SOME RUMEN CILIATES HAVE ENDOSYMBIOTIC METHANOGENS

Most of the small ciliate protozoa, including Dasytricha ruminantium a nd Entodinium spp. living in the rumen of sheep, were found to have in tracellular bacteria. These bacteria were not present in digestive vac uoles. They showed characteristic coenzyme F420 autofluorescence and t hey were detected with a rhodamine-labelled Archaea-specific oligonucl eotide probe. The measured volume percent of autofluorescing bacteria (1%) was close to the total volume of intracellular bacteria estimated from TEM stereology. Thus it is likely that all of the bacteria livin g in the cytoplasm of these ciliates were endosymbiotic methanogens, u sing H-2 evolved by the host ciliate to form methane. Intracellular me thanogens appear to be much more numerous than those attached to the e xternal cell surface of ciliates.