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Abstract

A novel hydrogenotrophic methanogen, designated strain MRE50(T), was isolated from a methanogenic consortium, which was originally established from an Italian rice field soil. Cells were non-motile rods, 1.3-2.8 μm long and 0.4-0.7 μm wide. Coccoid cells were also observed in cultures at the late-exponential phase of growth. Strain MRE50(T) grew at 37-55 °C (optimally at 45 °C), at pH 6-7.8 (optimally at pH 7.0) and in the presence of 0-20 g NaCl l(-1). The isolate utilized H(2)/CO(2) and formate for growth and methane production. Phylogenetic analyses of the 16S rRNA gene and the methanogen-specific marker gene mcrA showed that strain MRE50(T) is affiliated with the order Methanocellales, previously known as uncultured archaeal group Rice Cluster I. Based on both 16S rRNA gene and mcrA gene sequences, strain MRE50(T) was related most closely to Methanocella paludicola SANAE(T). Levels of sequence similarity were 92.5 and 86.1 %, respectively, indicating that strains MRE50(T) and Methanocella paludicola SANAE(T) represent different species within the genus Methanocella. In addition, although these strains shared phenotypic properties including cell morphology and substrate utilization, they differed with respect to susceptibility to antibiotics, and temperature and NaCl ranges for growth. Given the phenotypic differences and the distinct phylogenetic placement of the new isolate relative to the type species of the genus Methanocella, strain MRE50(T) is considered to represent a novel species of the genus Methanocella, for which the name Methanocella arvoryzae sp. nov. is proposed. The type strain is MRE50(T) (=NBRC 105507(T) =DSM 22066(T)).