RESUMO
Recent metagenomic studies have identified numerous lineages of hydrogen-dependent, obligately methyl-reducing methanogens. Yet, only a few representatives have been isolated in pure culture. Here, we describe six new species with this capability in the family Methanosarcinaceae (order Methanosarcinales), which makes up a substantial fraction of the methanogenic community in arthropod guts. Phylogenomic analysis placed the isolates from cockroach hindguts into the genus Methanimicrococcus (M. hacksteinii, M. hongohii, and M. stummii) and the isolates from millipede hindguts into a new genus, Methanolapillus (M. africanus, M. millepedarum, and M. ohkumae). Members of this intestinal clade, which includes also uncultured representatives from termites and vertebrates, have substantially smaller genomes (1.6-2.2 Mbp) than other Methanosarcinales. Genome reduction was accompanied by the loss of the upper part of the Wood-Ljungdahl pathway, several energy-converting membrane complexes (Fpo, Ech, and Rnf), and various biosynthetic pathways. However, genes involved in the protection against reactive oxygen species (catalase and superoxide reductase) were conserved in all genomes, including cytochrome bd (CydAB), a high-affinity terminal oxidase that may confer the capacity for microaerobic respiration. Since host-associated Methanosarcinales are nested within omnivorous lineages, we conclude that the specialization on methyl groups is an adaptation to the intestinal environment.