Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments

Pyrite (FeS₂) and iron monosulfide (FeS) play a central role in the sulfur and iron cycles of marine sediments. They may be buried in the sediment or oxidized by O₂ after transport by bioturbation to the sediment surface. FeS₂ and FeS may also be oxidized within the anoxic sediment in which NO₃⁻, Fe(III) oxides, or MnO₂ are available as potential electron acceptors. In chemical experiments, FeS₂ and FeS were oxidized by MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide (Schippers and Jørgensen, 2001). Here we also show that in experiments with anoxic sediment slurries, a dissolution of tracer-marked ⁵⁵FeS₂ occurred with MnO₂ but not with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor. To study a thermodynamically possible anaerobic microbial FeS₂ and FeS oxidation with NO₃⁻ or amorphous Fe(III) oxide as electron acceptor, more than 300 assays were inoculated with material from several marine sediments and incubated at different temperatures for > 1 yr. Bacteria could not be enriched with FeS₂ as substrate or with FeS and amorphous Fe(III) oxide. With FeS and NO₃⁻, 14 enrichments were obtained. One of these enrichments was further cultivated anaerobically with Fe²⁺ and S⁰ as substrates and NO₃⁻ as electron acceptor, in the presence of ⁵⁵FeS₂, to test for co-oxidation of FeS₂, but an anaerobic microbial dissolution of ⁵⁵FeS₂ could not been detected. FeS₂ and FeS were not oxidized by amorphous Fe(III) oxide in the presence of Fe-complexing organic compounds in a carbonate-buffered solution at pH 8. Despite many different experiments, an anaerobic microbial dissolution of FeS₂ could not be detected; thus, we conclude that this process does not have a significant role in marine sediments. FeS can be oxidized microbially with NO₃⁻ as electron acceptor. O₂ and MnO₂, but not NO₃⁻ or amorphous Fe(III) oxide, are chemical oxidants for both FeS₂ and FeS.