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.