Nitrate-dependent anaerobic methane oxidation and chemolithotrophic denitrification in a temperate eutrophic lake

While the emissions of methane (CH4) by natural systems have been widely investigated, CH4 aquatic sinks are still poorly constrained. Here, we investigated the CH4 cycle and its interactions with nitrogen (N), iron (Fe) and manganese (Mn) cycles in the oxic-anoxic interface and deep anoxic waters of a small, meromictic and eutrophic lake, during two summertime sampling campaigns. Anaerobic CH4 oxidation (AOM) was measured from the temporal decrease of CH4 concentrations, with the addition of three potential electron acceptors (NO3-, iron oxides (Fe(OH)3) and manganese oxides (MnO2)). Experiments with the addition of either 15N-labeled nitrate (15N-NO3-) or 15N-NO3- combined with sulfide (H2S), to measure denitrification, chemolithotrophic denitrification and anaerobic ammonium oxidation (anammox) rates, were also performed. Measurements showed AOM rates up to 3.8 µmol CH4 L-1 d-1 that strongly increased with the addition of NO3- and moderately increased with the addition of Fe(OH)3. No stimulation was observed with MnO2 added. Potential denitrification and anammox rates up to 63 and 0.27 µmol N2 L-1 d-1, respectively, were measured when only 15N-NO3- was added. When H2S was added, both denitrification and anammox rates increased. Altogether, these results suggest that prokaryote communities in the redoxcline are able to efficiently use the most available substrates.