We examined the influence of endosymbiotic methanogens on the growth and metabolic profile of the anaerobic ciliate Trimyema compressum. When we isolated the ciliate, it possessed a large number of endosymbiotic Methanobacterium-like methanogens inside the cell. The culture was transferred to fresh medium with Lactobacillus sp. cells as food bacteria. T. compressum grew to give a maximum cell number of 3300 cells/ml without loss of the endosymbionts over 30 passages of transfer. Acetate and methane were major end products with smaller amounts of propionate and butyrate. However, after further continued cultivation, the number of endosymbiotic methanogens started to decrease and they eventually disappeared. The growth and metabolic profile of the ciliate were changed significantly as the symbiotic association deteriorated. The major fermentation products were shifted to butyrate. These results suggest that the endosymbiotic methanogens which function as hydrogen scavengers are not indispensable for the growth and survival of T. compressum, although the growth yield decreased slightly in the absence of methanogens, and the ciliate is capable of fermenting food bacteria without endosymbiotic methanogens by changing its metabolic pathway.
Yamada, K., Kamagata, Y., & Nakamura, K. (1997). The effect of endosymbiotic methanogens on the growth and metabolic profile of the anaerobic free-living ciliate Trimyema compressum. FEMS Microbiology Letters, 149(1), 129–132. https://doi.org/10.1016/S0378-1097(97)00068-2