The impact of light on microbial iron(II) oxidation in freshwater sediments.

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Biogeochemical iron cycling has been recognized as an important reaction network that affects a series of environmentally relevant processes in sediments. Iron(II)-oxidizing and iron(III)-reducing bacteria control to large extent the turnover and the (im)mobilization of iron. In the classical view of the sedimentary biogeochemical iron cycle iron(II)-oxidizing bacteria are fed by an iron(II) flux that originates from the zone of microbial iron(III) reduction, typically located in deeper depth of redox stratified sediments. Although studied for open water systems, the process of iron(III) photoreduction and the consequent provision of iron(II) has so far been neglected for sedimentary systems. In the proposed study, we hypothesize that iron(III) photoreduction provides additional iron(II) as substrate for the iron(II)-oxidizing community in the top sunlit layers of sediments – in a zone that is currently characterized by the oxidative processes and the formation of iron(III). Along these lines we will measure light and geochemical parameters (oxygen, dissolved iron(II), H2O2 and pH) with microsensors and iron(III) mineralogy (Mössbauer and acid extraction) on high spatial and temporal resolution as a function of illumination in freshwater sediment cores. In addition to that we will examine how natural organic matter drives iron(III) photoreduction and the availability of iron(II) for microbial oxidation in sedimentary systems. Furthermore, reactive oxygen species will be detected as a function of aeration and their effect on the stability and availability of iron(II) for microbial oxidation will be assessed. The proposed research project will open doors towards a so far neglected reaction mechanisms and heads towards revision of our current understanding of the biogeochemical iron cycle and related substrate fluxes across redox gradients in sediments.