Co-reduction of Fe(III) and S0 drives Fe-S biomineral formation and phosphate mobilisation

Abstract

Microbially mediated iron and sulfur cycling have impacted redox transitions and the bioavailability of nutrients throughout Earth's history. Here, we incubated Geobacter sulfurreducens in the co-presence of ferrihydrite and S 0 at pH 6.5, 7.2 or 8.0. Microbial reduction of Fe(III) and S 0 resulted in a shift from ferruginous (Fe 2þ -rich) to sulfidic (sulfide-rich) conditions and the precipitation of mackinawite, greigite and vivianite. The initial pH controlled the timing of the ferruginous-sulfidic transition and the relative abundance and crystallinity of the formed minerals. Vivianite formation was attributed to phosphate initially added to the medium. Phosphate showed a dynamic cycle, with low dissolved concentrations initially due to sorption to ferrihydrite, followed by vivianite precipitation under ferruginous conditions, and a significant release under sulfidic conditions. Co-reduction of Fe(III) and S 0 therefore regulates Fe-S biomineral formation and P bioavailability, which could be particularly important to consider in microbial mats and the sulfate-poor Archean ocean.