Phototrophic Fe(II)-Oxidizers Can Use Multiple (In)Organic Substrates Simultaneously

Anoxygenic phototrophs are metabolically flexible bacteria. However, it is unclear how Fe(II) oxidation is affected by the simultaneous availability of organic or inorganic substrates. We therefore cultivated six green- and purple non-sulfur photoferrotrophs from freshwater and marine environments with 2 mM Fe(II), amended with 0.6 mM organic co-substrate or H₂. We found that photoferrotrophs generally utilized organics and Fe(II) simultaneously, and that two marine strains preferred Fe(II) over H₂. For some photoferrotrophs, Fe(II) oxidation rates were significantly enhanced by acetate, lactate, and glucose, whereas H₂ typically slowed Fe(II) oxidation. All phototrophic Fe(II)-oxidizers oxidized 80%-100% of the Fe(II) by the end of the experiment except for R. robiginosum, where Fe(II) oxidation reached only 52%-73%. We further observed for the first time that R. palustris TIE-1 can grow with butyrate and glucose and that R. iodosum can utilize glucose. Comparison of genomic predictions with experimentally observed substrate consumption revealed discrepancies in four strains. Overall, our results demonstrate that Fe(II) remains an important electron donor despite the availability of alternative substrates, suggesting that maintaining photoferrotrophy is a common trait of photoferrotrophs and likely contributes to iron cycling in natural environments.