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Methanocella paludicola gen. nov., sp. nov., a methane-producing archaeon, the first isolate of the lineage ‘Rice Cluster I’, and proposal of the new archaeal order Methanocellales ord. nov.

¹ Department of Environmental Systems Engineering, Nagaoka University of Technology, Nagaoka, Niigata 940-2188, Japan
² Subground Animalcule Retrieval (SUGAR) Program, Extremobiosphere Research Center, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa 237-0061, Japan
³ Institute for Biological Resources and Functions, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8566, Japan
⁴ Department of Social and Environmental Engineering, Hiroshima University, Hiroshima 739-8527, Japan
⁵ Department of Civil and Environmental Engineering, Tohoku University, Sendai 980-8579, Japan
⁶ Research Institute of Genome-Based Biofactory, National Institute of Advanced Industrial Science and Technology (AIST), Sapporo, Hokkaido 062-8517, Japan

Correspondence
Hiroyuki Imachi
imachi{at}jamstec.go.jp

A novel mesophilic, hydrogenotrophic methanogen, strain SANAE^T, was isolated from an anaerobic, propionate-degrading enrichment culture, which was originally established from rice paddy soil. The cells were non-motile, Gram-negative and rod-shaped (1.8–2.4 µm long by 0.3–0.6 µm wide). Growth of strain SANAE^T was observed at 25–40 °C, with an optimum temperature range for growth of 35–37 °C. The pH range for growth was 6.5–7.8, with an optimum at pH 7.0. The salinity range for growth was 0–1 g NaCl l^–1 (0–17 mM). The isolate was able to utilize H₂/CO₂ and formate for growth and methane production. The G+C content of the genomic DNA was 56.6 mol%. Based on comparative 16S rRNA gene sequence analysis, strain SANAE^T was affiliated with a clone lineage of the Archaea, Rice Cluster I (RC-I), placing it between the orders Methanosarcinales and Methanomicrobiales within the class ‘Methanomicrobia’. 16S rRNA gene sequence similarities between strain SANAE^T and members of Methanosarcinales were in the range 80.0–82.8 %, and those between the strain and members of Methanomicrobiales ranged from 77.5 to 82.4 %. In addition to 16S rRNA gene analysis, sequence analysis of the mcrA gene (encoding the subunit of methyl-coenzyme M reductase, a key enzyme in the methane production pathway) also showed that strain SANAE^T was affiliated with the RC-I lineage. Here, we propose the name Methanocella paludicola gen. nov., sp. nov. for the isolate, the first of the RC-I lineage. The type strain is SANAE^T (=JCM 13418^T=NBRC 101707^T=DSM 17711^T). In addition, we also propose the status of order for the RC-I lineage, for which we propose the name Methanocellales ord. nov.

The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene and mcrA gene sequences of strain SANAE^T are AB196288 and AB300467, respectively.

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