Fe(II) oxidation by metabolically flexible phototrophs under complex geochemical conditions
Metabolism, Biochemistry and Genetics of Microorganisms
Final Report Abstract
The overarching goal of this project was to identify and quantify the activity of phototrophic Fe(II)-oxidizing bacteria in the environment when organic substrates are present and bioavailable simultaneously to Fe(II). We were able to identify preferences for organic substrates for a variety of phototrophic Fe(II)-oxidizing bacteria and also determine the extent to which organics (different fatty acids and glucose) or Fe(II) are used as substrates. Additionally, all Fe(III) minerals that formed during phototrophic Fe(II) oxidation in the presence and absence of organic co-substrates were identified as short-range ordered Fe(III) (oxyhydr)oxides very similar to ferrihydrite. This suggests that the cooccurrence of organic substrates is influencing phototrophic Fe(II) oxidation rates but not the identity of Fe(III) minerals formed as a reaction product. Results from field experiments demonstrated that a diverse community of potential phototrophic Fe(II)-oxidizing bacteria are also present in the environment such as in an iron(II)-rich stratified water column of a freshwater lake system. Geochemical parameters, in particular the chemocline between anoxic Fe(II)-rich bottom waters and oxic surface waters, were identified as the main parameter determining the abundance of a phototrophic Fe(II)-oxidizing community. In-situ and ex-situ incubations confirmed that the availability and the concentration of organic co-substrates can significantly lower the impact of the phototrophic Fe(II)-oxidizing community can have on the environmental iron cycle. Not only because these bacteria prefer organic substrates over Fe(II) but also because organic substrates were fueling microbial Fe(III) reduction constantly replenishing the Fe(II) pool. Only under organic-poor conditions, phototrophic Fe(II) oxidation seemed to considerably contribute to the oxidative side of the iron cycle. Especially when light was sufficiently available. Additionally, we could demonstrate that not only dissolved Fe(II), as it is commonly used in incubation experiments, but also solid-phase Fe(II) minerals (e.g. siderite) can represent an additional source for inorganic carbon and Fe(II) which was presumably overlooked in the scientific community so far. With the results from this project we could contribute to a better understanding of phototrophic Fe(II) oxidation in natural environments when organic substrates are co-occurring with Fe(II). This will help to elucidate the role of the phototrophic Fe(II)-oxidizing community on the iron cycle in future laboratory or field studies.
