Ligands In Seawater Research Articles

Trace metal dissolution kinetics of East Asian size-fractionated aerosols in seawater: The effect of a model siderophore

Aerosol soluble metals are considered bioavailable to marine phytoplankton and may influence phytoplankton growth, their community structure, and material cycling in the ocean. The dissolution is controlled by the physical and chemical properties of aerosols and various atmospheric, physical, and biogeochemical processes before and after depositing in the surface water. Among these complicated processes, the interaction of aerosol metals with organic ligands in seawater is one of the major factors. In this study, we systematically investigated the dissolution kinetics of trace metals from fine (0.45–0.95 μm) and coarse (>7.2 μm) aerosols in seawater for 30 days under conditions with or without adding a model Fe organic ligand, siderophore desferrioxamine B (DFB). We found that most of the soluble metals in the fine aerosols were leached rapidly within the first hour. In terms of the coarse aerosols, with DFB addition, the fractional solubility of Fe increased substantially from 0.1 to 10%, and the solubilities of most other metals generally increased by about 2-fold within 30 days. Without DFB addition, we found that most soluble metals were still gradually released over time in coarse aerosols, except particle-reactive metals, Fe and Pb. We further observed strong linear correlations between dissolved metals (Al, Fe, Mn, Co, Ni, and Zn) and silicate concentrations for DFB addition with coarse aerosols, with metal to silicate ratios comparable to those of typical aluminosilicates. Quantitatively, the aluminosilicate-associated dissolved metals accounted for >85% of total dissolved Fe, Al, and Co, and 40–60% for total dissolved Mn and Ni with the addition of DFB for 30 days. By comparing dissolved metal to Al ratios with lithogenic ratios, the metals leached in the first hour for both coarse and fine aerosols originated from anthropogenic sources, and lithogenic aerosols gradually became the dominant soluble metal source for coarse aerosols after 1 h, extending to 720 h. The findings of this study exhibit the importance of the time-dependent interaction of lithogenic aerosols with organic ligands in seawater. The residence time of aerosol particles and the concentrations of available organic ligands in the euphotic zone are key factors affecting aerosol dissolved metal availability to marine phytoplankton in the surface ocean.

Slow Kinetics of Iron Binding to Marine Ligands in Seawater Measured by Isotope Exchange Liquid Chromatography-Inductively Coupled Plasma Mass Spectrometry.

Current understanding of dissolved iron (Fe) speciation in the ocean is based on two fundamentally different approaches: electrochemical methods that measure bulk properties of a heterogeneous ligand pool and liquid chromatography mass spectrometry methods that characterize ligands at a molecular level. Here, we describe a method for simultaneously determining Fe-ligand dissociation rate constants (kd) of suites of naturally occurring ligands in seawater by monitoring the exchange of ligand-bound 56Fe with 57Fe using liquid chromatography-inductively coupled mass spectrometry. Values of kd were determined for solutions of ferrichrome and ferrioxamine E. In seawater, the dissociation rate constant of ferrichrome (kd = 10 × 10-8 s-1) was greater than that of ferrioxamine E (kd = 3.6 × 10-8 s-1). The rates for both compounds were over twice as fast in seawater compared with pure water, suggesting that seawater salts accelerate dissociation. Isotope exchange experiments on organic extracts of natural seawater indicated that ligand-binding sites associated with chromatographically unresolved dissolved organic matter exchanged Fe more quickly (kd = 1.8 × 10-5 s-1) than amphibactin siderophores (kd = 2.15 × 10-6 s-1) and an unidentified siderophore with m/z 709 (kd = 9.6 × 10-6 s-1). These findings demonstrate that our approach can bridge molecular-level ligand identification with kinetic and thermodynamic metal-binding properties.

Comparing CLE-AdCSV applications using SA and TAC to determine the Fe-binding characteristics of model ligands in seawater

Abstract. 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.

Wide-range detection of Cu-binding organic ligands in seawater using reverse titration

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.

Fast iron speciation in seawater by catalytic Competitive Ligand Equilibration-Cathodic Stripping Voltammetry with tenfold sample size reduction

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.

Reactive iron isotope signatures of the East Asian dust particles: Implications for iron cycling in the deep North Pacific

Iron released from wind-blown dust has been widely suggested to be an important nutrient source for oceanic primary productivity. Once introduced into the seawater, the reactive Fe might get dissolved and impart its isotope signature into the water column. Characterization of the reactive iron isotope signatures of eolian dust will thus help to understand the budget of Fe in the global ocean. Yet, there are few systematic studies on the Fe isotope systematics of the dust sources across large spatial scale. Here, we report Fe isotope data from HCl leaching experiment on surficial samples from deserts and loess deposits of different locations spanning >3000 km spatially in east Asia, one of the most important dust source regions. Our data suggest most of the samples have relatively similar leaching signatures (δ56Fe = –0.09±0.07‰, n=9, 2 SD). To understand the kinetics of Fe release during proton promoted leaching of natural particles, a synthetic model taking into account of different dissolution dynamics of easily dissolvable Fe and silicate Fe was developed. The model was tested by continuous leaching of two representative pelagic sediment samples recovered from core LL44-GPC3 in the North Pacific. When introducing only one tunable parameter (i.e., the initial dissolution rate of Fe from hydro(oxides), with other parameters constrained by the dissolution kinetics), the model successfully reproduces leachate δ56Fe evolution and agrees reasonably well with Fe speciation in the studied marine sediment samples. We thus suggest that the leaching process can be well characterized by our kinetic model, while Fe isotopes in the initial leachates of the samples are dominated by endmember composition of the easily dissolvable Fe species. The small but distinct fractionation of Fe isotopes in the desert/loess samples from the parental upper continental silicate materials are suggested to record signatures of the secondary Fe species produced during weathering in the oxic environment. The significantly lighter Fe isotope compositions in the initial leachates of the North Pacific pelagic sediment samples compared to the east Asian desert/loess deposits indicate apparent isotope exchange of the dust reactive components with seawater dissolved species. The sedimentary light reactive Fe isotope signatures in turn point toward the contribution of sources characterized by low δ56Fe to the deep North Pacific and/or preferential uptake of 56Fe by seawater ligands from the dust reactive components particularly at the seawater-sediment interface.

Iron complexation by phenolic ligands in seawater

Iron is an essential micronutrient for phytoplankton and can limit primary production in the ocean. Fe chemistry is highly controlled by its interaction with organic complexes (>99%). It is still unknown which organic compounds produced by cells have the ability to bind Fe. Within the pool of organic ligands, polyphenols are known to be exudated by marine diatoms and, in this study, the role of three polyphenols ((±) – catechin, sinapic acid and gallic acid) was studied in terms of dissolved Fe complexation via kinetic and titration approaches, and also their role as a source of Fe(II) in seawater. The results demonstrated that these three polyphenols are weak L2-type Fe-binding ligands according to the conditional stability constant, computed by using the kinetic approach (log K′Fe′L = 8.86–9.2), where the formation rate constant (kf) was 3.1·105–4.2·105 M−1 s−1 and the dissociation rate constant (kd) was 2.43·10−4–4.4·10−4 s−1. The conditional stability was also computed from the titration approach with log K′Fe′L from 8.6 to 9.5. These studied ligands also regenerated Fe(II) in seawater from 0.05% to 11.92%. The results obtained in this study suggest that polyphenols increase the persistence of dissolved Fe and should be considered as an important Fe-binding ligands in seawater to better understand the global biogeochemical cycles.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Exploring the Potential Role of Terrestrially Derived Humic Substances in the Marine Biogeochemistry of Iron

There is a widely recognised link between iron bioavailability and phytoplankton growth in large tracts of the World Ocean. Iron bioavailability is thus pivotal to the removal of atmospheric carbon via the ocean’s ‘biological pump’. To evaluate future scenarios of global climate change, we must therefore understand better how the bioavailability of iron in seawater is linked to processes controlling its supply, chemical speciation and removal from the water column. Much of the research on iron inputs to the open ocean has focused on atmospheric, benthic (shelf sediments) and hydrothermal (mid-ocean) sources of iron. The conventional wisdom has been that riverine iron sources are negligible because many of the major world rivers exhibit extensive removal of dissolved iron by flocculation processes during estuarine mixing. However, recent studies have revealed that a fraction of iron associated with peatland-derived humic and fulvic acids may survive aggregation processes in the freshwater-seawater mixing zone, and thus be exported offshore. This review is a synthesis of available data and information for and against the hypothesis that land-derived humic substances exert a significant control on the marine biogeochemical cycle of iron. From the outset, it is shown that this hypothesis can neither be verified nor disproved at present, in part due to analytical difficulties in characterising the all-important marine colloidal phase. Evidence is then presented on the likely chemical nature and structure of iron-binding humic ligands along with implications for the lateral transport of iron in surface waters and its participation in carbon stabilisation in marine sediments. This is followed by a discussion of photochemically and microbially mediated processes acting on terrestrial humic substances and a discussion of how terrestrial humic substances may influence the biological availability of iron. The review finishes with a presentation of measurement technologies and approaches that could be used to assess (i) how iron-binding ligands in seawater relate to land-derived humic substances and (ii) how terrestrial humics may influence the global carbon cycle indirectly by influencing the processes that control the supply and maintain the pool of ‘dissolved’ iron in the ocean.

Evaluation of Immobilized Metal-Ion Affinity Chromatography and Electrospray Ionization Tandem Mass Spectrometry for Recovery and Identification of Copper(II)-Binding Ligands in Seawater Using the Model Ligand 8-Hydroxyquinoline

Complexation by organic ligands dominates the speciation of iron (Fe), copper (Cu), and other bioactive trace metals in seawater, controlling their bioavailability and distribution in the marine environment. Several classes of high-affinity Fe-binding ligands (siderophores) have been identified in seawater but the chemical structures of marine Cu-complexing ligands remain unknown. Immobilized metal-ion affinity chromatography (IMAC) allows Cu ligands to be isolated from bulk dissolved organic matter (DOM) in seawater and separated into fractions which can be characterized independently using electrochemical and spectroscopic techniques. Attempts have been made to combine IMAC with electrospray ionization mass spectrometry (ESI-MS) to characterize marine Cu ligands, but results have proven inconclusive due to the lack of tandem mass spectrometry (MS/MS) data to confirm ligand recovery. We used 8-hydroxyquinoline (8-HQ), a well-characterized model ligand that forms strong 1:2 metal:ligand complexes with Cu2+ at pH 8 (log β2 = 18.3), to evaluate Cu(II)-IMAC and ESI-MS/MS for recovery and identification of copper(II)-complexing ligands in seawater. One-litre samples of 0.45µm-filtered surface seawater were spiked with 8-HQ at low concentrations (up to 100 nM) and fractionated by IMAC. Fractions eluted with acidified artificial seawater were desalted and re-suspended in methanol via solid-phase extraction (SPE) to obtain extracts suitable for ESI-MS analysis. Recovery of 8-HQ by Cu(II)-IMAC was confirmed unambiguously by MS/MS and found to average 81% based upon accurate quantitation via multiple reaction monitoring (MRM). Cu(II)-IMAC fractionation of unspiked seawater using multiple UV detection wavelengths suggests an optimal fraction size of 2 mL for isolating and analyzing Cu ligands with similar properties.

The Ferrojan Horse Hypothesis: Iron-Virus Interactions in the Ocean

Iron is an essential nutrient and the sub-nanomolar concentrations of iron in open ocean surface waters are often insufficient to support optimal biological activity. More than 99.9% of dissolved iron in these waters is bound to organic ligands, yet determining the identity of these ligands in seawater remains a major challenge. Among the potential dissolved organic ligands in the colloidal fraction captured between a 0.02 µm and a 0.2 µm filter persists an extremely abundant biological candidate: viruses, most of which are phages (viruses that infect bacteria). Recent work in non-marine model systems has revealed the presence of iron ions within the tails of diverse phages infecting Escherichia coli. Based on these findings and the presence of conserved protein motifs in marine phages, here we present several lines of evidence to support the hypothesis that phages are organic iron-binding ligands in the oceans. With average concentrations of 10^7 phages per milliliter surface seawater, we predict that phages could contain up to 0.7 pM iron, a value equivalent to as much as 70% of the colloidal fraction of organically complexed dissolved iron in the surface ocean. Additionally, the production and uptake of siderophores, a strategy that bacteria have developed for assimilating iron, renders cells vulnerable to phage infection due to the dual-function of these cell surface receptors. Iron ions present in phage tails enable phages to exploit their bacterial host’s iron-uptake mechanism via the “Ferrojan Horse Hypothesis” proposed herein, where the apparent gift of iron leads to cell lysis. Finally, if host iron stores are recycled during the assembly of progeny phages, as much as 14% of the cellular iron released into the water column upon lysis would already be incorporated into new phage tails. The potential role of phages as iron-binding ligands has significant implications for both oceanic trace metal biogeochemistry and marine phage-host interactions.

Bioavailability and Electroreactivity of Zinc Complexed to Strong and Weak Organic Ligands.

Laboratory experiments have established the importance of complexation by organic ligands in determining the bioavailability of trace metals to marine phytoplankton, while electrochemical measurements with field samples have demonstrated that a large fraction of bioactive trace metals are complexed to strong organic ligands in seawater. Using the model organic ligands, EDTA and histidine, we show a quantitative correspondence between the bioavailability of Zn to the diatom Thalassiosira weissflogii, and its reduction at -1.2 V (vs Ag/AgCl) on a hanging mercury drop electrode. Equilibrium calculations and polarographic data indicate that Zn bound in inorganic complexes and the 1:1 Zn-histidine complex, but not in the 1:2 Zn-histidine complex or the Zn-EDTA complexes, is taken up by the organism and reduced at the electrode surface, confirming a previous report of the bioavailability of weak Zn complexes. Electrochemical measurements of Zn speciation in seawater do not generally reveal the presence of weak (and potentially bioavailable) complexes; but such measurements (particularly by Anodic Stripping Voltammetry) should nonetheless often provide good estimates of the bioavailable Zn concentrations. These results can likely be generalized to other bioactive divalent trace metals.

Development of an improved ligand mimetic calibration system for the analysis of iron(III) in seawater using the iron(III) chalcogenide glass ion selective electrode: A combined mechanistic and analytical study

Further to previous work on a seawater ligand mimetic calibration system for the electroanalysis of iron(III) in seawater using the iron chalcogenide glass ion-selective electrode (ISE), this study utilized alternative blends of synthetic ligands, in order to fine-tune and optimize the calibration response characteristics of this sensor for the electroanalysis of free iron(III) in seawater. Herein, an alternative calibration system (ACS) was derived using a mixture of 10−4M iron(III) chloride, 10−4M ethylenediaminetetraacetic acid (EDTA), 10−3M copper(II) sulphate, 5×10−3M ethylenediamine (EN) and 0.60M sodium chloride yielding a Nernstian response of about 30mV decade−1. The electroanalysis of free iron(III) in seawater using the ACS generated a free iron(III) level commensurate with the predicted organic and inorganic speciation of iron(III) in seawater. Furthermore, electrochemical impedance spectroscopy (EIS), synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy have demonstrated that the surface reaction processes of this membrane in the ACS are comparable to those experienced in a natural seawater matrix. Ultimately, this study demonstrated that the ACS can mimic the surface chemistry and concomitant potentiometric response characteristics of the iron(III) sensor in seawater, enabling reliable electroanalyses of free iron(III) in seawater.

Photochemistry of marine and fresh waters: A role for copper–dissolved organic matter ligands

The photodegradation of marine dissolved organic matter (DOM) plays a critical role in the global carbon cycle. The photodegradation of both DOM and particulate organic matter (POM) leads to the formation of carbon monoxide and/or the production of labile organic matter that can be rapidly biodegraded. DOM also regulates metal speciation, particularly important for the bioavailability of nutrient metals required by phytoplankton. Copper is one of these essential nutrients but is also toxic in elevated concentrations. Its speciation is regulated by DOM photodegradation however, DOM–copper ligands are not well characterized because of their low concentrations in natural waters. Copper immobilized metal affinity chromatography (IMAC) was used to isolate high and low affinity DOM ligands from both Pacific near-shore seawater and freshwater from the Black River (NC). Laser flash photolysis (LFP) was used to characterize excited state species from these fractions and to identify excited state species in the open ocean through a depth profile from 5 to 4532m (North Atlantic, Bermuda Atlantic Time Series Station (BATS)). LFP generated transients characteristic of the solvated electron (τ=3.5±0.1μs), a microbial-sourced triplet excited state (τ=1.1μs±0.1μs), and a long-lived transient, DOM*, with a millisecond lifetime. Unlike the seawater triplet excited state, a copper ligand from Black River water generated a triplet excited state with a lifetime of 3.4±0.1μs. Fractions were also characterized by excitation–emission matrix fluorescence spectroscopy (EEMs), UV–visible, and 1H NMR spectroscopy. EEMs spectra correlated the marine excited state species with the protein-like, microbial fraction while the freshwater transients were associated with chromophoric DOM (CDOM). 1H NMR with spectral database matching identified ninety-seven compounds as potential sources of these excited state species in the Pacific seawater IMAC fractions. Aqueous extracts of marine macro-algae (Sargassum natans (Atlantic Ocean), Macrocystis pyrifera), and surf grass (Phyllospadix torreyi) were analyzed to investigate potential additional sources of these excited state transients. To our knowledge, this is the first study to characterize DOM–copper ligands in seawater using a combination of EEMs, NMR and laser flash photolysis.

Adsorption behavior of cadmium ion onto synthetic ferrihydrite: effects of pH and natural seawater ligands

Batch experiments were conducted to study the adsorption behavior of cadmium ion onto the synthetic ferrihydrite. The adsorbent was characterized using X-ray diffraction and N2 adsorption–desorption measurements. The effects of pH value and the major seawater anions (nitrate, carbonate, chloride and sulphate) at their natural concentrations on the adsorption of cadmium ion were investigated. The results showed that the adsorption of cadmium ion was highly pH-dependent and suppressed in the presence of chloride, sulphate and nitrate, while carbonate was found to enhance the adsorption of cadmium ion over the examined range of 4.0–7.5. The maximum percentage of cadmium ion adsorbed can reach 89.96 % at pH 7.5. It is proposed that the competition for binding sites played a key role in the reduction of cadmium ion adsorption by nitrate, chloride and sulphate. X-ray diffraction spectroscopic data for cadmium adsorbed on ferrihydrite showed that CdCO3(s) was formed on ferrihydrite; the formation of CdCO3 precipitation or surface precipitation in carbonate system may be the reason for the increase of cadmium adsorption.

A critical look at the calculation of the binding characteristics and concentration of iron complexing ligands in seawater with suggested improvements

Environmental context The low concentration of iron in the oceans limits growth of phytoplankton. Dissolved organic molecules, called ligands, naturally present in seawater, bind iron thereby increasing its solubility and, consequently, its availability for biological uptake by phytoplankton. The characteristics of these ligands are determined indirectly with various mathematical solutions; we critically evaluate the underlying method and calculations used in these determinations. Abstract The determination of the thermodynamic characteristics of organic Fe binding ligands, total ligand concentration ([Lt]) and conditional binding constant (K′), by means of titration of natural ligands with Fe in the presence of an added known competing ligand, is an indirect method. The analysis of the titration data including the determination of the sensitivity (S) and underlying model of ligand exchange is discussed and subjected to a critical evaluation of its underlying assumptions. Large datasets collected during the International Polar Year, were used to quantify the error propagation along the determination procedure. A new and easy to handle non-linear model written in R to calculate the ligand characteristics is used. The quality of the results strongly depends on the amount of titration points or Fe additions in a titration. At least four titration points per distinguished ligand group, together with a minimum of four titration points where the ligands are saturated, are necessary to obtain statistically reliable estimates of S, K′ and [Lt]. As a result estimating the individual concentration of two ligands, although perhaps present, might not always be justified.

Trace Metal Interactions with Marine Phytoplankton

AbstractLarge spatial and temporal differences in both trace metal concentrations and chemical speciation in the sea have led to wide variations in biological availability of metals and their effects on phytoplankton. Trace metals are usually taken up by algae via the formation of coordination complexes with specialized transport ligands in their outer membranes, and metal uptake is determined by the interplay between redox, complexation, or oxide dissolution reactions of metals in seawater and ligand-exchange reactions at these sites. Some metals, such as copper and zinc, are heavily chelated by organic ligands in seawater, and their biological availability is determined by the concentrations of free metal ions or of kinetically labile inorganic species (free ions plus inorganic complexes). Once inside cells, trace metals influence metabolism primarily as a consequence of the role of many of these metals (Fe, Mn, Zn, Cu, Co, Mo, Ni) as essential cofactors in metalloenzymes. Trace metals may also inhibit ...

Voltammetric characterisation of macroalgae-exuded organic ligands (L) in response to Cu and Zn: a source and stimuli for L

Environmental context Identifying the source and stimuli responsible for organic ligands in seawater is crucial to understanding trace metal availability. Voltammetric techniques were employed to characterise the water chemistry of seaweed cultures exposed to low levels of Cu or Zn over 7 days. The results suggest that seaweeds are a potential source of metal complexing ligands and Cu and Zn appear to stimulate ligand production; further research is required to determine if this is applicable to macroalgae and metals outside this study. Abstract It is widely accepted that organic ligands control metal speciation in seawater, although little is known about their source, stimuli or identity. To gain insight on the possible environmental controls of metal complexing ligands (L), three brown macroalgae common in Irish waters (Ascophyllum nodosum, Fucus vesiculosus and Laminaria hyperborea) were cultured under low levels of Cu (0, 7.86 or 15.7nM) or Zn (0, 15.2 or 91.7nM) exposure. Seaweed chlorophyll-a fluorescence (Fv/Fm), metal speciation (Cu, Zn), complexing ligands (LM), conditional stability constants (logK′LM), glutathione (GSH), cysteine (Cys) and seaweed metal contents were monitored over 7 days. Although there was no effect on the internal seaweed metal concentrations, Cu and Zn additions significantly altered the water chemistry of each culture. Metal additions increased the total dissolved metal concentrations for all three species. Significantly higher [LM] values in cultures with added metals than the relevant controls point to both metals as stimuli of L production. All species released ligands in response to Cu or Zn exposure, indicating each seaweed is a relevant source of L. Comparison of logK′ values to those of previously determined ligands provides little evidence that the ligands reported here belong to the compounds identified as L in the literature.

Automated on-line flow-injection ICP-MS determination of trace metals (Mn, Fe, Co, Ni, Cu and Zn) in open ocean seawater: Application to the GEOTRACES program

Large programs in chemical oceanography currently require the determination of multiple trace elements in hundreds to thousands of open ocean seawater samples. For example, the international GEOTRACES program would benefit greatly from rapid, precise, automated, multi-element analytical methods for multiple trace metals in seawater. An automated, on-line extraction, flow-injection ICP-MS method is presented here for simultaneous determination of Mn, Fe, Co, Ni, Cu and Zn in open ocean seawater samples. A fully automated commercially available system buffers the pH of the samples on-line and extracts the metals of interest from 9mL of seawater onto a chelation resin column, which is then eluted (1.6M HNO3) directly into a magnetic sector ICP-MS. The resulting transient peaks elute in 45μL (~30s) yielding a preconcentration factor of ~200, and the total analysis cycle requires 8.75min per sample. Elemental concentrations are calculated from integrated peak areas using a combination of isotope dilution and matrix-matched standard curves. Procedural blanks determined at periodic intervals (every 10 samples) throughout each sample run are used for blank correction and for calculating detection limits (range 0.3 to 16pmolkg−1 seawater for these elements). Replicate analyses of a Ross Sea in-house consistency standard and four GEOTRACES reference seawater materials (Atlantic and Pacific, surface and deep water) are used to demonstrate precision of 1–3% (RSD) and very good agreement with reference consensus values for all elements except Co. Determined Co concentrations are ~40% lower than consensus values, consistent with the presence of strong Co ligands in seawater, and the absence of a UV-oxidation step in this method. As illustrated by oceanographically consistent profiles at the GEOTRACES Atlantic and Pacific baseline stations, this method provides highly precise measurement of trace metals over the full range of concentrations occurring in the global ocean. Moreover, the full methodology, including flow-injection hardware and control software, is readily transferable to other users.

Humic substances may control dissolved iron distributions in the global ocean: Implications from numerical simulations

This study used an ocean general circulation model to simulate the marine iron cycle in an investigation of how simulated distributions of weak iron‐binding ligands would be expected to control dissolved iron concentrations in the ocean, with a particular focus on deep ocean waters. The distribution of apparent oxygen utilization was used as a proxy for humic substances that have recently been hypothesized to account for the bulk of weak iron‐binding ligands in seawater. Compared to simulations using a conventional approach with homogeneous ligand distributions, the simulations that incorporated spatially variable ligand concentrations exhibited substantial improvement in the simulation of global dissolved iron distributions as revealed by comparisons with available field data. The improved skill of the simulations resulted largely because the spatially variable ligand distributions led to a more reasonable basin‐scale variation of the residence time of iron when present at high concentrations. The model results, in conjunction with evidence from recent field studies, suggest that humic substances play an important role in the iron cycle in the ocean.

Detection of Iron Ligands in Seawater and Marine Cyanobacteria Cultures by High-Performance Liquid Chromatography–Inductively Coupled Plasma-Mass Spectrometry

Organic ligands dominate the speciation of iron in the ocean. Little is known, however, about the chemical composition and distribution of these compounds. Here, we describe a method to detect low concentrations of organic Fe ligands using reverse-phase high-performance liquid chromatography (HPLC) tandem multicollector inductively coupled plasma mass spectrometry. This technique can be used to screen seawater and marine cultures for target compounds that can be isolated and structurally characterized. Sensitive detection (<1 picomole Fe) is achieved using an iron-free HPLC system to reduce background Fe levels, minimizing (40)Ar(16)O(+) interferences on (56)Fe with a hexapole collision cell, and introducing oxygen into the sample carrier gas to prevent the formation of reduced carbon deposits that decrease sensitivity. This method was tested with a chromatographic separation of five trace metal complexes that represent the polarity range likely found in seawater. Good separation was achieved with a 20 min water/methanol gradient, although sensitivity decreased by a factor of 2 at high organic solvent concentrations. Finally, Fe ligand complexes were detected from the organic extract of surface South Pacific seawater and from culture media of the siderophore producing cyanobacteria Synechococcus sp. PCC 7002.