Water column anammox and denitrification in a temperate permanently stratified lake (Lake Rassnitzer, Germany)

We studied microbial N-2 production via anammox and denitrification in the anoxic water column of a restored mining pit lake in Germany over an annual cycle. We obtained high-resolution hydrochemical profiles using a continuous pumping sampler. Lake Rassnitzer is permanently stratified at ca. 29 m depth, entraining anoxic water below a saline density gradient. Mixed-layer nitrate concentrations averaged ca. 200 mu mol L-1, but decreased to zero in the anoxic bottom waters. In contrast, ammonium was < 5 mu mol L-1 in the mixed layer but increased in the anoxic waters to ca. 600 mu mol L-1 near the sediments. In January and October, N-15 tracer measurements detected anammox activity (maximum 504 nmol N(2)L(-1)d(-1) in (NH4+)-N-15-amended incubations), but no denitrification. In contrast, in May, N-2 production was dominated by denitrification (maximum 74 nmol N-2 L-1 d(-1)). Anammox activity In May was significantly lower than in October, as characterized by anammox rates (maximum 6 vs. 16 nmol N, L-1 d(-1) in incubations with (NO3-)-N-15), as well as relative and absolute anammox bacterial cell abundances (0.56% vs. 0.98% of all bacteria, and 2.7 x 10(4) vs. 5.2 x 10(4) anammox cells mL(-1), respectively) (quantified by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) with anammox bacteria-specific probes). Anammox bacterial diversity was investigated with anammox bacteria-specific 16S rRNA gene clone libraries. The majority of anammox bacterial sequences were related to the widespread Candidatus Scalindua sorokinii/brodae cluster. However, we also found sequences related to Candidatus S. wagneri and Candidatus Brocadia fulgida, which suggests a high anammox bacterial diversity in this lake comparable with estuarine sediments. (C) 2009 Elsevier GmbH. All rights reserved.