Protozoan stimulation of anaerobic microbial activity: enhancement of the rate of terminal decomposition of organic matter

Anaerobic microcosms were established in which plant material was degraded by the co-operative interaction of various bacterial metabolic types, and the production of CH<inf>4</inf> and sulfide was monitored. The introduction of the anaerobic ciliate <it>Metopus palaeformis</it> resulted in a reduction of bacterial abundance but an increase in the rate of CH<inf>4</inf> and sulfide production. The enhanced bacterial activity (resulting in an increased rate of bacterial CH<inf>4</inf> production) was positively correlated with the number of ciliates present. The endosymbiotic methanogens of <it>M. palaeformis</it> did not contribute significantly to the observed increase in CH<inf>4</inf> production. It was concluded that protozoan excretions such as organic acids (acetate, propionate) were most likely responsible. Protozoa have previously been shown to enhance bacterial activity in aerobic systems, but this is the first report of the phenomenon in anaerobic systems. Bacterial stimulation by protozoan grazing signifies the importance of anaerobic protozoa in the natural environment, not just in controlling bacterial numbers but also in vitalising ecosystem function.