Methanobacterium arbophilicum sp. nov. An obligate anaerobe isolated from wetwood of living trees

The isolation and characterization of a new methanogenic bacterium,Methanobacterium arbophilicum, is described. Isolation from wetwood enrichment cultures, that were obtained from methane-positive trees, required a medium containing inorganic salts, vitamins, and an atmosphere consisting of an 80∶20 mixture of hydrogen-carbon dioxide. Isolates ofM. arbophilicum were gram-positive, non-motile short rods that occurred singly, in pairs, or chains. The organism was found to be an autotroph and a strict anaerobe, and to have a pH optimum of 7.5–8.0. The optimal temperature for growth was 30 to 37C, the maximum being 45C and the minimum about 10C. The organism had obligate growth requirements for H₂ and CO₂, and organic compounds greatly stimulated growth. The generation time in shake flask culture was about 17 hr in mineral salts medium and about 13 hr in complex medium. The DNA base composition was 27.5 mol % GC.