Denitrification, anaerobic ammonium oxidation, and dissimilatory nitrate reduction to ammonium in an East African Great Lake (Lake Kivu)

We investigated anaerobic nitrogen (N) cycling in the water column of Lake Kivu, a deep meromictic tropical lake in East Africa. Data were collected at one station in the Northern Basin and one in the Southern Basin, during two sampling campaigns (June 2011—dry season, and February 2012—rainy season). Short-term incubations of sulfide-free water with 15N-labeled substrates revealed high potential denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates (up to 350 and 36 nmol N produced L−1 h−1, respectively), while anaerobic ammonium oxidation (anammox) was lower (up to 3.3 nmol N produced L−1 h−1). However, anammox rates were 15 nmol N produced L−1 h−1 when 15 NH4+ was added at depths where NH4+ concentrations were very low (< 1 μmol L−1). With the addition of 5 μmol L−1 of 15 NO3− and 10 μmol L−1 of H2S, denitrification and anammox were stimulated in the Northern Basin, while the increase of DNRA rates was less notable. In the Southern Basin, the addition of H2S decreased denitrification rates, probably because of competition with DNRA, which increased, while no effect was observed on anammox. This study puts into evidence the co-occurrence of denitrification, anammox and DNRA, for the first time in a great tropical lake, and underlines the spatial heterogeneity of these processes. Contrary to numerous reports in literature, we show that anammox can significantly occur in presence of H2S, suggesting that the contribution of anammox in the N cycle may be underestimated.