Organic Matter Complexation Promotes Fe(II) Oxidation by the Photoautotrophic Fe(II)-Oxidizer Rhodopseudomonas palustris TIE-1

Fe(II)–organic matter (Fe(II)–OM) complexes are present in the photic zone of aquatic environments and due to their reactivity may play an important role in biogeochemical cycling. Complexation of Fe has been shown to influence the rates and extent of many chemical and microbial redox reactions. However, it is currently unknown whether, how fast, and to which extent Fe(II)–OM complexes can be oxidized by anoxygenic photoautotrophic Fe(II)-oxidizing microorganisms which are widespread in photic habitats and use electrons from Fe(II) to fix CO2. Here, we used the photoautotrophic Fe(II)-oxidizer Rhodopseudomonas palustris TIE-1 to demonstrate that Fe(II) complexation by OM significantly accelerated the rates of Fe(II) oxidation by strain TIE-1 compared to the oxidation of nonorganically bound, free Fe(II), although a fraction of the Fe(II) present as Fe(II)–OM complexes seemed to resist microbial oxidation. Analysis of Fe–OM aggregate sizes showed that the remaining, nonoxidized Fe(II) and almost all of the Fe(III) in the Fe(II)–humic and Fe(II)–fulvic acid oxidation products were in the form of colloids (3–200 nm). In summary, this study shows that Fe(II)–OM complexes can be oxidized microbially in the photic zone, and the complexation of Fe(II) by OM controls the kinetics and extent of Fe(II) oxidation