When using CO as the electron donor instead of formate in an enrichment of methyl-reducing methanogens from a hypersaline soda lake in Siberia resulted in a methane-forming binary culture whereby CO was utilized by a bacterium and not a methanogenic partner. The bacterial isolate, strain ANCO1, forms a deep-branching phylogenetic lineage of the level of a new family within the Firmicutes class "Natranaerobiia" which members originate exclusively from soda lakes.