ABSTRACT. One of the two methanogenic endosymbionts of the giant sapropelic amoeba Pelomyxa palustris was isolated in pure culture. The cells were slender non‐motile rods (3 × 0.4 μm), sometimes occurring in chains of 3–4 cells. Ultrathin sections revealed a Gram‐positive cell wall and conically pointed ends with mesosome‐like structures in the cytoplasm. The isolate had a generation time of 10 and 12 h during growth on H2/CO2 and formate, respectively. The optimum growth temperature was 40°C and the optimum pH was 7.8. The G+C content of its DNA was found to be 37.7% mole percent. The isolate was identified as Methanobacterium formicicum.
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