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The GEOTRACES program has greatly expanded measurements of dissolved trace metal concentrations across ocean basins, but to understand the behavior and cycling of metals and their impacts on primary productivity, we must understand the chemical forms in which they are present in the environment. Organic ligands play a central role in the speciation and cycling of trace metals in the marine environment, controlling their chemical reactivity and bioavailability. Here, we present an overview of the contributions the GEOTRACES program has made to understanding ocean metal speciation through advancing our knowledge of the distribution, sources, and sinks of metal-binding organic ligands across the global ocean, particularly for iron. Detailed assessments and intercalibration of the speciation methods most commonly applied have allowed integration of metal-binding ligand measurements across datasets. Work to characterize specific ligand groups within the wider pool of dissolved organic matter, along with their sources and sinks, is starting to unravel the role of metal-binding organic ligands in global biogeochemical cycles. Recent advances in complementary analytical techniques using liquid chromatography and mass spectrometry present a molecular picture of metal speciation and bioavailability—and also pose new questions. Moving forward, we need to address knowledge gaps in our understanding of how metal speciation and complexation relates to bioavailability in order to recognize the impacts of ocean metal distributions and cycling on marine productivity and the global carbon cycle. 

The method of competitive ligand exchange followed by adsorptive cathodic stripping voltammetry (CLEAdCSV) allows for the determination of dissolved iron (DFe) organic speciation parameters, i.e., ligand concentration (LFe) and conditional stability constant (log Kcond Fe′L ). Investigation of DFe organic speciation by CLEAdCSV has been conducted in a wide range of marine systems, but aspects of its application pose challenges that have yet to be explicitly addressed. Here, we present a set of observations and recommendations to work toward establishing best practice for DFe organic speciation measurements using the added ligand salicylaldoxime (SA). We detail conditioning procedures to ensure a stable AdCSV signal and discuss the processes at play during conditioning. We also present step-by-step guidelines to simplify CLE-AdCSV data treatment and interpretation using the softwares ECDSoft and ProMCC and a custom spreadsheet. We validate our application and interpretation methodology with the model siderophore deferoxamine B (DFO-B) in a natural seawater sample. The reproducibility of our application and interpretation methodology was evaluated by running duplicate titrations on 19 samples, many of which had been refrozen prior to the duplicate analysis. Nevertheless, 50% of the duplicate analyses agreed within 10% of their relative standard deviation (RSD), and up to 80% within 25% RSD, for both LFe and log Kcond Fe′L . Finally, we compared the sequential addition and equilibration of DFe and SA with overnight equilibration after simultaneous addition of DFe and SA on 24 samples. We found a rather good agreement between both procedures, with 60% of samples within 25% RSD for LFe (and 43% of samples for log KcondFe′L ), and it was not possible to predict differences in LFe or log KcondFe′L based on the method applied, suggesting specific association/dissociation kinetics for different ligand assemblages. Further investigation of the equilibration kinetics against SA may be helpful as a potential way to distinguish natural ligand assemblages.