ENDOSYMBIOTIC METHANOGENIC BACTERIA IN ANAEROBIC CILIATES - SIGNIFICANCE FOR THE GROWTH EFFICIENCY OF THE HOST

Endosymbiotic methanogenic bacteria of three species of anaerobic ciliates (Plagiopyla frontata, Metopus contortus, and M. palaeformis) were inactivated with the specific methanogen inhibitor 2-bromoethanesulfonic acid. The absence of endosymbiont methanogens reduced growth rate and growth yield by about 30% in P. frontata and M. contortus, while no significant change in fitness was observed in M. palaeformis. In Plagiopyla the growth rate constant is not affected by an artificially increased pH2 neither in normal nor in methanogen-free ciliates. The energetic advantage conferred by endosymbiont methanogens in Plagiopyla and in Metopus contortus probably is due to excretion of organic material from the bacteria at the expense of bacterial reproduction. It is unlikely that the maintenance of a low pH2 within the cells due to H2-consumption by the bacteria is important to the ciliates.