Mn and Cu speciation in the Tropical Atlantic: influences of Saharan dust, phytoplankton exudation and ROS (H2O2 and O2)

• Applicant: Dr. Peter Leslie Croot
• Subject Area: Oceanography
• Term: from 2009 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 138443788

Copper (Cu) and Manganese (Mn) are two trace metals that show contrasting behaviours in their surface ocean biogeochemical cycles, but are still strongly linked through shared processes. While Cu is an extremely toxic element to marine phytoplankton, particularly pico-phytoplankton such as Prochlorcoccus and Synechococcus that dominate oligotrophic tropical regions, contrastingly Mn is a required element for all phytoplankton due to its role in photosystem II and in many redox enzymes. Mn2+ and Cu2+ share the same metal uptake system in most phytoplankton with the toxic Cu2+ strongly favoured over Mn2+ by non-specific transporters. Cu and Mn differ greatly in their speciation in open ocean seawater with Mn present mostly as the Mn2+ aquo ion, while Cu is dominated by strong organic Cu complexing ligands that are probably produced by phytoplankton in response to Cu stress. The two metals also differ in their redox cycling in the surface ocean as particulate MnO2 can be photoreduced to produce soluble Mn2+ which is slowly oxidized back to Mn(IV) via Mn(III). While Cu2+ complexes may be reduced directly by photochemistry or via reaction with photoproduced H2O2 to Cu+, which is rapidly oxidized back to Cu2+ with possibly a small fraction stabilised as Cu+ via complexation with thiol ligands present in seawater. In this proposal, through a combination of field work and laboratory studies, we will investigate the speciation and redox cycling of Cu and Mn in the oligotrophic surface tropical ocean near the Cape Verde islands which are heavily impacted by Saharan dust. This study seeks to elucidate the role of photoredox cycling on the distribution and biogeochemical cycling of Cu and Mn in the surface tropical ocean.

• DFG Programme: Research Grants