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

The fate of nitrogen in natural environments is controlled by anaerobic nitrate-reducing processes by which nitrogen is removed as N 2 or retained as NH 4 + . These processes can potentially be driven by oxidation of reduced inorganic compounds at oxic-anoxic interfaces. Several studies have investigated the use of Fe 2+ 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 Fe 2+ oxidation. Anaerobic sediment slurries were supplemented with 15 N-substrates and electron donors (Fe 2+ 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 Fe 2+ addition in 7 of 10 slurry experiments and in some cases, denitrification was concomitantly reduced. The determined kinetic parameters (V max and K m ) for Fe 2+ -driven DNRA were 4.7 µmol N L −1 d −1 and 33.8 µmol Fe 2+ L −1 , respectively and reaction stoichiometry for Fe 2+ :NH 4 + (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 Fe 2+ concentrations may be exploited by DNRA organisms and have an inhibitory effect on denitrification, thus Fe 2+ 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 . [ABSTRACT FROM AUTHOR]