Abundance and distribution of anaerobic protozoa and their contribution to methane production in Lake Cadagno (Switzerland)

In the uupermost layers of the anoxic sediment in Lake Cadagno, 9 different species of anaerobic protozoa were identified. The total number of these organisms was about 580 cells?ml−1 sediment. Most pf these protozoa contained endosymbiotic methanogenic bacteria which in total amounted to 106 methanogens?ml−1 sediment. In addition to the methanogenic endosymbionts, cells of Metopus setosus and Caenomorpha lata also contained a non‐fluorescent bacterial rod inside the cytoplasm. In some individual cells of C. lata this second type of endosymbiotic bacterium was sometimes the only endosymbiont observed. Contrary to earlier suggestions, anaerobic protozoa do not seem to play a major role in methane production at least in Lake Cadagno. No significant methane production due to the anaerobic protozoa and their methanogenic endosymbionts was found in situ. Isolated ciliates and amoebae produced methane at 12°C, but not at 6°C, probably as a result of temperature limitation. In the sediment of Lake Cadagno sulfate reduction seemed to be the dominant terminal degradation process.