Iron constitutes one of the most abundant elements in the earth crust, and the second most abundant element that is redox-active in the near-surface aqueous environments. Although, it has long been recognised that microorganisms participate in the global iron cycle, the interconnection between biotic and abiotic reduction and oxidation processes in an ecosystem has not yet been demonstrated. This research proposal deals with the functioning of the iron biogeochemical cycle in a freshwater ecosystem that is influenced by biotic activity coupled with purely chemical processes. A complete description of the physicochemical parameters and redox gradients in the littoral sediment of Lake Constance will be performed. As a second step a modelling approach based on these field data is proposed, in order to simulate the variations in the energetical budget available to abundant microbes and their competition with the abiotic reaction processes throughout the entire iron cycle. The presented project is expected to provide important information for environmental and ecological studies, as well as engineering projects in the field of waste- or polluted natural water treatment.