The role of sulfides and thiols in the marine copper cycle

Copper (Cu) is an essential micronutrient that facilitates many significant biogeochemical processes in marine microbes and is also a well-recognised toxic agent for algal cells. Total Cu concentrations in open oceans are low (approximately 1-3 nM) while they can reach tens of nM in coastal systems. It is believed that only Cu in its free form, (hydrated Cu2+) is the bioavailable form with concentrations as low as 10pM of Cu2+ being a potent toxicant to marine microbes. Extensive complexation (>99.9%) of Cu with organic ligands maintain free Cu at low safe levels and with on-going ocean acidification, the concentration of Cu2+ is predicted to significantly increase. Understanding of the copper ligand pool is thus of prime importance.
Microbes are able to counteract the toxic effects of Cu2+ concentrations by producing thiols. Thiols are organic compounds that contain a sulfhydryl group (S-H) and are able to form highly stable complexes with metals, including copper. Thiols exist in a number of different forms such as glutathione (GSH), cysteine, thioglycolic acid and possibly many more that have not yet currently been identified in the marine system. They are produced by the biology in surface waters and may be one of the main complexing agent regulating the bioavailability of copper in surface waters. In addition to forming strong complexes with organic ligands, copper ions also complex with sulphide species, complexation that is most predominant in reducing environment such as hydrothermal systems .
In this project, we are using voltammetry as a highly sensitive analytical technique that allows detection in the sub nM levels of thiols and/or sulfide compounds as well as the determination of Cu speciation parameters (ligand concentrations and stability constants) obtained during titration experiments. A series of experiments will be carried out looking into the biogeochemical behavior of thiols, probing their stability and complexation capability towards Cu in presence/absence of lights and at various temperatures. Comparison of copper complexation parameters with thiols complexing abilities will assess the role of thiols in controlling copper organic speciation in well oxygenated surface waters. We are also interested in better understanding the parameters that might affect the complexation of copper with sulphide, e.g. temperature, in hydrothermal system and assess how this complexation might affect the transport of copper away from the vents. This information will further our understanding on the roles of thiols and sulphide in controlling the copper biogeochemical cycle.