An anaerobic ciliate as a natural chemostat for the growth of endosymbiotic methanogens.

An anaerobic ciliate (Metopus palaeformis) with methanogen endosymbionts was grown in batch culture. The growth of both partners was monitored for four ciliate generations following re-feeding. Methanogens consistently accounted for approximately 1% of total consortium biovolume. Minor variations in ciliate and methanogen biovolume were closely correlated. Methanogens completed one to two generations per ciliate generation. The growth of methanogens in these ciliates is analogous to the growth of microbes in a chemostat, with methanogen growth rate rapidly converging on a rate that is equal to the 'dilution rate' provided by the expanding ciliate cell volume. Methanogen digestion was insignificant in growing ciliates and no other loss processes were recognised. We conclude that the growth rates of ciliates and methanogens are approximately equivalent. The only exception occurs in ciliates showing no net growth, when substrate supply within the ciliate is still sufficient to promote measurable growth of methanogens. The methanogens within each ciliate appear to divide simultaneously and irrespective of their size. This phenomenon is not apparently integrated with events in the host cell cycle. It may give the methanogens some selective advantage in securing their persistence in successive ciliate generations.