Methanolinea mesophila sp. nov., a hydrogenotrophic methanogen isolated from rice field soil, and proposal of the archaeal family Methanoregulaceae fam. nov. within the order Methanomicrobiales

A novel mesophilic, hydrogenotrophic methanogen, designated strain TNR^T, was isolated from an anaerobic, propionate-degradation enrichment culture that was originally established from a rice field soil sample from Taiwan. Cells were non-motile rods, 2.0-6.5 μm long by 0.3 μm wide. Filamentous (up to about 100 μm) and coccoid (about 1 μm in diameter) cells were also observed in cultures in the late exponential phase of growth. Strain TNR^T grew at 20-40°C (optimally at 37°C), at pH 6.5-7.4 (optimally at pH 7.0) and in the presence of 0-25 g NaCl l^-1 (optimally at 0 g NaCl l^-1). The strain utilized H₂/CO₂ and formate for growth and produced methane. The G+C content of the genomic DNA was 56.4 mol%. Based on sequences of both the 16S rRNA gene and the methanogen-specific marker gene mcrA, strain TNR^T was related most closely to Methanolinea tarda NOBI-1^T; levels of sequence similarities were 94.8 and 86.4%, respectively. The 16S rRNA gene sequence similarity indicates that strain TNR^T and M. tarda NOBI-1^T represent different species within the same genus. This is supported by shared phenotypic properties, including substrate usage and cell morphology, and differences in growth temperature. Based on these genetic and phenotypic properties, strain TNR^T is considered to represent a novel species of the genus Methanolinea, for which the name Methanolinea mesophila sp. nov. is proposed; the type strain is TNR^T (5NBRC 105659^T5DSM 23604^T). In addition, we also suggest family status for the E1/E2 group within the order Methanomicrobiales, for which the name Methanoregulaceae fam. nov. is proposed; the type genus of family is Methanoregula.