Organic Cu-complexation at the shallow marine hydrothermal vent fields off the coast of Milos (Greece), Dominica (Lesser Antilles) and the Bay of Plenty (New Zealand)

Abstract

Hydrothermal vents at the seafloor release large volumes of metal-rich fluids into the ocean. Some of these metals are biologically essential (such as Fe), while others may be toxic (e.g. Cu). Organisms living at these habitats produce small organic molecules, ligands that are able to form complexes with different metals, to either enhance their bioavailability or to decrease their toxicity. While some deep-sea vents have been studied with respect to metal complexation, comparable studies at shallow marine hydrothermal vent systems are rather rare. In this study, total dissolved Cu concentrations ([Cu]T) and corresponding Cu binding ligand concentrations ([L]Cu) at the shallow water hydrothermal vent fields off the coast of Milos (Greece), Dominica (Lesser Antilles) and the Bay of Plenty (New Zealand) were examined by a voltammetric ligand titration, using competitive ligand equilibration–adsorptive stripping voltammetry (CLE–AdCSV).Milos, Dominica and the Bay of Plenty are three very different environments and our data show that they are relatively depleted in total dissolved Cu, compared to deep-sea hydrothermal vents where [Cu]T and [L]Cu concentrations up to 400nM and 4000nM have been found, respectively. In this data set, [Cu]T rises up to 21.53nM, however, most samples are in the range of 0.5nM–3nM. [L]Cu varies between 2.07nM and 23.9nM, with one exception, reaching 56.8nM. Conditional stability constants (Log K) range between 11.57 and 14.01.Assuming that the three investigated sites are representative for shallow marine hydrothermal vents, the Cu-flux into the ocean due to complexation appears to be relatively low compared to deep-sea systems. Our data indicate a stable complexation of Cu in most cases, which may lower the toxic effect of Cu on these mixed photic–chemosynthetic communities. However one pore water sample from Dominica showed an excess [Cu]T over [L]Cu, which would create a highly toxic environment for most marine organisms. We further show that ligand–Cu ratios and complex stability constants are in a similar range as the ones at deep-sea vents, which may indicate similar complexation parameters and processes at shallow and deep-sea hydrothermal vents.