Methanonatronarchaeum thermophilum AMET1 Genome sequencing and assembly. Methanonatronarchaeum thermophilum strain:AMET1

Methanogenic archaea are major players in the global carbon cycle. The phylum Euryarchaeota includes diverse groups of methanogens that are interspersed with non-methanogenic branches in the evolutionary tree. So far methanogens inhabiting hypersaline environments have been identified only within the order Methanosarcinales. These halophilic methanogens employ the bacterial type of osmoprotection based on the "salt-out" strategy. We report the discovery of a novel deep lineage of extremophilic methanogens that inhabit hypersaline lakes and can be considered as "methanogenic haloarchaea", and present a comprehensive analysis of two nearly complete genomes from this group. Within the phylum Euryarchaeota, these isolates form a novel, class-level lineage that is most closely related to the class Halobacteria and for which we propose the name "Methanonatronarchaea". Their extreme halophily and the absence of organic osmolytes indicate that "Methanonatronarchaea" employ the "salt-in" osmotic strategy. Their growth requires FeS or sediments from hypersaline lakes. The novel methanogens belong to heterotrophic methyl reducers that utilize C1-methylated compounds as electron acceptors and formate or hydrogen as electron donors. The genomes contain an incomplete (and apparently inactivated) set of genes encoding the upper branch of methyl group oxidation to CO2, the membrane-bound heterosulfide reductase and cytochromes. The presence of these genes clearly differentiates this novel group from all known methyl reducing methanogens. The discovery of extremely halophilic, methyl-reducing methanogens related to haloarchaea sheds new light on the origin of methanogenesis and shows that the ability of methanogens to live in salt-saturating conditions is not limited to the classical methylotrophic pathway.