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Article << Previous     |     Next >>   Contents Vol 4(2)

Organic complexation of copper in deep-sea hydrothermal vent systems

Sylvia G. Sander A D, Andrea Koschinsky B, Gary Massoth C, Matthew Stott C, Keith A. Hunter A

A Marine and Freshwater Chemistry, Department of Chemistry, University of Otago, PO Box 56, Dunedin, New Zealand.
B Geosciences and Astrophysics, School of Engineering and Science, Jacobs University Bremen, Campus Ring 8, Bremen D-28759, Germany.
C GNS Sciences, Wairakei Research Center, 114 Karetoto Road, Wairakei, New Zealand.
D Corresponding author. Email: sylvias@chemistry.otago.ac.nz
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Environmental context. Deep-sea hydrothermal vents represent a natural habitat for many extremophile organisms able to cope with extreme physical and chemical conditions, including high loads of heavy metals and reduced gases. To date, no information is available on the level and role of organic complexation of metals in these systems, which will have consequences on the bioavailability and precipitation or mineralisation of metals. In this work, we give evidence for the presence of organic molecules, including thiols, capable of forming complexes with copper strong enough to compete against sulfide present at high levels in hydrothermal systems.

Abstract. Here we report, for the first time, that strong organic complexation plays an important role in the chemical speciation of copper in hydrothermal vent systems including medium temperature outlets, diffuse vents with an adjacent hydrothermal biocommunity, and local mixing zone with seawater. Samples from three deep-sea hydrothermal vent areas show a wide concentration range of organic copper-binding ligands, up to 4000 nM, with very high conditional stability constants (log KCu′L = 12.48 to 13.46). Measurements were usually made using voltammetric methods after removal of sulfide species under ambient seawater conditions (pH 7.8), but binding still occurs at pH 4.5 and 2.1. The voltammetric behaviour of our hydrothermal samples is compared with that of glutathione (GSH) a known strong Cu-binding ligand, as a representative of an organic thiol. Our results provide compelling evidence for the presence of organic ligands, including thiols, which form complexes strong enough to play an important role in controlling the bioavailability and geochemical behaviour of metal ions around hydrothermal vents.

Keywords: copper complexation, electrochemistry (analysis), geochemistry (organic), hydrothermal vents, thiols.

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