at which all members of the genus Methanobacterium cannot grow, we conclude that strain RMAST belongs to the genusMethanothermobacter. In fact, optimum growth temperature is an important physiological trait discriminating the genus Methanobacterium (37–45 uC) from the genus Methanothermobacter (55–65 uC) (Boone, 2000; Boone et al., 2001).