Organic ligands in natural waters complex metal ions, which are thereby mobilized and transported between different reservoirs within marine systems. The ligands consist of a pool of uncharacterized macromolecular organic material derived from different sources (e.g., phytoplankton exudates, bacterial utilization/transformation of organic materials, humic and fulvic substances, anthropogenic organic contaminants etc). Studies focusing on the speciation of metal ions, and organic matter distributed among different pools of dissolved, colloidal and particulate phases, often provide valuable information on the nature and properties of organic substances and metals in marine systems. Metal associations with organic matter can be characterized as surface complexation reactions, whereby cations associate with negatively charged functional groups of the organic molecule (or "surface groups” on particulate matter). These likely include -COOH, -OH, and R-N-R or R-S-R groups distributed within the organic matrix. However, the direct sources of strong complexing ligands controlling metal speciation in seawater are still not sufficiently understood.

The biological availability of a trace metal behaving as either a required nutrient (e.g., Cu, Zn, Co) or a toxin (e.g., Hg, Pb, Cd) is dependent on its chemical form (speciation). It is generally assumed that free metal ions exhibit a higher toxicity to aquatic biota than metal ions bound to organic molecules forming “inert” organic complexes. Complexation by organic ligands generally decreases metal uptake, simultaneously enhancing their residence time in surface waters, and these ligands are seldom well-characterized, partly due to persistent analytical difficulties.

Interactions of metal ions and organic ligands are influenced by different environmental processes such as climate change, including elevated temperatures, different rainfall regimes and ocean acidification. The introduction of metal ions deriving from newly emerging 21st-century technologies also unbalance the system. The growing release of technology-critical elements (TCEs) (Ga, Ge, In, Nb, Ta, Te Tl), platinum group elements (PGEs) and rare earth elements (REEs) requires comprehension of their mobility, reactivity and chemical transformations in the environment, which are critically dependent on their chemical forms. Concentrations of these elements at ultra-trace levels make their speciation analysis challenging, while information on their environmental behavior and fate, particularly in seawater, is limited. Potential reactions of metals with the organics in seawater are mostly unexplored, whereas this could be of special interest for their targeted characterization.

Original research and review articles are invited in this Research Topic, demonstrating recent advances, approaches and findings on aspects of:

• Speciation / fractionation of metals (including TCEs, PGEs, REEs), metalloids and organic matter in open ocean, shelf seas and estuaries under changing climate and anthropogenic conditions.

• Characterization of organic ligands complexing trace metals.
• Impacts of metal complexation with organics on their bioavailability and toxicity to flora and fauna.
• Organic matter - metal interactions in biogeochemical cycling.
• Biotic and abiotic factors influencing metal speciation.
• Development and application of analytical methods and tools (including metal separation and recovery; sampling techniques).
• Modeling of metal speciation: thermodynamics, kinetics and uncertainty.

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Frontiers in Marine Science

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