the first to perform hierarchal clustering of a trace metal speciation dataset and enabled the distinction of 6 clusters across season, depth, and region of the SoG, highlighting the influence of freshwater and open ocean ligand sources, conservative mixing dynamics, and particulate Cu concentrations on dCu speciation within estuarine basins.
... Amongst the available AL's, only 1-nitroso-2-naphthol (NN) has so far been used in sea-ice samples (Boyé et al., 2001;Lannuzel et al., 2015;Genovese et al., 2018). Although recent work in seawater made use of 2,3-dihydroxynaphthalene (DHN) (Rivaro et al., 2019), most published studies have focussed on 2-(2-thiazolylazo)-pcresol (TAC) and salicylaldoxime (SA) Thuróczy et al., 2012;Buck et al., 2018;Buck et al., 2012;Caprara et al., 2016). Overall, the accurate choice of the artificial ligand and its concentration is still debated because of the different assumptions and possible errors . ...
... Previous comparisons suggest that TAC (10 μM) only partially detects L belonging to the humic pool (Slagter et al., 2019). Similarly, NN (10 μM) is not suitable for humic substances because of out-competition (Laglera et al., 2011) and SA (25 μM) might overestimate L concentration (Slagter et al., 2019) or logK' (Buck et al., 2012). ...
It is widely accepted that iron (Fe)-binding organic ligands play a crucial role in Fe distribution in the marine environment and thus in Fe biogeochemistry. Although Competitive Ligand Equilibration – Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) is a well-established technique to investigate Fe chemical speciation in marine samples, several impediments still need to be addressed. These include the extrapolation of laboratory measurements to in-situ conditions, the harmonization of the analytical procedures used, and the applicability of the methods over salinity ranges wider than seawater (e.g., sea ice). This work focusses on the calibration of 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-naphthol (NN), along the salinity range 1–90, and titration of natural samples at two different temperatures (4 °C and 20 °C). The artificial ligand concentration was 10 μM for TAC and 5 μM for SA and NN. Calibrations showed that increasing salinity caused a decrease in the conditional stability constants (logK'Fe’AL) for NN and SA (although different behaviours were noted for the two species FeSA and FeSA2). Less accuracy was noted using TAC, which behaved inconsistently outside the 21 < S < 35 range, and its use is therefore discouraged in fresh and highly saline waters. Titrations of natural samples showed that only SA covered the salinity range selected, up to 78, and its use is therefore recommended in sea-ice studies. The side reaction coefficient (logα'Fe’AL) of each artificial ligand was found to be influenced by temperature differently: logα'Fe’SA was higher at lower temperature (4 °C), whereas logα'Fe’SA2 and logα'Fe’NN3 increased with increasing temperature (to 20 °C). Although titrations performed at 4 °C highlighted that the uncomplexed Fe fraction was 14% lower than at 20 °C, with potential consequences on primary productivity, the percentage of natural Fe complexed was >99%. Future investigations should consider the analysis of the samples at a temperature as close as possible to in-situ conditions to reduce the potential temperature effects.
... Previous studies have also reported excess ligands in the Pacific Ocean water column [14][15][16][17][18][19][20][21][22][23][24][25][26] . An excess of [D-Fe] relative to [D-L] has been demonstrated in deep waters (> 2000 m) 18,20 , suggesting a variation in dissolved Fe speciation between the Pacific Ocean's upper and deep waters. ...
... However, it was suggested that using the CLE-ACSV method with TAC is inefficient for detecting Fe complexation with humic substances 29 . Indeed, dissimilarity in the CLE-ACSV method results has been reported for deep water in the Pacific Ocean 16 and the Arctic Ocean 30 . Generally, the CLE-ACSV method using 10 μM of TAC reagent underestimates ligand concentrations. ...
Iron (Fe) is well known as a limiting factor to control primary productivity especially in high-nutrient and low chlorophyll area such as the subarctic Pacific. The solubility of Fe is believed to be controlled by its complexation with natural organic ligands, while the distribution of organic ligands is poorly understood. Here, we report that dissolved (< 0.2 µm) organic ligands were unevenly distributed between the western and eastern stations in the subarctic Pacific. The concentration of dissolved organic ligands around the lower part of subarctic Pacific intermediate water was higher in the western station, suggesting that Fe complexation with these organic ligands supports a lateral transport within the water mass. However, a more detailed size-fractionated treatment indicated no significant difference in the soluble (< 1000 kDa) ligands’ distribution between the western and eastern stations. These results suggest that organic and inorganic colloid formations are potentially essential for Fe transport mechanisms in the subarctic Pacific.
... Similarly, bioavailability of Cu in situ is controlled by organic ligands (Bruland et al., 1991). In many regions, up to 99% of total Cu is complexed by organic ligands well beyond the mixed layer (Coale andBruland, 1988, 1990;Blake et al., 2004;Shank et al., 2004;Moffett and Dupont, 2007;Buck et al., 2012;Stockdale et al., 2015). These compounds act as a natural trace metal buffering system; where ligand concentration exceeds total Cu concentration, the system avoids potentially toxic conditions, and where ligand concentration is exceeded by Cu concentration, the system can approach the [Cu 2+ ] toxicity thresholds of certain organisms (Brand et al., 1986;Buck et al., 2012). ...
... In many regions, up to 99% of total Cu is complexed by organic ligands well beyond the mixed layer (Coale andBruland, 1988, 1990;Blake et al., 2004;Shank et al., 2004;Moffett and Dupont, 2007;Buck et al., 2012;Stockdale et al., 2015). These compounds act as a natural trace metal buffering system; where ligand concentration exceeds total Cu concentration, the system avoids potentially toxic conditions, and where ligand concentration is exceeded by Cu concentration, the system can approach the [Cu 2+ ] toxicity thresholds of certain organisms (Brand et al., 1986;Buck et al., 2012). Relatively little is known about the chemical structure of organic ligands in the marine environment (Schwarzenbach et al., 2003). ...
The cupric ion (Cu2+) plays a dual role as both nutrient and toxicant to freshwater and marine phytoplankton, functioning in multiple photochemical processes, as well as reactive oxygen species (ROS) production. This duality has been investigated through a variety of methods to determine the consequences of natural and anthropogenic copper introduction to algal ecosystem composition. Studies conducted over the past few decades have described the growth responses of many unique organisms to copper availability. Such observations are critical for describing the global distributions of major phytoplankton species in terms of global trace metal abundance; however, highly variable experimental practices impede direct inter-study comparison. The aim of this systematic review is to summarize the available data regarding the effects of copper concentration on diverse marine phytoplankton growth rates. Through extensive literary comparison, 143 studies were systematically reviewed, and data on copper concentrations and growth rates reported were extracted. From the data available, we conclude that trends in phytoplankton sensitivity to copper are mainly driven by a single study. We discuss the obstacles to inter-study comparison and detail both the concurring and conflicting results to date, with an emphasis on taxonomic trends and methodologies employed. Finally, we present the first copper sensitivity measurements for marine unicellular diazotrophs using three representative strains of the unicellular nitrogen-fixing cyanobacterium, Crocosphaera watsonii.
... Previous studies on Cu(II) speciation in the seawater have revealed that the conditional stability constants are in the range between 9 and 15 (Sunda and Ferguson 1983;Moffett and Zika 1987;Donat and van den Berg 1992;Yokoi and van den Berg 1992). It was commonly established that a strong ligand (L 1 ) class has a log K between 13 and 14, while a weak ligand (L 2 ) class has a log K between 10 and 12 (Ellwood and van den Berg 2001;Buck et al. 2012;Donat et al. 1994). This indicated the presence of a weak ligand (L 2 ) complex in our study area, and most of these organic ligands are complexed to Cu(II) ions, because the concentration of L T in the area is always in excess to that of dissolved Cu(II) (Fig. 5). ...
The vertical distribution of Cu(II) in dissolved and particulate phases was determined in the seawater at Pulau Redang, Terengganu, in April 2019. Our present results have shown a similar concentration of total dissolved Cu(II) at the upper layer (< 15 m depth) and bottom layer (> 15 m depth). At the upper layer, its concentration ranged between 10.05 and 20.81 nM, and at the bottom layer, it ranged between 4.37 and 24.03 nM. On the other hand, we found a low concentration of Cu(II) at the upper (0.05-0.88 nM) and bottom (0.05-0.51 nM) layers in the particulate phase. The distribution coefficient (K D) analysis has indicated that most of the Cu(II) existed in the dissolved phases throughout the water column during the study period. Our analysis on dissolved Cu(II) speciation has revealed the presence of weak ligand class (L 2), and its concentration ([L T ] = 4.55-39.76 nM) has always been in excess of the dissolved Cu(II) concentration throughout the water column. The [L T ]/[dCu] ratio (> 1.0) indicated that an existence of a saturated state condition and more than 99.0% of Cu(II) ion is organically complex to these natural organic ligands. This present study highlights that most of the Cu(II) exists in a dissolved organic complex at the tropical coastal waters. The presence of excess natural organic ligands as a chelator to the Cu(II) increases its solubility in the dissolved phase.
... This leaching procedure has been used extensively to investigate the labile fraction of Fe and other trace elements in marine aerosols (e.g., Bridgestock et al., 2016;Jickells et al., 2016). However, the relationship between trace element dissolution in this medium and in seawater is unclear Meskhidze et al., 2019), with lower solubility in seawater (due to higher pH) probably offset by strong organic complexation (Campos and van den Berg, 1994;Ellwood and van den Berg, 2000;Buck et al., 2012). Elemental concentrations were measured using inductively coupled plasma mass spectroscopy (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Natural History Museum, London (Bridgestock et al., 2016;. ...
Anthropogenic activities have significantly enhanced atmospheric metal inputs to the ocean, which has potentially important consequences for marine ecosystems. This study assesses the potential of Zn and Cu isotope compositions to distinguish between natural and anthropogenic atmospheric inputs of these metals to the surface ocean. To this end, the isotopic compositions of Zn and Cu in aerosols collected from the eastern tropical Atlantic Ocean on the GEOTRACES GA06 cruise are examined. Enrichment factors and fractional solubility measurements indicate the presence of a significant anthropogenic component in the aerosols collected furthest from the North African dust plume for both Zn and Cu. The mean δ⁶⁵CuNIST SRM 976 for the fully digested aerosols is +0.07 ± 0.39 ‰ (n = 9, 2 SD), which is indistinguishable from the lithogenic value, and implies that Cu isotopes are not an effective tracer of aerosol sources in this region. The mean δ⁶⁶ZnJMC-Lyon value for the aerosols that underwent a total digestion is +0.17 ± 0.22 ‰ (n = 11, 2 SD). The aerosols leached with ammonium acetate have similar Zn isotope compositions, with a mean of +0.15 ± 0.16 ‰ (n = 7, 2 SD). The aerosols were collected in a region with prevalent mineral dust but, despite this, exhibit isotopically lighter Zn than lithogenic Zn with δ⁶⁶Zn ≈ +0.3 ‰. When coupled with the previously published Pb isotope data, the aerosols exhibit coupled Zn-Pb isotope systematics that are indicative of mixing between mineral dust (δ⁶⁶Zn = +0.28 ‰ and ²⁰⁶Pb/²⁰⁷Pb = 1.205) and anthropogenic emissions (δ⁶⁶Zn = −0.22 ‰ and ²⁰⁶Pb/²⁰⁷Pb = 1.129). This demonstrates the potential of Zn isotopes to trace atmospheric Zn inputs from anthropogenic sources to the surface ocean.
... Thus, although the emissions of combustion-derived Fe and other metals are much lower than mineral dust metals, they supply more bioavailable metals than the mineral dust and are especially important in high anthropogenic emission regions [58,59]. The speciation and bioavailability of some biologically important metals in seawater, i.e., Fe, Cu, and Co, are largely controlled by biogenic ligands [60]. The dissolution process can be either rapid or gradual, i.e., Zn, Co, and Cd dissolve faster than other metals, and Ni, Cu, and Mn dissolve slower [59]. ...
Atmospheric deposition is recognized as a significant source of nutrients in the surface ocean. The East Asia region is among the largest sources of aerosol emissions in the world, due to its large industrial, agricultural, and energy production. Thus, East Asian aerosols contain a large proportion of anthropogenic particles that are characterized by small size, complex composition, and high nutrient dissolution, resulting in important influences on marine microbes and biogeochemical cycles in the downwind areas of the northwest Pacific Ocean (NWPO). By using remote sensing, modeling, and incubation experimental methods, enhanced primary production due to the East Asian aerosol input has been observed in the NWPO, with subsequent promotion and inhibition impacts on different phytoplankton taxa. Changes of bacterial activity and diversity also occur in response to aerosol input. The impact of East Asian aerosol loadings is closely related to the amount and composition of the aerosol deposition as well as the hydrological condition of the receiving seawater. Here, we review the current state of knowledge on the atmospheric nutrients and the effects of the East Asian aerosols on microbes in the NWPO region. Future research perspectives are also proposed.
... Copper is a bioessential element and its distribution in seawater is controlled by biological recycling and abiotic scavenging (see review in [71]). Copper is known to be strongly complexed by organic ligands [72,73], even though the organic ligands are not yet well characterized. ...
Geochemical behaviour and bio-availability of trace metals are closely related to their physical fractionation and chemical speciation. The DGT speciation technique allows the challenging assessment of labile concentrations of Mn, Cd, Cu, Ni, V, As, and REY in ocean waters. In this first deep-water in situ study of DGT-lability, we demonstrate the approach in bottom waters of the Clarion-Clipperton Zone in the central NE Pacific. In the dissolved fraction (<0.2 μm), 70% to 100% of Cd, Ni, V, and REY, but only 25% of Cu and less than 50% of As were determined, reflecting their prevailing dominance of organic vs. inorganic complexation. This study demonstrates the applicability and sensitivity of DGT-passive samplers for trace metals as a suitable technique in monitoring of anthropogenic activities, such as deep seabed mining, as well as for natural process studies in abyssal environments.
... Although the importance of iron-binding ligands to iron speciation and distributions in the ocean has been well recognized, the identity of these ligands, their sources and the conditions in which they are produced remain unclear (Tagliabue et al. 2017). Iron-binding ligands in seawater have been most commonly measured by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) (Gledhill and van den Berg 1994;Rue and Bruland 1995;Buck et al. 2012;Bundy et al. 2015). This electrochemical method groups ligands into classes operationally defined by their binding affinity for iron, with the L1 class being the strongest iron-binding ligands and L2 and below being progressively weaker ligands (Hunter and Boyd 2007;Gledhill and Buck 2012;Bundy et al. 2014Bundy et al. , 2015. ...
Siderophores are strong iron-binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron-binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron-replete, low-macronutrient North Pacific Subtropical Gyre (NPSG) through the North Pacific Transition Zone (NPTZ) to the iron-deplete, high-macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0 - 13.9 pM) than the SAFZ (1.2 - 5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust-derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins and iron-free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Additionally, the widespread genetic potential for siderophore biosynthesis and uptake across latitude and the variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin and salmochelin transporter genes at different latitudes suggest that particular microbes actively produce and use siderophores, altering siderophore distributions and the bioavailability of iron across the North Pacific.
... The importance of organic complexation of many metals in modifying their chemical speciation, solubility, and bioavailability in seawater is well documented, e.g., for Co (87), Cu (88), Cd (89), Fe (90), and Zn (91). Our knowledge of this organic binding has been gained from Competitive Ligand Exchange-Cathodic Stripping Voltammetry titrations (92). With few exceptions, these titrations have been carried out at buffered pH values close to that of seawater. ...
Anthropogenic emissions to the atmosphere have increased the flux of nutrients, especially nitrogen, to the ocean, but they have also altered the acidity of aerosol, cloud water, and precipitation over much of the marine atmosphere. For nitrogen, acidity-driven changes in chemical speciation result in altered partitioning between the gas and particulate phases that subsequently affect long-range transport. Other important nutrients, notably iron and phosphorus, are affected, because their soluble fractions increase upon exposure to acidic environments during atmospheric transport. These changes affect the magnitude, distribution, and deposition mode of individual nutrients supplied to the ocean, the extent to which nutrient deposition interacts with the sea surface microlayer during its passage into bulk seawater, and the relative abundances of soluble nutrients in atmospheric deposition. Atmospheric acidity change therefore affects ecosystem composition, in addition to overall marine productivity, and these effects will continue to evolve with changing anthropogenic emissions in the future.
... The surface values average about 1 nM and are thought to be low enough to limit phytoplankton production in some remote regions (Peers et al., 2005;Takeda et al., 2014). Like other bioactive metals, strong organic ligands complex dissolved Cu and greatly reduce the concentration of the inorganic chemical species (Cu') that are the most reactive and biologically available (Coale and Bruland, 1988;Buck et al., 2012). ...
Copper (Cu) concentration is greatly reduced in the open sea so that phytoplankton must adjust their uptake systems and acclimate to sustain growth. Acclimation to low Cu involves changes to the photosynthetic apparatus and specific biochemical reactions that use Cu, but little is known how Cu affects cellular metabolic networks. Here we report results of whole transcriptome analysis of a plastocyanin‐containing diatom, Thalassiosira oceanica 1005, during its initial stages of acclimation and after long‐term adaptation in Cu‐deficient seawater. Gene expression profiles, used to identify Cu‐regulated metabolic pathways, show down‐regulation of anabolic and energy‐yielding reactions in Cu‐limited cells. These include the light reactions of photosynthesis, carbon fixation, nitrogen assimilation and glycolysis. Reduction of these pathways is consistent with reduced growth requirements for C and N caused by slower rates of photosynthetic electron transport. Up‐regulation of oxidative stress defense systems persists in adapted cells, suggesting cellular damage by increased reactive oxygen species (ROS) occurs even after acclimation. Copper deficiency also alters fatty acid metabolism, possibly in response to an increase in lipid peroxidation and membrane damage driven by ROS. During the initial stages of Cu‐limitation the majority of differentially regulated genes are associated with photosynthetic metabolism, highlighting the chloroplast as the primary target of low Cu availability. The results provide insights to the mechanisms of acclimation and adaptation of T. oceanica to Cu deficiency. This article is protected by copyright. All rights reserved.
... For accurate assessment of its potential toxicity to biota, the knowledge of the free ion concentration, which is considered as the key indicator [15,16], is needed. In aqueous systems, Cu is present at nM levels but is extensively complexed to natural organic ligands, strongly reducing its free ion concentration [17,18]. Presence of organic ligands is therefore of main significance in assessing Cu bioavailability [19][20][21][22]. ...
Copper (Cu) is a bio-essential trace element that is of concerns due to its potential toxicity at concentrations commonly encountered in coastal waters. Here, we revisit the applicability of Cu(II) ion selective electrode (Cu-ISE) based on a jalpaite membrane for the measurement of Cufree in seawater. At high total Cu concentration (>0.1 mM), (near)Nernstian slope was obtained and determination of Cufree down to fM levels was possible. However, this slope decreases with decreasing total Cu concentration (e.g. 7 mV/decade at 15 nM total Cu) making the use of a common single calibration approach unreliable. To solve this problem, we carried out several calibrations at different levels of total Cu (15 nM - 1 mM) and ethylenediamine (EN: 5 μM - 15 mM) and fitted the calibration parameters (slope and intercept) as a function of total Cu using the Gompertz function (a meta-calibration approach). The derived empirical equations allowed the determination of Cufree at any total Cu concentration above 20 nM (determination of Cufree at lower total Cu levels is prevented by the dissolution of the electrode). We successfully tested this meta-calibration approach in UV digested seawater in presence of a synthetic ligand (EN), isolated natural organic matter (humic acid, HA) and in a natural estuarine sample. In each case, our meta-calibration approach provided a good agreement with modeled speciation data (Visual MINTEQ), while standard single approach failed. We provide here a new method for the direct determination of the free Cu ion concentration in seawater at levels relevant for coastal waters.
... In contrast, despite having D w similar to TAC, The use of NN results in both a lower [L] and log K Fe L . The application of multiple methods as suggested by Buck et al. (2012) thus requires careful consideration. Nevertheless, our results showed that the one ligand model captured a similar trend in [L t ] for all three AL, and [L] increased from Fram Strait to the Norske Trough to the Westwind Trough (Figure 2). ...
Competitive ligand exchange – adsorptive cathodic stripping voltammetry (CLE-AdCSV) is a widely used technique to determine dissolved iron (Fe) speciation in seawater, and involves competition for Fe of a known added ligand (AL) with natural organic ligands. Three different ALs were used, 2-(2-thiazolylazo)-p-cresol (TAC), salicylaldoxime (SA) and 1-nitroso-2-napthol (NN). The total ligand concentrations ([Lt]) and conditional stability constants (log K′Fe’L) obtained using the different ALs are compared. The comparison was done on seawater samples from Fram Strait and northeast Greenland shelf region, including the Norske Trough, Nioghalvfjerdsfjorden (79N) Glacier front and Westwind Trough. Data interpretation using a one-ligand model resulted in [Lt]SA (2.72 ± 0.99 nM eq Fe) > [Lt]TAC (1.77 ± 0.57 nM eq Fe) > [Lt]NN (1.57 ± 0.58 nM eq Fe); with the mean of log K′Fe’L being the highest for TAC (log ′KFe’L(TAC) = 12.8 ± 0.5), followed by SA (log K′Fe’L(SA) = 10.9 ± 0.4) and NN (log K′Fe’L(NN) = 10.1 ± 0.6). These differences are only partly explained by the detection windows employed, and are probably due to uncertainties propagated from the calibration and the heterogeneity of the natural organic ligands. An almost constant ratio of [Lt]TAC/[Lt]SA = 0.5 – 0.6 was obtained in samples over the shelf, potentially related to contributions of humic acid-type ligands. In contrast, in Fram Strait [Lt]TAC/[Lt]SA varied considerably from 0.6 to 1, indicating the influence of other ligand types, which seemed to be detected to a different extent by the TAC and SA methods. Our results show that even though the SA, TAC and NN methods have different detection windows, the results of the one ligand model captured a similar trend in [Lt], increasing from Fram Strait to the Norske Trough to the Westwind Trough. Application of a two-ligand model confirms a previous suggestion that in Polar Surface Water and in water masses over the shelf, two ligand groups existed, a relatively strong and relatively weak ligand group. The relatively weak ligand group contributed less to the total complexation capacity, hence it could only keep part of Fe released from the 79N Glacier in the dissolved phase.
... The consistency between the two analytical methods is encouraging as it shows that utilizing a 5 μM SA concentration for GP16 samples does not preclude comparison with GA03 samples, which utilized a 2.5 μM SA concentration. This is consistent with previous intercalibration exercises for Cu organic speciation (Bruland et al., 2000;Buck et al., 2012), which included the methods used here and obtained comparable results, especially for free Cu ion determinations, with predictable differences in ligand concentration and log K CuL, Cu 2+ cond across different analytical windows. ...
Samples for organic copper (Cu)-binding ligand characterization were collected along the 2013 U.S. GEOTRACES Pacific (GP16) cruise transect from Peru to Tahiti. Full depth profiles of Cu speciation were collected across a dynamic range in oceanographic conditions including a highly productive coastal region, an oxygen deficient zone, a high nutrient low chlorophyll (HNLC) region, an oligotrophic region and a hydrothermal vent plume. Surface waters from Peru to Tahiti exhibited elevated dissolved Cu and ligand concentrations near Peru and then decrease in concentration (< 1 nM) offshore toward the oligotrophic waters. There was also an apparent shelf sediment source of strong Cu-binding ligands near the coast of Peru. Throughout most of the transect dissolved Cu and ligand concentrations were lower in the upper waters and increased with depth, with the highest concentrations near the ocean bottom. The hydrothermal vent sampled during the cruise did not seem to be a source for dissolved Cu but there was a slight elevation of Cu-binding ligands at the vent site. Similar vertical patterns in Cu-binding ligands were seen in both the GP16 dataset and the North Atlantic GEOTRACES (GA03) cruise, with notable differences in deep waters of the Pacific. The older water masses of the Pacific were highlighted by higher concentrations of dissolved Cu, Cu binding ligands, and the free Cu ion (Cu²⁺) relative to the deep Atlantic. Excess Cu ligands in both GP16 and GA03 point to a possible fraction of Cu accumulating in the deep Pacific that is inert to ligand exchange, suggesting older waters might contain a high fraction of unreactive Cu.
... See reviews by Kenney and Rosenzweig (2018) and Johnstone and Nolan (2015) for detailed information. detection and has recently been described as "the primary technique employed for measuring metal-binding ligands and determining metal speciation in seawater" (Buck et al., 2012). 2 It is also used in freshwater research (Xue and Sigg, 2002), though since freshwater limits of detection are more forgiving, for freshwater absorbance detection methods are also applicable. ...
There is an increasing need to study the effects of trace metal micronutrients on microorganisms in natural waters. For Fe, small Fe-binding ligands called siderophores, which are secreted from cells and bind Fe with high affinity, have been demonstrated to modulate bioavailability of this critical nutrient. Relatively little is known about secretion of strong Cu-binding ligands (chalkophores) that may help organisms navigate the divide between Cu nutrition and toxicity. A barrier to environmental chalkophore research is a lack of literature on chalkophore analysis. Here we report the development of a quantitative, high-throughput approach to chalkophore screening based on a popular competitive-ligand binding assay for siderophores wherein ligands compete for metal in a chromogenic ternary complex of chrome azurol sulfonate-metal-surfactant. We developed the assay for high-throughput analysis using a microplate reader. The method performance is slightly better than that of comparable screening approaches for siderophores. We find that levels of other metals in natural samples may be capable of causing matrix interferences (a neglected source of analytical uncertainty in siderophore screening) and that for our method this can be overcome by standard additions. In this respect the high-throughput nature of the technique is a distinct advantage. To demonstrate practical use, we tested samples from field mesocosm studies that were set up with and without Cu and Fe amendments; we find trends in results that are logical in the environmental context of our application. This approach will be useful in areas such as risk assessment for a rapid survey of metal speciation and bioavailability; investigators who perform structural studies might also benefit from this approach to rapidly screen and select samples with high Fe/Cu binding capacity for further study.
... Several techniques can be used to determine TM speciation and assess the bioavailability of TM (Feldmann et al., 2009). Amongst those, electrochemical stripping techniques such as Anodic Stripping Voltammetry (ASV) (Garnier et al., 2004;Gibbon-Walsh et al., 2012;Omanović et al., 2015a;Omanović et al., 1996) or Competitive Ligand Exchange Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) (Buck et al., 2012;Pižeta et al., 2015) are widely used to measure the concentrations of reactive (electrochemically labile) and free metal ion, respectively, as well getting insights into the presence and strength of complexes/ligands. In-situ passive sampling techniques such as Diffusive Gradients in Thin-films (DGT) are also very popular because of their perceived simplicity, multi-elemental capabilities, and in-situ application, i.e. directly in the water column (Amato et al., 2014;Baeyens et al., 2018;Cindrić et al., 2017;Davison and Zhang, 1994;Davison and Zhang, 2012;Degryse et al., 2009;Menegário et al., 2017;Omanović et al., 2015b;Peijnenburg et al., 2014;Shiva et al., 2016;Warnken et al., 2009;Zhang and Davison, 2015). ...
Understanding the potential bioavailability of trace metals (TM) in marine systems is of prime importance to implement adapted regulations and efficiently protect our coastal and estuarine waters. In this study Diffusive Gradients in Thin films (DGT) technique with two different pore size was used to evaluate the potentially bioavailable fractions (DGT-labile) of Cd, Co, Cu, Ni, Pb and Zn at various depths of a highly stratified estuary (the Krka River estuary, Croatia) both in winter and summer. DGT-labile concentrations were compared to (1) total dissolved concentrations, (2) concentrations of labile species measured by anodic stripping voltammetry (ASV-labile) for Cu and (3) concentrations derived by chemical speciation modelling. High correlation between dissolved and DGT-labile concentrations was found for all metals, except for Zn where contamination problems prevented reliable conclusions. Percentages of DGT-labile fractions over total dissolved concentrations were (AVG ± SD): 92 ± 3%, 64 ± 2%, 23 ± 5%, 61 ± 3% and 57 ± 6% for Cd, Pb, Cu, Ni and Co, respectively. No significant difference was found between trace metal concentrations measured with an open pore and restricted pore devices, implying the predominance of kinetically labile metal complexes smaller than 1 nm. For Cu, ASV-labile and DGT labile concentrations were highly correlated (0.97) with ASV-labile concentration being around 35% lower than that of the DGT-labile. Modelling of chemical speciation reliably predicted dynamic (free, inorganic and part of organic complexes) concentration of Cd, whereas dynamic concentrations of Cu and Pb were underestimated by 32% and 65%, respectively. In view of the relative simplicity of DGT devices, they are well suited for the monitoring effort of coastal waters, informing on potentially bioavailable concentrations of TM and thereby, helping to achieve good environmental status of coastal waters, as stipulated within the EU Water Framework Directive.
... The FPE bottles for dFe speciation were cleaned by soaking first in a 1% soap (Citranox; Fisher) bath for 1 week, followed by at least a month in a 25% hydrochloric acid (HCl, trace-metal grade, Fisher); once clean, the bottles were rinsed and filled with Milli-Q (>18.2 MΩ·cm) until use [26]. Arctic seawater from Baffin Bay (BBA), Canadian Arctic Archipelago (CCA), and Beaufort Sea (BS) was collected as part of the Canadian GEOTRACES Arctic Program (legs 2 and 3, Aug-Sep 2015). ...
Phytoplankton growth in large parts of the world ocean is limited by low availability of dissolved iron (dFe), restricting oceanic uptake of atmospheric CO2. The bioavailability of dFe in seawater is however difficult to appraise since it is bound by a variety of poorly characterized organic ligands. Here, we propose a new approach for evaluating seawater dFe bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. We utilized seven phytoplankton species of diverse classes, sizes, and provenances to probe for dFe bioavailability in 12 seawater samples from several ocean basins and depths. All tested phytoplankton acquired organically bound Fe in any given sample at similar rates (after normalizing to cellular surface area), confirming that multiple, Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. These phytoplankton-based uptake rate constants allowed us to compare water types, and obtain a grand average estimate of seawater dFe bioavailability. Among water types, dFe bioavailability varied by approximately four-fold, and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. Compared with well-studied Fe complexes, seawater dFe is more available than model siderophore Fe, but less available than inorganic Fe. Exposure of seawater to sunlight, however, significantly enhanced dFe bioavailability. The rate constants established in this work, not only facilitate comparison between water types, but also allow calculation of Fe uptake rates by phytoplankton in the ocean based on measured dFe concentrations. The approach established and verified in this study, opens a new way for determining dFe bioavailability in samples across the ocean, and enables modeling of in situ Fe uptake rates by phytoplankton using dFe concentrations from GEOTRACES datasets.
... The equilibration times of two hours for natural ligands with Cu, and an additional hour with SA, are deemed sufficient for complete equilibration as demonstrated by thorough tests provided by Buck and Bruland (2005) for water samples from a similar coastal environment along a salinity gradient. This was recently confirmed by an intercomparison experiment (Buck et al., 2012). The samples were then equilibrated for another hour and subsequently measured on a Metrohm 757 VA Computrace using the following parameters: Purging with nitrogen (N 2 ) gas for 300 s, deposition of the SA-Cu-complex at À50 mV for 60 s, scanning from À50 mV to À600 mV at a sweep rate of 20 mV s À1 . ...
Subterranean estuaries (STEs) are land-ocean interfaces where meteoric fresh groundwater mixes with intruding seawater in a coastal aquifer, before discharging into the adjacent water column. In contrast to surface estuaries, STEs have the potential to amplify concentrations of constituents such as copper (Cu) and iron (Fe) due to long residence times and reductive dissolution of mineral phases along the groundwater flowpaths. However, oxidative precipitation of Fe and Mn at the sediment-water interface may scavenge many constituents again before they reach the coastal water column. Hence, the geochemical impact of the suboxic to anoxic submarine groundwater discharge (SGD) on the oxygenated coastal ocean relies on the capability of constituents such as Cu and Fe to stay in solution across redox boundaries. Here, we propose that dissolved organic matter (DOM) in the STE plays a pivotal role in the speciation of Cu and Fe through (i) fueling reductive dissolution and (ii) providing ligands to form stable metal-DOM complexes, increasing their transfer from the STE into the coastal ocean. We investigated the concentrations and speciation of Cu and Fe, and DOM chemical characteristics, in two beach STEs of a barrier island. By combining well-established techniques with novel quantification and speciation approaches from both the inorganic and organic geochemical realm (size-fractionation filtration, ferrozine detection, voltammetry, sequential DOM extraction, and ultra-high resolution mass spectrometry) we characterized metal-DOM associations down to the molecular level. Overall, pore water from both STEs was enriched with Cu and Fe compared to seawater, which indicated transfer potential for both trace metals across the sediment-water interface. However, Fe gradients from pore water to surface were steeper than those for Cu, indicating a larger net transfer of the latter compared to the former. Our voltammetry data showed that Cu was exclusively organically bound in both STEs and the water column, mostly in soluble form (<20 nm). The majority of >60 newly identified Cu-containing complexes had primarily aliphatic character and N and S in their molecular formulae resembling labile marine DOM, while two Cu-DOM complexes had polyphenol (“humic-like”) molecular formulae indicative of terrestrial vascular plant-derived material. In contrast to Cu, the Fe pool consisted of either reduced, soluble (<20 nm), likely free Fe(II) in the anoxic STE, or of larger colloids (<200 nm and >20 nm) in the fresh groundwater and seawater endmembers, likely as Fe(III)(hydr)oxides stabilized by DOM. Furthermore, while Fe and humic-like DOM seemed to share common sources, all directly identified mobile Fe-DOM complexes appeared to have marine origins. Therefore, organic forms of Fe in the STE may primarily consist of immobile humic-Fe coagulates, partially mobile Fe-nanocolloids, and mobile, N-containing, marine aliphatic Fe-complexes. Our study indicates that aliphatic, N-containing ligands may play an important role in the organic complexation and stabilization of Fe and particularly Cu in the STE, and enable them to cross redox boundaries at the sediment-water interface.
... Various analytical methods have been applied to study the conditional stability constants of trace metal -organic matter binding reactions such as ion selective electrode, anodic and cathodic stripping voltammetry, or fluorescence quenching (e.g. Buck et al., 2012;Pizeta et al., 2015). The results show that the conditional stability constants vary over a wide logarithmic range, indicating the diverse nature of the composition and structure of organic ligands in seawater. ...
Interactions between dissolved trace elements and organic ligands in seawater play an important role in ocean biogeochemistry, ranging from regulating primary production in surface waters to element cycling on basin-wide scale, with strong feedbacks to climate variability. In this study, we review different aspects in the field of marine trace elements and their organic ligands: recent instrumental innovation, factors that affect the fate of trace element complexes at the molecular level, spatial distribution of organic matter – trace element complexes in the ocean, modeling approaches as well as prospect in the scenarios of climate variability. We also assess the critical issues of parameterization in the numerical simulation that incorporate the trace elements – organic ligands interactions. Given the predicted climate changes, we examine the potential of exchange between inorganic and organic complexes for trace elements in different oceanic provinces.
... For example, iron (Fe) and copper (Cu) are among the elements that show highest affinities towards organic matter (Xue and Sigg, 2002;Ksionzek et al., 2018). Metals are complexed by a suite of competing inorganic or organic ligands that control their bioavailability and toxicity, and there is evidence for the presence of highly specific organic ligands in aquatic systems especially regarding essential trace elements like Cu and Fe (Ross et al., 2000;Buck et al., 2012;Waska et al., 2015). However, knowledge about the nature or identity of these ligands within DOM remains scarce (Xue and Sigg, 2002;Gaillardet et al., 2014). ...
The Amazon delivers a fifth of the global continental runoff and riverine dissolved organic carbon (DOC) to the ocean. Intensified biogeochemical processes are expected at the junction of the Amazon’s major blackwater tributary, the Rio Negro, and its parent, the Rio Solimões, due to large gradients in pH, conductivity, DOC and particle load. Dissolved organic matter (DOM) plays a major role in aquatic biogeochemical processes which are poorly understood on the molecular level. To gain insights into the potential role of DOM in non-conservative processes, we assessed dynamics of Cu, Fe and DOM by ultrahigh resolution mass spectrometry in: (1) endmembers, (2) regional samples and (3) laboratory mixing experiments under presence/absence of natural particles (> 0.2 μm). The relative abundances of 3,600 DOM molecular formulae were interpreted via multivariate statistics which revealed major dynamics in the DOM molecular composition. More than 40% of molecular formulae displayed conservative behavior even in the presence of natural particles, agreeing with bulk DOC behavior, but opposing the often-presumed non-conservative behavior of DOM. Another 16–27% of formulae fluctuated in FT-ICRMS signal intensity during mixing, but did not show consistent non-conservative behavior. Both rivers left a clear molecular imprint within the DOM of the Amazon, each being linked to > 800 molecular formulae. Characteristic for the Rio Negro was a dominance of phenolics with a wide molecular mass range (centered at ∼ 400 Da), and for the Rio Solimões more saturated but lower-molecular mass compounds (centered at ∼ 300 Da). Both Fe and Cu showed distinct non-conservative mixing patterns under particle presence. In the controlled mixing experiments including original particles at natural concentration, up to 0.5 μg L-1 Cu was released from the particles into solution at 20–40% blackwater contribution. Our molecular analysis revealed distinct DOM compositional changes in polyphenol- and nitrogen-containing formulae paralleling this release, suggesting links to desorption of potential ligands and charge-induced effects at particle surfaces caused by pH and conductivity changes in the course of mixing.
Cobalt distribution and speciation were quantified in space and time in the water column of a small, stratified, eutrophic lake, Linsley Pond, North Branford, CT. While scrupulously employing clean techniques, we used ligand exchange with dimethylglyoxime and cathodic stripping voltammetry to evaluate free and complexed forms of Co. Free aquo Co ion concentrations [Co2+] were found in the range from 0.014 to 0.28 nM from June to October 2018. Despite the orthograde distribution for total dissolved Co (0.42–3.34 nM), free Co2+ was higher in the epilimnion, decreasing with depth. Natural organic ligand concentrations [L] were in the span from 0.7 to 8.1 nM, with conditional stability constants (as logK) in the range from 9.43 to 11.13. Nearly all of the Co was complexed with highly selective ligands, and patterns suggest three controlling processes: (1) Co release from dissolving Mn (and perhaps Fe) oxides, (2) possible limitation by solubility of CoS(s), and (3) stabilization in solution via complexation by strong ligands. No correlation was observed between dissolved organic carbon and [L] in this study, suggesting that the ligands are not a simple subset of total dissolved organic matter, but may be specific compounds, perhaps S based. The hypothesis that the biological activity of plankton in Linsley Pond might be limited by micronutrient Co is only weakly supported. Cobalamin (VB12) measured via enzyme-linked immunosorbent assays ranged from 0.033 to 0.048 nM in this lake and does not follow a simple pattern with either total dissolved Co or Co2+, or with biological activity as indicated by chlorophyll levels.
Siderophores are strong iron‐binding molecules produced and utilized by microbes to acquire the limiting nutrient iron (Fe) from their surroundings. Despite their importance as a component of the iron‐binding ligand pool in seawater, data on the distribution of siderophores and the microbes that use them are limited. Here, we measured the concentrations and types of dissolved siderophores during two cruises in April 2016 and June 2017 that transited from the iron‐replete, low‐macronutrient North Pacific Subtropical Gyre through the North Pacific Transition Zone (NPTZ) to the iron‐deplete, high‐macronutrient North Pacific Subarctic Frontal Zone (SAFZ). Surface siderophore concentrations in 2017 were higher in the NPTZ (4.0–13.9 pM) than the SAFZ (1.2–5.1 pM), which may be partly attributed to stimulated siderophore production by environmental factors such as dust‐derived iron concentrations (up to 0.51 nM). Multiple types of siderophores were identified on both cruises, including ferrioxamines, amphibactins, and iron‐free forms of photoreactive siderophores, which suggest active production and use of diverse siderophores across latitude and depth. Siderophore biosynthesis and uptake genes and transcripts were widespread across latitude, and higher abundances of these genes and transcripts at higher latitudes may reflect active siderophore‐mediated iron uptake by the local bacterial community across the North Pacific. The variability in the taxonomic composition of bacterial communities that transcribe putative ferrioxamine, amphibactin, and salmochelin transporter genes at different latitudes further suggests that the microbial groups involved in active siderophore production and usage change depending on local conditions.
Iron (Fe) is an essential micronutrient to oceanic microalgae, and its dissolved fraction (DFe) is retained in surface waters by Fe-binding ligands. Previous work has suggested that ligands may also bind Fe within sea ice, although supporting data are limited. This study investigates distribution, concentration, and potential drivers of Fe-binding ligands in Antarctic sea ice, considering the ice type, location and season. Results suggest that the concentration of ligands (CL) varies throughout the year, both spatially and seasonally. The lowest CL (3.3–8.0 nM) and DFe concentrations (0.7–3.5 nM) were recorded in newly formed winter sea ice in the Weddell Sea, likely due to the early stage of sea-ice growth and low biological activity. The highest CL (1.7–74.6 nM), which follows the distribution of DFe (1.0–75.5 nM), was observed during springtime, in the Eastern Antarctic Sector. There, consistently higher values for CL in bottom ice depths were likely associated with enhanced algal biomass, while aeolian deposition may have acted as an additional source of DFe and ligands near Davis station. In summer, the senescence of ice algae and advanced sea-ice melting led to intermediate CL (1.0–21.9 nM) and DFe concentrations (0.6–13.3 nM) both on and off the East Antarctic coast. Regardless of time and location, >99% of DFe was complexed, suggesting that CL controls the distribution of DFe in sea ice. This study represents a first attempt at a year-round investigation of CL in sea ice, providing results that support the premise that sea ice acts as a potential biogeochemical bridge between autumn and spring phytoplankton blooms.
Organic complexation of Cu in estuarine waters in Japan was investigated using reverse titration competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV). We detected very strong Cu-binding organic ligands (L1) with conditional stability constants (KʹCuL1) of more than 1017. These ligands were successfully determined throughout the water column in Otsuchi Bay as well as in three rivers flowing into the bay. Organic ligands in the rivers had a concentration range of 2.6 nM–5.0 nM and log KʹCuL1 values between 17.5 and 18.6. The use of reverse titration enabled the detection of high concentrations of a weaker ligand class (L2) in river waters that ranged from 339 to 354 nM with log KʹCuL2 values between 11.9 and 12.1. We estimated that L1 was not derived from rivers or open ocean. Our incubation results indicated that the detected ligands did not increase with phytoplankton growth in Otsuchi Bay. Rather, coastal and benthic sediments may be major suppliers of L1 due to the enrichment of these ligands close to the sediment source. In contrast, the possible sources of weak Cu-binding organic ligands in Otsuchi Bay were estimated to be humic substances.
Organic Cu-binding ligands have a fundamental influence on Cu distributions in the global ocean and they complex >99% of the dissolved Cu in seawater. Cu-binding ligands however, represent a large diversity of compounds with distinct sources, sinks and chemical properties. This heterogeneity makes the organic Cu-binding ligand pool difficult to study at the global scale. In this review, we provide an overview of the diversity of compounds that compose the marine Cu-ligand pool, and their dominant sources and sinks. We also summarize the most common analytical methods to measure ligands in marine water column samples. Generally, ligands are classified according to their conditional binding strength to Cu. However, the lack of a common definition for Cu ligand categories has previously complicated data intercomparison. To address this, we provide a general classification for Cu-binding ligands according to their binding strength and discuss emerging patterns in organic Cu-binding ligand distributions in the ocean according to this classification. To date, there is no global biogeochemical model that explicitly represents Cu ligands. We provide estimates of organic Cu-binding ligand fluxes at key interfaces as first order estimates and a first step for future modeling efforts focused on Cu and Cu-binding ligands. Margin sediments and rivers are the most significant sources of copper binding ligands in the ocean while sedimentation, microbial uptake and photochemical degradation are the major sinks, suggests a synthesis of research on the cycling of copper ligands in the oceans.
The availability of iron (Fe) to marine microbial communities is enhanced through complexation by ligands. In Fe limited environments, measuring the distribution and identifying the likely sources of ligands is therefore central to understanding the drivers of marine productivity. Antarctic coastal marine environments support highly productive ecosystems and are influenced by numerous sources of ligands, the magnitude of which varies both spatially and seasonally. Using competitive ligand exchange adsorptive cathodic stripping voltammetry (CLE-AdCSV) with 2-(2-thiazolylazo)-p-cresol (TAC) as a competing artificial ligand, this study investigates Fe-binding ligands (FeL) across the continental shelf break in the Mertz Glacier Region, East Antarctica (64 - 67°S; 138 - 154°E) during austral summer of 2019. The average FeL concentration was 0.86 ± 0.5 nM Eq Fe, with strong conditional stability constants (Log KFeL) averaging 23.1 ± 1.0. The strongest binding ligands were observed in modified circumpolar deep water (CDW), thought to be linked to bacterial Fe remineralisation and potential siderophore release. High proportions of excess unbound ligands (L’) were observed in surface waters, as a result of phytoplankton Fe uptake in the mixed layer and euphotic zone. However, FeL and L’ concentrations were greater at depth, suggesting ligands were supplied with dissolved Fe from upwelled CDW and particle remineralisation in benthic nepheloid layers over the shelf. Recent sea-ice melt appeared to support bacterial production in areas where Fe and ligands were exhausted. This study is included within our newly compiled Southern Ocean Ligand (SOLt) Collection, a database of publicly available Fe-binding ligand surveys performed south of 50°S. A review of the SOLt Collection brings attention to the paucity of ligand data collected along the East Antarctic coast and the difficulties in pinpointing sources of Fe and ligands in coastal environments. Elucidating poorly understood ligand sources is essential to predicting future Fe availability for microbial populations under rapid environmental change.
Dissolved iron (dFe) and copper (dCu), the concentration and the conditional stability constants of organic binding ligands (LFe, LCu, log KcondFe3+L and log KcondCu2+L) were studied in the surface coastal waters of the Macaronesia region (Cape Verde, Canary Islands, and Madeira) using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV). Two oceanic stations were also studied: the Cape Verde Ocean Observatory (CVOO) and the European Station for Time Series in the Ocean in the Canary Islands (ESTOC). Dissolved Fe varied from 0.46 to 1.32 nM and LFe concentrations were between 0.56 and 2.96 nM. More than 98% of the total dFe was complexed with conditional stability constants (log KcondFe3+L) between 20.77 and 21.90 (L2-type ligands). Dissolved Cu concentrations ranged between 0.07 and 4.03 nM and the amount of LCu varied between 0.54 and 2.59 nM, with more than 99% of dCu organically complexed. The conditional stability constant (log KcondCu2+L) showed values between 13.40 and 14.42 (L1-type).
Due to biological activity and water mixing induced by the wind around the islands, dissolved metals and ligand concentrations were greater at the coastal stations than in oceanic water. Variations were observed between the eastern and western parts of Fogo, Tenerife and Gran Canaria. On the east coasts, the increase in dissolved metals and ligand concentrations were related to wind-induced water mixing. The results of this study will contribute to the knowledge about the impact of coastal areas on the Fe and Cu biogeochemical cycles.
Photochemical redox reactions of Cu(II) complexes of eight amino acid ligands (L) with nonpolar side chains have been systematically investigated in deaerated aqueous solutions. Under irradiation at 313 nm, the intramolecular carboxylate-to-Cu(II) charge transfer within Cu(II)–amino acid complexes leads to Cu(I) formation and the concomitant decomposition of amino acids. All amino acid systems studied here can produce ammonia and aldehydes except proline. For the 1:1 Cu(II) complex species (CuL), the Cu(I) quantum yields at 313 nm (ΦCu(I),CuL) vary by fivefold and in the sequence (0.10 M ionic strength at 25 °C) alanine (0.094) > valine (0.059), leucine (0.059), isoleucine (0.056), phenylalanine (0.057) > glycine (0.052) > methionine (0.032) > proline (0.019). This trend can be rationalized by considering the stability of the carbon-centered radicals and the efficient depopulation of the photoexcited state, both of which are dependent on the side-chain structure. For the 1:2 Cu(II) complex species (CuL2), the Cu(I) quantum yields exhibit a similar trend and are always less than those for CuL. The photoformation rates of ammonia, Cu(I), and aldehydes are in the ratio of 1:2.0 ± 0.2:0.7 ± 0.2, which supports the proposed mechanism. This study suggests that the direct phototransformation of Cu(II)–amino acid complexes may contribute to the bioavailable nitrogen for aquatic microorganisms and cause biological damage on cell surfaces in sunlit waters.
Organic ligands play a key role in the marine biogeochemical cycle of copper (Cu), a bio-essential element, regulating its solubility and bioavailability. However, the sources, abundance, and distribution of these ligands are still poorly understood. In this study, we examined vertical Cu speciation profiles from the South-East Atlantic (GEOTRACES section GA08). Profiles were collected from a range of ocean conditions, including the Benguela upwelling region, the oligotrophic South Atlantic Gyre, and the Congo River outflow. In general, the lack of a significant correlation between most of the parameters assessed here with Cu speciation data obscures the provenance of Cu-binding ligands, suggesting that Cu speciation in the South-East Atlantic is influenced by a complex interplay between biotic and abiotic processes. Nevertheless, the total dissolved Cu (CuT) illustrated an allochthonous origin in the working area, while Cu-binding ligands showed both an allochthonous and a biogenic, autochthonous origin. Pigment concentrations showed that the phylogeography of different microorganisms influenced the spatial features of the Cu-binding ligand pool in the South-East Atlantic. Allochthonous Cu-binding ligand sources in the upper water column are likely associated with dissolved organic matter which originated from the Congo River and the Benguela upwelling system. Deep water ligand sources could include refractory dissolved organic carbon (DOC), resuspended benthic inputs, and lateral advected inputs from the shelf margin. The degradation of L1-type ligands and/or siderophores in low oxygen conditions may also be a source of L2-type ligands in the deep. Free Cu ion levels (1.7 to 156 fM), the biologically available form of CuT, were below the putative biolimiting threshold of many marine organisms. Two classes of ligands were found in this study with total ligand concentrations ([LT]) ranging from 2.5 to 283.0 nM and conditional stability constants (logKCuL, Cu2+cond) ranging from 10.7 to 14.6. The Cu speciation values were spatially variable across the three subregions, suggesting that biogeochemical processes and sources strongly influence Cu speciation.
We used a combined ion pairing - organic matter speciation model (NICA-Donnan) to predict the organic complexation of iron (Fe) at ambient pH and temperature in the Celtic Sea. We optimized our model by direct comparison with Fe speciation determined by Adsorptive Cathodic Stripping Voltammetry using the added Fe-binding ligand 1-nitroso-2-naphthol (HNN) in the presence and absence of natural organic matter. We compared determined Fe speciation with simulated titrations obtained via application of the NICA-Donnan model with four different NICA parameter sets representing a range of binding site strengths and heterogeneities. We tested the assumption that binding sites scale to dissolved organic carbon (DOC) concentrations in marine waters. We found that a constant low DOC concentration resulted in an improved fit of our titration data to the simulated titrations, suggesting that inputs of autochthonous marine DOM may not increase the heterogeneity or concentrations of Fe binding sites. Using the optimal parameter set, we calculated pFe(III)´ (−log(∑Fe(OH)i³⁻ⁱ)) and apparent Fe(III) solubility (SFe(III)app) at ambient pH and temperature in the water column of the Celtic Sea. SFe(III)app was defined as the sum of aqueous inorganic Fe(III) species and Fe(III) bound to DOM formed at a free Fe (Fe³⁺) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). SFe(III)app was within range of the determined dissolved Fe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at our most offshore stations. Our study supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding by organic matter, although the overall number of metal binding sites in the marine environment may not be directly scalable to DOC concentrations.
Competitive ligand exchange–adsorptive cathodic
stripping voltammetry (CLE-AdCSV) is used to determine the conditional
concentration ([L]) and the conditional binding strength (logKcond) of
dissolved organic Fe-binding ligands, which together influence the
solubility of Fe in seawater. Electrochemical applications of Fe speciation
measurements vary predominantly in the choice of the added competing ligand.
Although different applications show the same trends, [L] and logKcond
differ between the applications. In this study, binding of two added ligands
in three different common applications to three known types of natural
binding ligands is compared. The applications are (1) salicylaldoxime (SA)
at 25 µM (SA25) and short waiting time, (2) SA at 5 µM (SA5), and
(3) 2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two
with overnight equilibration. The three applications were calibrated under
the same conditions, although having different pH values, resulting in the
detection window centers (D) DTAC > DSA25 ≥ SA5 (as logD
values with respect to Fe3+: 12.3 > 11.2 ≥ 11).
For the model ligands, there is no common trend in the results of
logKcond. The values have a considerable spread, which indicates that
the error in logKcond is large. The ligand concentrations of the nonhumic model ligands are overestimated by SA25, which we attribute to the lack
of equilibrium between Fe-SA species in the SA25 application. The
application TAC more often underestimated the ligand concentrations and the
application SA5 over- and underestimated the ligand concentration. The
extent of overestimation and underestimation differed per model ligand, and
the three applications showed the same trend between the nonhumic model
ligands, especially for SA5 and SA25. The estimated ligand concentrations for
the humic and fulvic acids differed approximately 2-fold between TAC and SA5
and another factor of 2 between SA5 and SA25.
The use of SA above 5 µM suffers from the formation of the species
Fe(SA)x (x>1) that is not electro-active as already suggested by
Abualhaija and van den Berg (2014). Moreover, we found that the reaction
between the electro-active and non-electro-active species is probably
irreversible. This undermines the assumption of the CLE principle, causes
overestimation of [L] and could result in a false distinction into more than
one ligand group.
For future electrochemical work it is recommended to take the above
limitations of the applications into account. Overall, the uncertainties
arising from the CLE-AdCSV approach mean we need to search for new ways to
determine the organic complexation of Fe in seawater.
Heavy metals are occurring in the aquatic environment as the result of natural or anthropogenic inputs, and depending on concentration, availability and resilience time, they can differently affect the animal wellness. Numerous studies reveal that more than 99% of metals in seawater are complexed with organic ligands suggesting the major role of organic complexation on metal behavior. Moreover, the amphilic character of marine natural organic matter makes this substance a relevant medium for interactions with charged and uncharged metal molecules. Here we review mechanisms and factors that control marine organic matter composition and its interactions with metals. Organic matter–metal complexes modify metal bioavailability and, in turn, change effects on living organisms.
Phytoplankton growth has been shown to be limited by a low supply of iron (Fe) in large parts of the world’s surface ocean. In oxic seawater, the thermodynamically favored Fe form is Fe(III), which is rapidly precipitated and scavenged out of solution. Iron bound to organic matter has been shown to dominate Fe speciation and to buffer dissolved Fe (DFe) concentrations over the solubility of inorganic Fe (Fe´). Our current knowledge of Fe speciation suggests that an excess binding capacity of organic matter relative to Fe typically exists in seawater, but the sources, nature and residence time of the Fe-binding ligand pool are still largely unclear. Organic speciation of Fe is usually determined via competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV), but its data interpretation has some limitations, e.g. the absence of pH and temperature dimensions. The oceans are currently experiencing acidification, warming, deoxygenation and stratification, and therefore it is important to understand the impact of changing seawater chemistry conditions (e.g. decreasing pH) on Fe speciation. Therefore, I applied an ion paring-organic matter model (NICA-Donnan), to thermodynamically calculate ambient Fe speciation and derive the apparent Fe(III) solubility (SFe(III)app). Iron speciation is calculated by the competition between inorganic complexes and organic complexation with the NICA-Donnan model, using DFe concentrations from various seawater samples. The SFe(III)app is calculated in a oversaturated system by setting an input of DFe(III) to 10 nmol L-1, at ambient ocean pH, temperature and dissolved organic carbon (DOC) concentrations. This will result in the precipitation of Fe hydroxide, as ferrihydrite assumed in our system. The SFe(III)app is defined as the sum of all aqueous inorganic species and Fe bound to organic matter at a free Fe (Fe3+) concentration equal to the limiting solubility of Fe hydroxide (Fe(OH)3(s)). I combined these predictions with observational DFe as well as Fe(II) data, to build a comprehensive picture on Fe speciation and DFe inventory in the ambient oceanic water column, with further feedbacks on primary productivity. In Chapter 3, I first calibrated predictions of Fe speciation with four different NICA parameter sets representing a range of binding sites strengths and heterogeneities, by comparing those predictions to determinations of Fe speciation via CLE-AdCSV in samples collected from the Celtic Sea. The results showed a constant low DOC concentration resulted in a slight improvement in the fit of titration data to the simulated titrations, suggesting that the changes in dissolved organic matter composition that occur alongside changes in DOC concentration dilute the Fe binding site pool. Using the optimized parameters, the calculated SFe(III)app was within the range of DFe concentrations observed after winter mixing on the shelf and in waters >1500 m depth at the furthest offshore stations. This supports the hypothesis that the ocean dissolved Fe inventory is controlled by the interplay between Fe solubility and Fe binding to organic matter. In Chapter 4, I further derived Fe(III) NICA constants for marine DOM from samples collected across the Peruvian shelf and slope, via the approach PEST-ORCHESTRA. Using the constants, the modelled SFe(III)app showed a ca. 2 fold increase in the oxygen minimum zone compared to surface waters. The increase results from a one order of magnitude decrease in H+ concentrations which impacts both Fe(III) hydroxide solubility and organic complexation. Using the Fe(II) measurements, I calculated the dissolved Fe(III) concentrations (DFe-FeII). The results highlight that the underlying distribution of ambient DFe(III) largely reflected the modelled SFe(III)app and an important role of ambient pH and temperature on the speciation and solubility of Fe. Finally, I investigated correlations of predicted SFe(III)app and measured DFe at ocean basin scales, using data obtained during a series of GEOTRACES cruises (Chapter 5). A similar trend was observed in the vertical distributions of horizontally averaged SFe(III)app and DFe. Combining the regression analysis and proportions of DFe relative to predicted SFe(III)app at the basin scale, the results suggest the distribution of DFe is not solely a function of sinking organic matter remineralization processes, but also regulated by relative changes in ambient pH and temperature. pH has a larger impact on SFe(III)app than DOM at basin scales, based on a solubility gradient of Fe hydroxide that is driven by ambient temperature. Therefore, consideration of the impact of pH and temperature on organic Fe complexation is as important for the speciation and solubility of Fe as the characterization of Fe-binding ligands, since the global distributions Fe-binding ligand (and DOC concentrations) are relatively invariant at the basin scale.
An optimization procedure was developed in this study using Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-AdCSV) to determine the concentration of dissolved Fe(III) in coastal water collected at Pulau Redang, Terengganu. The method was optimized for the UV-irradiation period for dissolved organic digestion, while the pH of the samples prior to UV-irradiation was determined. Additionally, the types and concentrations of the artificial ligands, followed by the sample equilibration period for sample measurements, were confirmed and performed. A small standard deviation of 0.002 was obtained in this experiment, indicating that the data obtained were precise when the optimized method was applied. Those optimized method was applied to the samples at Station 3, and the data obtained were compared with the previous study. Our present data showed that the concentration of dissolved Fe(III) was low, approximately 6-82 times lower than previously reported data. As our study area was affected by the Northeast monsoon (NEM), the phenomenon of strong turbulence during the post-monsoon event could have caused the resuspension of bottom sediment to the upper layer of seawater. Hence, the total suspended solids (TSS) level could have increased in the water column, suggesting that Fe(III) was mainly attached to the particulate phase rather than during the dissolved phase.
Competitive ligand exchange–adsorptive cathodic stripping voltammetry (CLE-AdCSV) is used to determine the conditional concentration ([L]) and the conditional binding strength (logKcond) of dissolved organic Fe-binding ligands, which together influence the solubility of Fe in seawater. Electrochemical applications of Fe speciation measurements vary predominantly in the choice of the added competing ligand. Although different applications show the same trends, [L] and logKcond differ between the applications. In this study, binding of two added ligands in three different common applications to three known types of natural binding ligands are compared. The applications are: 1) Salicylaldoxime (SA) at 25µM (SA25) and short waiting time, 2) SA at 5µM (SA5) and 3)2-(2-thiazolylazo)-ρ-cresol (TAC) at 10 µM, the latter two with overnight equilibration. The three applications were calibrated under the same conditions, although having different pH values, resulting in the detection window centers (D) DTAC > DSA25 ≥ SA5 (as log D values with respect to Fe3+: 12.3 > 11.2 ≥ 11). For the model ligands, there is no common trend in the results of logKcond. The values have a considerable spread, which indicates that the error in logKcond is large. The ligand concentrations of the non humic model ligands are overestimated by SA25 which we attribute to the lack of equilibrium between Fe-SA species in the SA25 application. The application TAC more often underestimated the ligand concentrations and the application SA5 over and under estimated the ligand concentration. The extent of overestimation and underestimation differed per model ligand, and the three applications showed the same trend between the non humic model ligands especially for SA5 and SA25. The estimated ligand concentrations for the humic and fulvic acids differed approximately 2 fold between TAC and SA5 and another factor of 2 between SA5 and SA25. The use of SA above 5 µM suffers from the formation of the species Fe(SA)x (x > 1) that is not electro-active as already suggested by Abualhaija and Van den Berg (2014). Moreover, we found that the reaction between the electro-active and non-electro-active species is probably irreversible. This undermines the assumption of the CLE principle, causes overestimation of [L] and could result in a false distinction into more than one ligand group. For future electrochemical work it is recommended to take the above limitations of the applications into account. Overall, the uncertainties arising from the CLE-AdCSV approach mean we need to search for new ways to determine the organic complexation of Fe in seawater.
Iron speciation in seawater is of the utmost importance as this element plays a central role in the regulation of primary productivity. Here we present the development of a CLE-CSV (Competitive Ligand Equilibration-Cathodic Stripping Voltammetry) procedure for iron speciation in seawater avoiding for the first time the use of the pH buffer (2,3-dihydroxynaphthalene is used as the added ligand, atmospheric oxygen as the catalytic enhancer and a 1 mL volume per sample aliquot). The unbuffered method was setup, validated by using known ligands and finally applied to the analysis of six seawater samples from the Ross Sea (Antarctica). The validation procedure demonstrated that ultratrace levels of ligands may be reliably determined and the application to seawater samples proved that the complex natural ligand pool can be detected with results undistinguishable from the ones obtained by the buffered procedure. The proposed method demonstrated a new principle in trace element speciation analysis by CLE-CSV, namely that the equilibration step may be performed at natural pH, whereas the pH may be set at its optimal value for sensitivity during analysis, thanks to the raise in pH at the electrode/solution interface caused by oxygen reduction. This change in paradigm paves the way to the investigation of iron speciation at natural pH in traditionally difficult samples that show circumneutral or slightly acidic pH values. The relevance of the here proposed approach to existing speciation procedures by CLE-CSV is also discussed.
The toxicity and bioavailability of Cu in seawater were estimated using its chemical speciation. In this study, the concentrations and conditional stability constants (K'CuL) of Cu complexes with Cu-binding organic ligands (L) in seawater were determined using the reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry method. The concentrations of strong and weak ligands (L1 and L2, respectively) were determined using reverse titration via the addition of Cu to seawater samples to achieve ligand saturation. Our results revealed that the reverse titration method can be successfully used for the detection of high concentrations of L2. Using the reverse titration method, we detected L2 concentrations that exceeded 80 and 120 nM in seawater samples from the subtropical western North Pacific Ocean and Otsuchi Bay Japan, respectively. The K′CuL1 and K′CuL2 values obtained using the reverse titration method were comparable with those obtained using the forward titration method. Therefore, the reverse titration method can be used to determine the chemical speciation of Cu in the ocean and coastal regions as supplement to the more conventional forward titration method.
The concentrations of bioactive trace metals (Fe, Cu, Mn, Zn, Co, Ni, Cd, and Pb), Fe-and Cu-binding organic ligands, and electroactive Fe-binding humic substances were measured in surface waters across the West Florida Shelf in the eastern Gulf of Mexico in June 2015 and in February-March 2018. Seasonal differences in dust deposition were associated with increased concentrations of Fe, Mn and Pb in offshore surface waters in June 2015 compared to Feb-Mar 2018. Total concentrations of Fe-binding ligands offshore were similar between seasons, and this ambient ligand pool acted to stabilize a portion of Fe delivered via dust deposition within the dissolved fraction. Apparent photoreduction of Cu-binding organic ligands in offshore summer surface waters led to bioavailable Cu²⁺ concentrations that could potentially inhibit the growth of some cyanobacteria species. The concentrations of Fe-binding humic-like ligands were better correlated with Cu-binding ligands than Fe-binding ligands, suggesting that terrestrially derived photoactive ligands may have a significant influence on the cycling of Cu-binding ligands across the West Florida Shelf. Intrusion of the Loop Current into the study region was observed in June 2015 and appeared to entrain bands of low salinity water from the northern Gulf of Mexico. This process brought elevated trace metals and Fe-binding humic-like ligands to the outer West Florida Shelf, which may then be delivered to the North Atlantic via the Loop Current-Florida Current-Gulf Stream system.
Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.
Iron speciation analysis in seawater is a fundamental step to understand the cycling of this element in oceanic waters, in view of its central role in regulating primary productivity and its connection to global planetary cycles. At present, analytical procedures are the bottleneck for speciation analysis, in term of both time and sample size requirement. Here we present a novel instrumental configuration for the speciation analysis of iron by the Competitive Ligand Equilibration-Cathodic Stripping Voltammetry (CLE-CSV) procedure. The new system features a 1 mL microcell and a silver wire pseudoreference enabling a tenfold reduction of the sample volume. 2,3-dihydroxynaphthalene was used as the complexing ligand and atmospheric oxygen as the catalytic enhancer because they ensured the best analytical performances in terms of detection capabilities. The side reaction coefficient for the FeDHN complex αFe’DHN was calibrated against EDTA and an average value of 9.25 for logK’Fe’DHN was calculated. The method was successfully validated in UV digested seawater using diethylenetriaminepentaacetic acid (DTPA), which has known stability constant for iron. The method was lastly applied to six samples from the Ross Sea water column (Antarctica), demonstrating its fit for purpose for the detection of trace amounts of iron ligands in seawater. Thanks to the employed instrumental configuration and the high sensitivity, the proposed method achieved a tenfold reduction in sample size, a tenfold increase in sensitivity compared with other methods employing DHN and halved the analysis time with respect to the fastest method reported in the literature. Half an hour is enough to measure a 12 point titration, making the analysis of at least three titrations per day feasible. It is expected that the application of this procedure will foster the sample throughput, thanks to the reduced analysis time, and make possible the analysis of limitedly available and challenging samples, like porewater and vent fluids via the tenfold reduction in sample size.
The size partitioning of dissolved trace metals is an important factor for determining reactivity and bioavailability of metals in marine environments. This, alongside the advent of more routine shipboard ultrafiltration procedures, has led to increased attention in determining the colloidal phase of metals such as Fe in seawater. While clean and efficient filtration, prompt acidification, and proper storage have long been tenets of trace metal biogeochemistry, few studies aim to quantify the kinetics of colloidal exchange and metal adsorption to bottle walls during storage and acidification. This study evaluates the effect of storage conditions on colloidal size partitioning, the kinetics of colloid exchange over time, and the timescale of bottle wall adsorption and desorption for dissolved Fe, Cu, Ni, Zn, Cd, Pb, Mn and Co. We report that preservation of dissolved size partitioning is possible only for Fe and only under frozen conditions. All metals except Mn and Cd show regeneration of the colloidal phase following its removal in as short as 14 h, validating the importance of prompt ultrafiltration. Adsorption of metals to bottle walls is a well-known sampling artifact often cited for Fe and assumed to be potentially significant for other metals as well. However, only Fe and Co showed significant proclivity to adsorption onto low density polyethylene bottle walls, sorbing a maximum of 91 and 72% over 40 months, respectively. After 20 weeks of acidification neither Fe nor Co desorbed to their original concentrations, leading to an acidified storage recommendation of 30 weeks prior to analyses following storage of unacidified samples for long periods of time. This study provides empirical recommendations for colloidal and dissolved trace metal methodology while also paving the way for much-needed future methods testing.
In this work, we performed electrochemical investigations of Fe-binding ligands in water samples collected in autumn 2011 along the Australian GEOTRACES southwestern Pacific section (GP13, between 153°E and 170°W longitude along the 30°S line East of Australia, 0-1000 m depth). We determined the capacity of the bulk organic ligands to complex Fe using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime as the competing ligand. Two categories of organic ligands, humic substances (HS-like) and catalytically active polymers (Cat. P) were electrochemically quantified in order to better define the bulk of Fe-binding ligands. Finally, Fe speciation results have been linked to oceanographic data, phytoplankton biomass, and two groups of cyanobacteria (Prochlorococcus and Synechococcus) which are prominent members of the phototrophic community in the study region. Across the section, higher total ligand concentrations over dissolved Fe concentrations were observed, as well as the predominance of “weak” Fe-binding ligands (log K’Fe’L < 12). Highest “excess ligands” were mostly concentrated in the upper layer of the water column, suggesting a direct link with biological activity. None of the two groups of organic ligands measured (HS-like and Cat. P) accounted for the bulk of the total Fe-binding ligands concentration, hindering a better characterization of the nature of in-situ Fe-binding ligands. Cat. P concentrations showed statistically significant positive correlations with all biomarker pigments and the abundance of Prochlorococcus, suggesting that this material, resembling polysaccharides, could be a good parameter to probe organic compounds from specific biological origin. Amongst the biological parameters, only Prochlorococcus was related to Fe’ concentrations.
The cycling of copper (Cu) and its isotopes in the modern ocean is controlled by the interplay of biology, redox settings, and organic complexation. To help build a robust understanding of Cu cycling in the modern ocean and investigate the potential processes controlling its behavior in the geological past, this study presents Cu abundance and isotope data from modern Peru Margin sediments as well as from a suite of ancient, mostly organic-rich, shales. Analyses of an organic-pyrite fraction extracted from bulk modern sediments suggest that sulphidation is the main control on authigenic Cu enrichments in this setting. This organic-pyrite fraction contains, in most cases, >50% of the bulk Cu reservoir. This is in contrast to ancient samples, for which a hydrogen fluoride (HF)-dissolvable fraction dominates the total Cu reservoir. With <20% of Cu found in the organic-pyrite fraction of most ancient sediments, interpretation of the associated Cu isotope composition is challenging, as primary signatures may be masked by secondary processes. But the Cu isotope composition of the organic-pyrite fraction in ancient sediments hints at the potential importance of a significant Cu(I) reservoir in ancient seawater, perhaps suggesting that the ancient ocean was characterized by different redox conditions and a different Cu isotope composition to that of the modern ocean.
The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs—the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings.
Expected final online publication date for the Annual Review of Marine Science Volume 12 is January 3, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Total dissolved Cu concentrations, concentrations and conditional stability constants of Cu-binding organic ligands, and the intensities of fluorescent dissolved organic matter (FDOM) components in seawater were determined in the East China Sea. Concentrations of organic ligands were determined using CLE-AdCSV with a 5 μM SA detection window. Total dissolved Cu concentrations ranged from 0.59 nM to 4.65 nM and were particularly high in surface waters with low salinity due to influences of the Changjiang Diluted Water. Concentrations of the stronger ligand (L 1 ) were between 0.65 and 6.92 nM and its distribution closely followed that of dissolved Cu. Four FDOM components were detected in surface waters, including two humic-like and two protein-like components. L 1 concentrations exhibited similar distribution patterns to the intensities of all four FDOM components. The concentrations of the weaker ligands (L 2 ) ranged from 2.86 nM to 31 nM, with the highest concentrations detected in the surface waters of stations unaffected by the Kuroshio Current. We found the buffering ability of Cu-binding organic ligands in the East China Sea to be sufficient for responding to atmospheric and anthropogenic Cu inputs and keeping the free cupric ion concentrations relatively constant.
The physicochemical speciation of dissolved iron (Fe) across natural dissolved Fe gradients in the oceanic and shelf domains of the southeastern Bering Sea was examined in surface and subsurface samples using competitive ligand exchange-adsorptive cathodic stripping voltammetry with the added ligand salicylaldoxime. Two ligand classes were measured in all samples, a stronger L<sub>1</sub> ligand class and a weaker L<sub>2</sub> ligand class. Conditional stability constants for both ligand classes were comparable between surface and subsurface samples, with mean $log K_{FeL_{1},Fe'}^{cond} = 11.5 \pm 0.3$ and mean $log K_{FeL_{2},Fe'}^{cond} = 10.3 \pm 0.3$ in surface samples, and mean $log K_{FeL_{1},Fe'}^{cond} = 11.4 \pm 0.2$ with a weaker ligand and mean $log K_{FeL_{1},Fe'}^{cond}$ , of $10.2 \pm 0.2$ in subsurface samples. The concentrations of dissolved Fe were strongly correlated with ambient stronger L<sub>1</sub> ligand concentrations for all samples with dissolved Fe concentrations greater than $0.2 nmol L^{-1}$ . In samples with dissolved Fe concentrations less than $0.2 nmol L^{-1}$ , large and variable excesses of L<sub>1</sub> ligand concentrations were measured, coincident with observed Fe stress or limitation on the ambient phytoplankton. These observations suggest that the phytoplankton community is readily able to access dissolved Fe from the $FeL_1$ complex, resulting in excess L<sub>1</sub> in these waters. The available speciation data from other sources indicate that a significant correlation exists between dissolved Fe and L<sub>1</sub> ligand concentrations in samples with intermediate dissolved Fe, and this is a seemingly ubiquitous feature of dissolved Fe cycling in the marine environment.
The trace metal iron (Fe) is now routinely included in state-of-the-art ocean general circulation and biogeochemistry models (OGCBMs) because of its key role as a limiting nutrient in regions of the world ocean important for carbon cycling and air-sea CO 2 exchange. However, the complexities of the seawater Fe cycle, which impact its speciation and bioavailability, are simplified in such OGCBMs due to gaps in understanding and to avoid high computational costs. In a similar fashion to inorganic carbon speciation, we outline a means by which the complex speciation of Fe can be included in global OGCBMs in a reasonably cost-effective manner. We construct an Fe speciation model based on hypothesised relationships between rate constants and environmental variables (temperature, light, oxygen, pH, salinity) and assumptions regarding the binding strengths of Fe complexing organic ligands and test hypotheses regarding their distributions. As a result, we find that the global distribution of different Fe species is tightly controlled by spatio-temporal environmental variability and the distribution of Fe binding ligands. Impacts on bioavailable Fe are highly sensitive to assumptions regarding which Fe species are bioavailable and how those species vary in space and time. When forced by representations of future ocean circulation and climate we find large changes to the speciation of Fe governed by pH mediated changes to redox kinetics. We speculate that these changes may exert selective pressure on phytoplankton Fe uptake strategies in the future ocean. In future work, more information on the sources and sinks of ocean Fe ligands, their bioavailability, the cycling of colloidal Fe species and kinetics of Fe-surface coordination reactions would be invaluable. We hope our modeling approach can provide a means by which new observations of Fe speciation can be tested against hypotheses of the processes present in governing the ocean Fe cycle in an integrated sense.
The speciation of iron was investigated in three shelf seas and three deep basins of the Arctic Ocean in 2007. The dissolved fraction (<0.2 m) and a fraction < 1000 kDa were considered here. In addition, unfiltered samples were analyzed. Between 74 and 83% of dissolved iron was present in the fraction < 1000 kDa at all stations and depth, except at the chlorophyll maximum (42-64%). Distinct trends in iron concentrations and ligand characteristics were observed from the shelf seas toward the central deep basins, with a decrease of total dissolvable iron ([TDFe] > 3 nM on the shelves and [TDFe] < 2 nM in the Makarov Basin). A relative enrichment of particulate Fe toward the bottom was revealed at all stations, indicating Fe export toward the deep ocean. In deep waters, dissolved ligands became less saturated with Fe (increase of [Excess L]/[Fe]) from the Nansen Basin via the Amundsen Basin toward the Makarov Basin. This trend was explained by the reactivity of the ligands, higher (log alpha > 13.5) in the Nansen and Amundsen basins than in the Makarov Basin (log alpha < 13) where the sources of Fe and ligands were limited. The ligands became nearly saturated with depth in the Amundsen and Nansen Basins, favoring Fe removal in the deep ocean, whereas in the deep Makarov Basin, they became unsaturated with depth. Still here scavenging occurred. Although scavenging of Fe was attenuated by the presence of unsaturated organic ligands, their low reactivity in combination with a lack of sources of Fe in the Makarov Basin might be the reason of a net export of Fe to the sediment.
Current knowledge about the role of iron-binding organic ligands in the ocean and their role in determining the biogeochemistry of this biologically active element has been summarised. Some electrochemical measurements suggest the presence of at least two ligand types, a strong binding ligand L1 found mainly in the mixed layer and a weaker ligand L2 found mainly in deep water. Speciation of FeIII is dominated by L1 in the mixed layer and L2 in the deep ocean. There is some evidence that L1 is siderophore-like and is specifically generated by marine microbes (i.e. heterotropic bacteria and cyanobacteria). We suggest that this is a specific biological mechanism for sequestering iron in the mixed layer that developed early in the ocean's history (Archaean period, 2500-3500 million years BP), whereas the more ubiquitous L2 ligand only arose at the close of the Proterozoic (500-2500 million years BP) when eukaryotic organisms evolved to switch on the ocean's biological pump, allowing L2 ligands to form from the oxidation of sinking biological particles. This development coincided with the complete oxygenation of the ocean's interior which removed the iron-binding sulfide ion and allowed maintenance of the ocean's iron inventory. These speculations are accompanied by various suggestions about avenues for future research to better understand iron biogeochemistry.
Complexation of iron (III) with natural organic ligands was investigated during a mesoscale iron enrichment experiment in the western subarctic North Pacific (SEEDS II). After the iron infusions, ligand concentrations increased rapidly with subsequent decreases. While the increases of ligands might have been partly influenced by amorphous iron colloids formation (12–29%), most in-situ increases were attributable to the <200 kDa fraction. Dilution of the fertilized patch may have contributed to the rapid decreases of the ligands. During the bloom decline, ligand concentration increased again, and the high concentrations persisted for 10 days. The conditional stability constant was not different between inside and outside of the fertilized patch. These results suggest that the chemical speciation of the released iron was strongly affected by formation of the ligands; the production of ligands observed during the bloom decline will strongly impact the iron cycle and bioavailability in the surface water.
The trace metal iron (Fe) is now routinely included in state-of-the-art ocean general circulation and biogeochemistry models (OGCBMs) because of its key role as a lim-iting nutrient in regions of the world ocean important for carbon cycling and air-sea CO 2 exchange. However, the complexities of the seawater Fe cycle, which impact its 5 speciation and bioavailability, are highly simplified in such OGCBMs to avoid high com-putational costs. In a similar fashion to inorganic carbon speciation, we outline a means by which the complex speciation of Fe can be included in global OGCBMs in a reason-ably cost-effective manner. We use our Fe speciation to suggest the global distribution of different Fe species is tightly controlled by environmental variability (temperature, 10 light, oxygen and pH) and the assumptions regarding Fe binding ligands. Impacts on bioavailable Fe are highly sensitive to assumptions regarding which Fe species are bioavailable. When forced by representations of future ocean circulation and climate we find large changes to the speciation of Fe governed by pH mediated changes to redox kinetics. We speculate that these changes may exert selective pressure on phy-15 toplankton Fe uptake strategies in the future ocean. We hope our modeling approach can also be used as a "test bed" for exploring our understanding of Fe speciation at the global scale.
The speciation of trace metals in seawater based on the voltammetric (DPASV) titration of ligands by metal addition is considered. The method allows the determination of the total metal amount, the labile fraction, the ligand concentration and the related conditional stability constant. Analytical problems related to sample contaminations during sampling, filtration and storage, displacement of complexing equilibria in freeze storage, the kinetics of the reaction of complexation and the potential kinetic lability of organic complexes are discussed and possible solutions presented. Data on quality control tests carried out to verify the accuracy of the laboratory procedure for trace metal determination in seawater are reported.
The supply of iron, relative to that of the macronutrients nitrate, phosphate, and silicic acid, plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes. The presence or absence of a broad continental shelf influences the supply of iron, The iron input to central California upwelling waters varies spatially and can be characterized by two end-member regimes. One end member, which includes Monterey Bay and extending north to Pt. Reyes, is an iron-replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron (>10 nM) entrained together with high concentrations of nitrate and silicic acid. In these iron-replete regions, extensive blooms of large diatoms deplete macronutrient concentrations, which results in correspondingly high chlorophyll a concentrations. The other end member, located to the south of Monterey Bay off the Big Sur coast, is an iron-deplete regime where upwelling is focused offshore of a narrow continental shelf. Upwelled waters in the Big Sur region are characterized by low dissolved and particulate iron concentrations (<1 nM), together with high concentrations of nitrate and silicic acid. Extremely low iron concentrations, unused nitrate and silicic acid, and a low abundance of large diatoms characterize surface waters in these iron-deplete regions, and thus represent coastal upwelling, high-nutrient, low-chlorophyll systems limited by the micronutrient iron.
The goals of this dissertation are to better understand the sources and the Cu binding ability of ligands that control Cu toxicity in estuaries and harbors, where elevated Cu concentrations have caused documented toxic effects on microorganisms, fish, and benthic fauna. I modified and improved a commonly used approach to determine metal speciation (competitive ligand exchange adsorptive cathodic stripping voltammetry, CLE-ACSV). Using this new approach to chemical Cu speciation and an old approach to physical Cu speciation (filtration), I show that riverine humic substances, filtrable, recalcitrant and light absorbing molecules from degraded plant material, can account for all of the Cu binding in the Saco River estuary. This finding directly supports the hypothesis that terrestrial humic substances might be the most important source of Cu ligands for buffering Cu toxicity in coastal locations with freshwater inputs. However, fieldwork in coastal waters with large inputs of both Cu and suspended colloids (Boston Harbor, Narragansett Bay, and two ponds on Cape Cod) shows that some Cu present in these samples is inert to our competitive ligand exchange method for at least 48 hours. These results support the hypothesis that a significant fraction of the Cu present in these samples is physically sequestered in colloidal material, with the remaining fraction complexed by humic substances. Previous studies of Cu speciation were not able to distinguish between strongly complexed Cu and inert Cu, and our analytical approach should be used further to determine the role of colloids in Cu speciation in all natural waters.
Chromophoric dissolved organic matter (CDOM) was surveyed along the 160°W transect from the equatorial to the subarctic Pacific. CDOM characteristics were evaluated through the measurement of fluorescence intensity at 320 nm excitation and 420 nm emission and of absorption coefficient (a) at 320 nm, both indicative of marine humic-like CDOM. In the surface layer (̃200 m), different levels but similar optical characteristics of CDOM were found among the oceanic regions studied. In the mesopelagic (200-1000 m) and abyssal layers (1000 m- bottom), levels of fluorescence intensities were linearly correlated to those of absorption coefficients, with a similar slope between these layers. However, the intercept of the linear relationship between the two optical parameters was significantly lower for the mesopelagic layer than for the abyssal layer. Differences between intercepts were consistent with the transport of optically distinct CDOM to the mesopelagic layer through the formation of the North Pacific intermediate water (NPIW). At wavelengths shorter than 300 nm, the absorption coefficients of CDOM in the surface layer systematically deviated from the natural logarithmic relationships between absorption coefficients and wavelengths in the 320-350 nm range. This class of CDOM, estimated using absorption coefficient at 275 nm, was defined as surface-specific CDOM and may be derived from biocomponents. Surface-specific CDOM was higher in the relatively young mode water than in old NPIW, suggesting that this class of CDOM is semi-labile. © 2009, by the American Society of Limnology and Oceanography, Inc.
In the Eastern North Atlantic Ocean iron (Fe) speciation was investigated in three size fractions: the dissolvable from unfiltered samples, the dissolved fraction (<0.2 μm) and the fraction smaller than 1000 kDa (<1000 kDa). Fe concentrations were measured by flow injection analysis and the organic Fe complexation by voltammetry. In the research area the water column consisted of North Atlantic Central Water (NACW), below which Mediterranean Overflow Water (MOW) was found with the core between 800 and 1000 m depth. Below 2000 m depth the North Atlantic Deep Water (NADW) proper was recognised. Dissolved Fe and Fe in the <1000 kDa fraction showed a nutrient like profile, depleted at the surface, increasing until 500–1000 m depth below which the concentration remained constant. Fe in unfiltered samples clearly showed the MOW with high concentrations (4 nM) compared to the overlying NACW and the underlying NADW, with 0.9 nM and 2 nM Fe, respectively. By using excess ligand (Excess L) concentrations as parameter we show a potential to bind Fe. The surface mixed layer had the highest excess ligand concentrations in all size fractions due to phytoplankton uptake and possible ligand production. The ratio of Excess L over Fe proved to be a complementary tool in revealing the relative saturation state of the ligands with Fe. In the whole water column, the organic ligands in the larger colloidal fraction (between 0.2 μm and 1000 kDa) were saturated with Fe, whereas those in the smallest fraction (<1000 kDa) were not saturated with Fe, confirming that this fraction was the most reactive one and regulates dissolution and colloid aggregation and scavenging processes. This regulation was remarkably stable with depth since the alpha factor (product of Excess L and K′), expressing the reactivity of the ligands, did not vary and was 1013. Whereas, in the NACW and the MOW, the ligands in the particulate (>0.2 μm) fraction were unsaturated with Fe with respect to the dissolved fraction, thus these waters had a scavenging potential.
Copper(II) complexation in the upper water column was studied at the Bermuda Atlantic Time Series Station (BATS) from January 1992 to March 1993, and in the southern Sargasso Sea in April 1992, using adsorptive cathodic stripping voltammetry (ACSV). Copper titration data, analysed using a one ligand model, indicated that speciation was dominated by a strong ligand or ligand class, with a conditional stability constant of 1013.2. Total concentrations of copper and ligand were very similar, ranging from 0.9 nM to 2 nM. Concentrations of free cupric ion (Cuf2+) varied widely in the upper water column depending on whether Cu exceeded the ligand concentration or vice versa. Temporal and spatial variability in these parameters showed trends with hydrographic and biological parameters consistent with biological production and near-surface photochemical decomposition of the strong ligand. Under well stratified oligotrophic conditions, such as those prevailing year round at the southern Sargasso station and in the summer and autumn at BATS, the ligand showed a subsurface maxima coinciding roughly with the chlorophyll maximum. Ligand concentrations decreased below total Cu concentrations in the mixed layer, leading to pronounced increases in Cuf2+ concentrations. However, samples collected at BATS during or following periods of intense vertical mixing and biological activity showed excess ligand concentration throughout the upper water column and extremely low cupric ion concentrations. The spatial and temporal variability of the strong ligand at BATS is similar to that observed by Coale and Bruland (1990) in the NE Pacific, suggesting that Cu speciation in both regions is controlled by common processes. However, in that study, copper concentrations were always below the ligand concentration, so cupric ion concentrations did not display the great variability observed in the Sargasso Sea in this study.
Cu speciation was characterized at three stations in the sub arctic NW Pacific and Bering Sea using cathodic stripping voltammetry with the competing ligands benzoylacetone and salicylaldoxime. A single ligand model was fit to the titration data, yielding concentrations throughout the water column of ∼3–4nM, and conditional stability constants ranging from 1012.7 to 1014.1, this range being partly due to the choice of competing ligand. Free Cu2+ in surface waters was 2–4×10−14M, in close agreement with values reported by previous workers in the NE Pacific using anodic stripping voltammetry (ASV). However, those results showed that complexation by strong organic ligands becomes unimportant below 200–300m, while our data indicated Cu is strongly complexed to depths as great as 3000m. Free Cu2+ concentrations in surface waters reported here and in previous work are close to the threshold value where Cu can limit the acquisition of Fe by phytoplankton.
During the Kerguelen Ocean and Plateau compared Study (KEOPS; January–February 2005) cruise, the area southeast of the Kerguelen Archipelago in the Indian sector of the Southern Ocean was investigated to identify the mechanisms of natural iron fertilization of the Kerguelen Plateau. In this study, the organic speciation of Fe is described. Samples were determined immediately on board using competing ligand-adsorptive cathodic stripping voltammetry (CL-AdCSV). The dissolved organic ligands were always in excess of the dissolved Fe concentration, increasing the residence time in the water column and the potential availability for phytoplankton. The concentration of the dissolved organic ligands ranged from 0.44 to 1.61nEq of M Fe (=complexation site for Fe), with an average concentration of 0.91nEq of M Fe (S.D.=0.28, n=113) and a mean logarithm of conditional stability constant (logK′) of 21.7 (S.D.=0.28, n=113). A second weaker dissolved organic ligand group was detected in 32% of the samples, with Fe-binding characteristics at the edge of the detection window of the applied method.The occurrence of the highest concentrations of dissolved organic ligands in the wind-mixed surface layer and near the sediment at the bottom of the water column indicated that both phytoplankton and the sediment act as sources. Both sources are in concert with the general conclusions from the KEOPS research on the sources of Fe, where Fe was regenerated, organic Fe-binding ligands were formed in the upper layers, and both Fe and ligands were supplied by the sediment.
A highly sensitive voltammetric technique was developed to examine Fe speciation in seawater. The technique involves adding an Fe(III)-complexing ligand, salicylaldoxime, which competitively equilibrates with inorganic and organic Fe(III) species in ambient seawater. The Fe(III)-salicylaldoxime complex then is measured by adsorptive cathodic stripping voltammetry (ACSV). This new method revealed that 99.97% of the dissolved Fe(III) in central North Pacific surface waters is chelated by natural organic ligands. The total concentration of Fe-binding ligands is approximately 2 nM, a value greatly in excess of ambient dissolved iron concentrations. The titration data can be modeled as consisting of two classes of Fe-binding ligands, a strong ligand class (L1) with an average surface-water concentration equal to 0.44 nM with a conditional stability constant KL1Fe′cond = 1.2 × 1013 M−1, and a weaker ligand class (L2) with an average concentration equal to 1.5 nM with KL2Fe′cond = 3.0 × 1011 M−1. The low concentration of dissolved Fe present in surface waters (~ 0.2 nM), coupled with the excess of strong Fe-chelators, results in extremely low equilibrium concentrations of dissolved inorganic iron, [Fe′] ≈ 0.07 pM. In the deeper waters there is a 2 nM excess of Fe-binding ligands with a stability constant similar to that of the L2 class of ligands observed in surface waters, resulting in dissolved Fe(III) existing primarily in the chelated form in deep waters as well. The stability constants of the natural ligands are comparable to the model ligands desferal, a siderophore, and the prosthetic heme group, protoporphyrin-IX. The high degree of organic complexation of iron makes it critically important to reevaluate our perceptions of the marine biogeochemistry of iron and the mechanisms by which biota can access this chelated Fe.
Dissolved iron (Fe) speciation in the Columbia River plume, the San Francisco Bay plume, and the Columbia River estuary was investigated using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with the added ligand salicylaldoxime. A stronger L₁-type Fe-binding ligand class was measured in all surface samples, and in the Columbia River estuary. A weaker L₂-type ligand class was present in the far-field Columbia River plume and the San Francisco Bay plume but was not observed in the low-salinity (S = 1.4-22.5) waters of the near-field Columbia River plume or estuary. Concentrations of total dissolved Fe were correlated with the concentrations of the stronger L₁-type ligand in nonestuarine (S > 13) surface samples. Leachable particulate (>0.4 µm) Fe concentrations in the Columbia River plume were measured to supplement existing data from the San Francisco Bay plume. There is a large concentration of readily leachable particulate Fe in the two plumes, yet it is the concentration of ambient L₁-type ligands that appears to dictate the concentration of dissolved Fe in these waters and, consequently, the supply of dissolved Fe to neighboring coastal waters. The correlation between dissolved Fe and L₁ ligand concentrations in both plume waters, as well as in California Current and upwelled surface waters, suggests that this relationship will persist in other coastal environments and should be considered when evaluating and modeling coastal Fe cycling and supply.
Accuracy of the Scatchard linearization data processing method for competitive ligand exchange (CLE)–cathodic stripping voltammetry (CSV) measurements of seawater Fe-organic complexation in two-ligand case is examined with idealized Fe titrations data sets that are simulated using preset values of ligand parameters (conditional binding constants and total ligand concentrations). The results reveal substantial inherent artifacts for this method. An examination of patterns by which these artifacts vary with changes in Fe-binding strength of natural ligands relative to that of the added ligand suggests that the artifacts result not only from underestimated voltammetric sensitivity, but also from inadequate separation of individual ligand's contribution to Fe complexation at each titration point. For idealized simulated titration data, these artifacts can be eliminated by a procedure that combines non-linear regression with Turoczy and Sherwood's iteration. The method is demonstrated by reproducing preset values of ligand parameters over a diverse range of organic ligand alpha coefficients and by modeling titration data determined by CLE–CSV for seawater samples collected from the Eastern Bering Sea. Error analysis suggests that the sensitivity of model-derived ligand parameters to the error in the titration data is a strongly non-linear function of ligand parameters. Accurate measurement of titration data is thus required to use this method for accurate CSV measurements of seawater Fe-organic complexation.
The complexation of zinc by natural organic ligands in the upper 600 m of the central North Pacific was determined with differential pulse anodic stripping voltammetry at a thin mercury film, rotating glassy-carbon disk electrode. Of the dissolved zinc in surface waters >98% was bound in strong complexes (log Klcond, ="z+ = 11 .O) by relatively zinc-specific organic ligands existing at low concentrations (1.2 nM). Although the vertical distribution of this ligand class is relatively uniform, total dissolved zinc varies from concentrations close to 0.1 nM in surface waters of the central North Pacific to 3 nM at 500 m. Complexation of dissolved zinc by this class of organic ligands causes the concentration of inorganic zinc to vary from 2 pM in surface waters to 2 nM at 500 m, a 1 ,OOO-fold variation. The free zinc ion activity, expressed as pZn, is calculated to be of the order of 12.7 in the surface waters of the central North Pacific. This low free zinc ion activity may influence distributions of oceanic and neritic phytoplankton species.
Copper titrations were conducted at sea with differential pulse anodic stripping voltammetry to examine the degree to which copper was associated with organic ligands. Greater than 99.7% of the total dissolved copper in surface waters of the central Northeast Pacific shallower than 200 m was estimated to be associated with strong organic complexes. Below 200 m, increasing proportions of inorganic or labile copper spccics were observed. At middepths (1,000 m), about 50-70% of the total dissolved copper was in the organically complexed form. Whereas total copper varies by a factor of only three from the surface to middepths (0.6-I.8 nM), copper complexation gives rise to extremely low cupric ion activities in surface waters ({Cu!'} = 1.4 x lo-l4 M) and higher values at middepth ({Cuz-I} = lo- I1 M)-a variation of three orders of magnitude. Two classes of copper- binding ligands were found to be responsible for this complexation: an extremely strong ligand class (log K'cond (cu,j = 11.5 J at low concentrations (- 1.8 nM) -which dominated copper complexation in the surface waters and decreased with depth, and a weaker class of ligands (log K'cond(Cu3 = 8.51 at higher concentrations (8-10 nM) which was observed throughout the water column and showed no apparent structure in its vertical distribution. These findings have significant implications concerning the toxicity and bioavailability of copper in open ocean systems.
Copper complexation measurements in estuarine organic-rich samples with differential pulse anodic stripping voltammetry (DPASV) and differential pulse cathodic stripping voltammetry (DPCSV) show a relatively large scatter in the data, due to a high organic matter content. Two methods for the estimation of the complexation parameters according to the Langmuir isotherm are compared: the linear transformation of Van den Berg/Ruzic and the non-linear method according to Wilkinson.
For DPASV measurements in estuarine samples the values of the conditional stability constant (K'), estimated with the linearization method are systematically lower than those estimated with the Wilkinson method. The Van den Berg/Ruzic linearization is more susceptible to outliers or data points that do not follow the model of the Langmuir isotherm. For DPCSV measurements, where the data followed the Langmuir isotherm more closely, this difference could not be demonstrated.
The non-linear method is to be preferred because this method is more suited to the error structure of the data, which is of constant absolute magnitude. Moreover it offers the advantage that the standard error of the estimated parameters can be calculated consistently.
Procedures and theory are presented to determine complexing capacities for nickel in seawater using cathodic stripping voltammetry of complexes with dimethylglyoxime (DMG). Values for [beta]NI(DMG)2 are determined for seawater of several salinities by ligand competition with EDTA. Preliminary measurements on samples from various locations reveal the presence of low concentrations (2-4 nM) of strong complexing ligands with values for the conditional stability constant, log K′NiL, between 17.3 and 18.7.
Procedures and theory are presented to determine complexing capacities for nickel in seawater using cathodic stripping voltammetry of complexes with dimethylglyoxime (DMG). Values for βNI(DMG)2 are determined for seawater of several salinities by ligand competition with EDTA. Preliminary measurements on samples from various locations reveal the presence of low concentrations (2-4 nM) of strong complexing ligands with values for the conditional stability constant, log K′NiL, between 17.3 and 18.7.
Environmental Context. The theoretical basis on which Town and van Leeuwen (Environ. Chem. 2005, 2, 80) dispute current ideas on the speciation of iron in seawater is valid only for the simplified condition of the binding of ionic Fe3+ with an ionic organic ligand. The possibilities of different pathways for complex formation and dissociation involving mixed hydroxide–organic species, or of different redox states, were not considered. The mismatch with experimental reality shows that the simplification is incorrect.
The theory is discussed which describes the distribution of copper ions between a weak ion exchanger, as exemplified by MnO2, and natural organic complexing material in seawater. Application of this theory and experimental procedures are outlined in part II of this series. It is apparent from the theory that titration with Cu2+ of one or more organic complexing ligands can be graphically represented by straight lines; slope and y-axis intercept provide information on the conditional stability constants and the ligand concentrations. Model calculations show that measurement of metal complexation at ligand concentrations higher than normally present in seawater may produce erroneous results because of possible changes in the metal to ligand ratio in the complexes. It is therefore advisable to measure metal complexation in the original, unaltered, water sample.
Humic substances (HS), the main hydrophobic component of dissolved organic matter, are present in all natural waters and are known to interact strongly with trace metals by complexation and co-precipitation. Traditionally, the role assigned to HS in iron cycling was as scavengers via flocculation in brackish waters. Their iron binding properties have recently been studied by competing ligand equilibration with detection by cathodic stripping voltammetry (CLE/CSV), the standard method to obtain complexing capacities and conditional stability constants. According to the stability and solubility of Fe–HS complexes obtained in seawater, HS could have a crucial role in iron cycling in deep oceanic and coastal seawaters. An attempt to study HS iron complexing characteristics in seawater with a variety of different artificial iron ligands (AL) revealed unexpected complications which have implications for previous complexation studies of fresh, brackish and coastal waters. For some AL, the sensitivity can be enhanced catalytically via addition of an oxidant, usually bromate, which was found to cause interference in the form of an overlapping peak due to Fe–HS complexes. Our data shows that iron complexation with HS is not detected by CSV in the presence of either NN (1-nitroso-2-naphthol), due to out-competition at the NN concentration required by the analytical method, or of TAC (2-(2-thiazolylazo)-4-methylphenol) probably due to interactions of the HS binding groups with TAC.
A new method, based on the direct detection of iron-humic substance (HS) species by cathodic stripping voltammetry (CSV), is used to determine the iron binding capacity and complex stability of fulvic acid (FA), humic acid (HA), and the natural HS in the seawater. The FA binds 16.7 ± 2.0 nmol iron (Fe) (mg FA)-1, whereas the HA and the marine HS bind 32 ± 2.2 nmol Fe (mg FA)21. The complex stabilities are (log K'Fe'HS values) 10.6 for FA and 11.1 for HA and coastal HS. Measurements of coastal waters (Irish Sea) show that the HS occur in a widespread fashion, the HS concentration decreasing with increasing salinity and occurring at levels of 400 (at salinity 30) to 70 (at salinity 34) μg L-1. A sample from the deep Pacific was found to contain 36 μg HS L-1, amounting to 4% of the dissolved organic matter. Comparative measurement of the total iron complexing capacity by CSV with competitive ligand exchange showed that the natural HS can account for the entire ligand concentration in the shallow coastal and deep ocean waters tested. Measurements of the iron solubility showed that FA added to seawater and the HS in coastal waters maintain iron in solution at a level just below the iron binding capacity. The preliminary data for the open ocean indicate that the same may be true for HS in deep ocean waters. The data are consistent with a mechanism by which iron is transported from land to sea, associated with land-derived HS. © 2009, by the American Society of Limnology and Oceanography, Inc.