Contribution by the methanogenic endosymbionts of anaerobic ciliates to methane production in Dutch freshwater sediments

Biogenic methane contributes substantially to the atmospheric methane concentration and thus to global warming. This trace gas is predominantly produced by strictly anaerobic methanogenic archaea, which thrive in the most divergent ecological niches, e. g. paddy fields, sediments, landfills, and the digestive tract of various animals. Methanogenic archaea also live as endosymbionts in the cytoplasm of anaerobic protozoa. In marine sediments these endosymbionts can contribute up to 90% to the overall rate of methanogenesis, whereas their role of in freshwater sediments is largely unknown. Here we describe the results of a one year's survey of the methanogenesis by endosymbiotic methanogens in four different Dutch freshwater sediments. The abundance of anaerobic protozoa, in particular ciliates, the methane production rates by the ecosystem and by the protists, and a number of abiotic parameters were measured. A novel method (heatshock for 5 min) for estimating the contribution by endosymbiotic methanogens was established. Our results reveal large fluctuations of ciliate abundance throughout the year, but on average, only minor contributions by methanogenic endosymbionts to the total methanogenesis in these environments