Fe(II) oxidation by metabolically flexible phototrophs under complex geochemical conditions

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Decades of previous work have demonstrated that the microbial formation of iron minerals in the environment has a crucial impact on global biogeochemical cycling. However, much of our fundamental understanding of microbial mineral-forming processes comes from experiments with pure cultures of microorganisms. Typically, these experiments are conducted with only one energy or carbon substrate at unnaturally high concentrations. In the environment, however, many different sources of substrate are commonly present at the same time, often with low concentrations. A fundamental knowledge gap exists as to how iron mineral-forming bacteria behave under these low, mixed substrate conditions. This is a particular problem when studying types of microorganisms with high metabolic flexibility which can use many different energy and carbon sources. In this project, we propose a multi-disciplinary approach to determine the effect of competing substrates on rates and extent of Fe(II) oxidation by the most metabolically flexible type of mineral-forming bacteria, anoxygenic phototrophic Fe(II)-oxidizers. We aim to develop a fundamental understanding of how both mineralogical and biochemical markers left by these bacteria are affected by the presence of substrates other than iron (i.e. Fe(II), acetate, glucose, H2). To achieve this, we will extensively characterize the substrate preference of a variety of anoxygenic phototrophic Fe(II)-oxidizers at environmentally relevant substrate concentrations (typically on the order of 10s-100s micrometer). We will then characterize the mineralogical and biochemical markers left by these bacteria. This will include extensive characterization of the “mineral fingerprint” (i.e. minerals and cell-mineral aggregates formed) using Mössbauer spectroscopy, miniaturized backscatter Mössbauer spectroscopy (MIMOS II), X-ray diffraction, confocal laser scanning microscopy (CLSM), cryogenic focussed ion beam scanning electron microscopy (cryo-FIB-SEM) and zeta potentials. We will complement this mineralogical analysis with proteomics to determine how the biochemistry of the cells varies under different mixed substrate conditions. This will enable us to establish an environmentally-relevant “molecular fingerprint” for the process of Fe(II) oxidation. Finally, we will combine both these mineralogical and microbiological approaches in marine sediment microcosms in order to determine the effect of competing substrate concentration on the activity of phototrophic Fe(II) oxidation in one of the natural environments in which they are found. Ultimately this unique combination of mineralogical and molecular approaches will provide a thorough understanding of the controls on Fe(III) mineral formation at environmentally relevant conditions, and develop a suite of markers with which to identify the contribution of phototrophic Fe(II)-oxidizing bacteria in the natural environment.