Phototrophic Fe(II) oxidation by Rhodopseudomonas palustris TIE-1 in organic and Fe(II)-rich conditions

Rhodopseudomonas palustris TIE-1 grows photoautotrophically with Fe(II) as an electron donor and photoheterotrophically with a variety of organic substrates. However, it is unclear whether R. palustris TIE-1 conducts Fe(II) oxidation in conditions where organic substrates and Fe(II) are available simultaneously. In addition, the effect of organic co-substrates on Fe(II) oxidation rates or the identity of Fe(III) minerals formed is unknown. We incubated R. palustris TIE-1 with 2 mM Fe(II), amended with 0.6 mM organic co-substrate, and in the presence/absence of CO₂. We found that in the absence of CO₂, only the organic co-substrates acetate, lactate and pyruvate, but not Fe(II), were consumed. When CO₂ was present, Fe(II) and all organic substrates were consumed. Acetate, butyrate and pyruvate were consumed before Fe(II) oxidation commenced, whereas lactate and glucose were consumed at the same time as Fe(II) oxidation proceeded. Lactate, pyruvate and glucose increased the Fe(II) oxidation rate significantly (by up to threefold in the case of lactate). ⁵⁷Fe Mössbauer spectroscopy revealed that short-range ordered Fe(III) oxyhydroxides were formed under all conditions. This study demonstrates phototrophic Fe(II) oxidation proceeds even in the presence of organic compounds, and that the simultaneous oxidation of organic substrates can stimulate Fe(II) oxidation.