Because phylogenetic analysis based on the 16S rRNA gene, the most commonly used and reliable phylogenetic marker gene, clearly shows that strain RMAST is more closely related to the genus Methanothermobacter than the genus Methanobacterium and because strain RMAST prefers a high temperature for growth (70 uC), at which all members of the genus Methanobacterium cannot grow, we conclude that strain RMAST belongs to the genus Methanothermobacter. 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).