Oxidation of Fe(II) carbonate (siderite) by anoxygenic phototrophic Fe(II)-oxidising bacteria

Abstract Microbial, anoxygenic phototrophic ferrous iron (Fe(II)) oxidation (pFeOx) plays an important role in biological iron cycling. The uptake and oxidation of dissolved Fe(II) species (Fe 2+ aq ) as an electron donor for pFeOx bacteria is well understood. In contrast, the oxidation of solid Fe(II)-bearing minerals by pFeOx is less well studied, with possible mechanisms including dissolution of the minerals followed by uptake and intracellular oxidation of Fe 2+ aq or extracellular electron transfer from solid Fe(II) minerals to the bacterial cells. We investigated the oxidation of the Fe(II)-bearing carbonate mineral siderite (FeCO 3 ) by an anoxygenic phototrophic Fe(II) oxidiser Rhodopseudomonas palustris TIE-1. We aimed to explain if oxidation was controlled by chemical dissolution kinetics or whether direct electron transfer was involved. Controlled dissolution experiments using increasing dissolved bicarbonate concentrations (0–300 mM HCO 3 – ), supported by geochemical modelling, demonstrated that R. palustris TIE-1 can oxidise up to 5-fold more Fe(II) when cells are in direct contact with siderite than would be expected if oxidation occurred through dissolution alone. These results suggest that anoxygenic phototrophic Fe(II)-oxidising bacteria have the capability to enhance carbonate dissolution or even access solid-phase Fe(II) in siderite as a source of electrons, especially when siderite dissolution is limited or suppressed by geochemical constraints.