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The fate of nitrogen is linked to iron(II) availability in a freshwater lake sediment

Journal article
Authors Elizabeth Robertson
Bo Thamdrup
Published in Geochimica et Cosmochimica Acta
Volume 205
Pages 84-99
ISSN 0016-7037
Publication year 2017
Published at
Pages 84-99
Language en
Links doi.org/10.1016/j.gca.2017.02.014
Keywords Denitrification, DNRA, Nitrogen, Sediment
Subject categories Geochemistry, Other Earth and Related Environmental Sciences, Microbiology

Abstract

© 2017 The Authors The fate of nitrogen in natural environments is controlled by anaerobic nitrate-reducing processes by which nitrogen is removed as N2or retained as NH4+. These processes can potentially be driven by oxidation of reduced inorganic compounds at oxic-anoxic interfaces. Several studies have investigated the use of Fe2+as an electron donor in nitrate reduction in bacterial cultures, however current information on this process in the environment is sparse. We aimed to determine whether nitrate-reducing processes in the freshwater Lake Almind (Silkeborg, Denmark) were linked to Fe2+oxidation. Anaerobic sediment slurries were supplemented with15N-substrates and electron donors (Fe2+and/or acetate) to characterize nitrate-reducing processes under environmentally relevant substrate concentrations and at higher concentrations traditionally used in microbial enrichment studies. Dissimilatory nitrate reduction to ammonium, DNRA, was stimulated by Fe2+addition in 7 of 10 slurry experiments and in some cases, denitrification was concomitantly reduced. The determined kinetic parameters (Vmaxand Km) for Fe2+-driven DNRA were 4.7 µmol N L−1 d−1and 33.8 µmol Fe2+ L−1, respectively and reaction stoichiometry for Fe2+:NH4+(8.2:1) was consistent with that of predicted stoichiometry (8:1). Conversely, under enrichment conditions, denitrification was greatly increased while DNRA rates remained unchanged. Increased Fe2+concentrations may be exploited by DNRA organisms and have an inhibitory effect on denitrification, thus Fe2+may play a role in regulating N transformations in Lake Almind. Furthermore, we suggest enrichment conditions may promote the adaptation or change of microbial communities to optimally utilize the available high substrate concentrations; misrepresenting metabolisms occurring in situ.

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