The relationship between cerium and manganese oxidation in the marine environment

Abstract

The microbially mediated oxidation of Ce(II1) and Mn(I1) in surface waters of Vineyard Sound, Massachusetts, has been studied to evaluate the relationship between these two processes under different experimental conditions. These data, combined with earlier observations showing that Ce(II1) and Mn(I1) specific oxidation rates covary over a wide range of environments, suggest a close mechanistic relationship between the two processes which contributes to the marked similarities in the geochemistry of each element. Oxidation of Ce(II1) and Mn(I1) at different oxygen concentrations, pH, and concentrations of Mn(II), Ce(III), and Pr(II1) was studied with radiotracers. For both elements, oxidation was inhibited in the absence of oxygen. Ce(II1) oxidation was inhibited by Mn(I1) and vice versa, probably due to competitive inhibition within a common oxidative pathway. Pr(III), a nonredox analog of Ce(III), did not inhibit Mn(I1) oxidation nearly as effectively as Ce(II1) did, indicating that inhibition was not exclusively due to competition at a common binding site but that a redox step must also be involved. Both reactions exhibited a modest dependence on pH which was much smaller than would be expected for nonbiological oxidation. The results are consistent with a common oxidative pathway for Ce and Mn and indicate that an alternative process, nonbiological Ce oxidation on freshly formed Mn oxides, is unlikely. The results also illustrate the potential usefulness of Ce(II1) and Pr(II1) as probes of Mn uptake and redox transformations in biological systems.