Methanogenesis in marine shallow water sediments: The quantitative role of anaerobic protozoa with endosymbiotic methanogenic bacteria

Anaerobic ciliates with endosymbiotic methanogenic bacteria are sometimes responsible for a substantial fraction (15–90%) of the production of CH4 in anaerobic detrital sediments and in the anoxic water column. In sandy sediments anaerobic protozoa playa relatively smaller role (typically < 2%) due to their low numbers and limited vertical distribution and since there is a relatively large production of methane by free-living methanogens associated with detrital particles even in the upper, sulphate-rich layers of the sediment. Under all circumstances protozoa playa small role for the terminal mineralisation in anaerobic biota since phagotrophs represent a second trophic level in an ecosystem with low growth efficiencies. In sulphate-poor anoxic habitats endosymbiotic methanogenesis will therefore always playa relatively small quantitative role.