3139Two methylotrophic bacteria having similar cell morphologies have been isolated from cultures of 'Methylobacterium ethanolicum' grown on methane. One of these, strain POC, is an obligate methanotroph containing the serine pathway and Type I1 intracytoplasmic membranes, which appears to be a 'Methylocystis' species. The second organism, strain H4-14, can fix N, and grows on a variety of substrates, including methanol, formate, ethanol, succinate, fructose and H2 + CO,; during growth on methanol, carbon is assimilated via the Calvin-Benson cycle.Strain H4-14 appears to be a Xanthobacter species. Mixtures of the two organisms formed stable associations during growth on methane, consisting of approximately 5-20% H4-14 and 80-95 % POC. Strain POC excreted biotin, which was required by strain H4-14. Our results suggest that 'Methylobacterium ethanolicum' cultures grown on methane consist of a syntrophic association of two methylotrophs.
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