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Fe(III) Photoreduction Producing Feaq2+ in Oxic Freshwater Sediment

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journal contribution
posted on 09.01.2020, 21:45 by Ulf Lueder, Bo Barker Jørgensen, Andreas Kappler, Caroline Schmidt
Iron­(III) (Fe­(III)) photoreduction plays an important role in Fe cycling and Fe­(II) bioavailability in the upper ocean. Although well described for water columns, it is currently unknown to what extent light impacts the production of dissolved Fe­(II) (Fe2+) in aquatic sediments. We performed high-resolution voltammetric microsensor measurements and demonstrated light-induced Fe2+ release in freshwater sediments from Lake Constance. Fe2+ concentrations increased up to 40 μM in the top 3 mm of the sediment during illumination in the visible range of light (400–700 nm), even in the presence of oxygen (100–280 μM). The Fe2+ release was strongly dependent on the organic matter content of the sediment. A lack of photoreduced Fe2+ was measured in sediments with concentrations of organic carbon <6 mg L–1. The simultaneous presence of sedimentary Fe­(III) photoreduction besides microbial and abiotic Fe2+ oxidation by oxygen suggests an active Fe redox cycling in the oxic and photic zone of aquatic sediments. Here, we provide evidence for a relevant contribution of Fe­(III) photoreduction to the bio-geochemical Fe redox cycle in aquatic freshwater sediments.