Contribution of microaerophilic iron(II)-oxidizers to iron(III) mineral formation

Neutrophilic microbial aerobic oxidation of ferrous iron (Fe(II)) is restricted to pH-circumneutral environments characterized by low oxygen where microaerophilic Fe(II)-oxidizing microorganisms successfully compete with abiotic Fe(II) oxidation. However, accumulation of ferric (bio)minerals increases competition by stimulating abiotic surface-catalyzed heterogeneous Fe(II) oxidation. Here, we present an experimental approach that allows quantification of microbial and abiotic contribution to Fe(II) oxidation in the presence or initial absence of ferric (bio)minerals. We found that at 20 μM O2 and the initial absence of Fe(III) minerals, an iron(II)-oxidizing enrichment culture (99.6% similarity to Sideroxydans spp.) contributed 40% to the overall Fe(II) oxidation within approximately 26 h and oxidized up to 3.6 × 10–15 mol Fe(II) cell–1 h–1. Optimum O2 concentrations at which enzymatic Fe(II) oxidation can compete with abiotic Fe(II) oxidation ranged from 5 to 20 μM. Lower O2 levels limited biotic Fe(II)...