The complete genome sequence of the methanogenic archaeon ISO4-H5 provides insights into the methylotrophic lifestyle of a ruminal representative of the Methanomassiliicoccales

Methane emissions from agriculture represent around 9% of anthropogenic greenhouse emissions. The single largest source of this methane is animal enteric fermentation, predominantly from ruminant livestock where it is produced mainly in their fermentative forestomach (or reticulo-rumen) by a group of archaea known as methanogens. In order to reduce methane emissions from ruminants, it is necessary to understand the role of methanogenic archaea in the rumen, and to identify their distinguishing characteristics that can be used to develop methane mitigation technologies. To gain insight into the role of methylotrophic methanogens in the rumen environment, the genome of a methanogenic archaeon, strain ISO4-H5, which was isolated from the ovine rumen and belonging to the order Methanomassiliicoccales, has been sequenced and compared to the genomes of other members of this order from bovine and human sources. Genomic analysis suggests ISO4-H5 is an obligate hydrogen-dependent methylotrophic methanogen, and is able to use methanol and methylamines as substrates for methanogenesis. Like other organisms within this order, ISO4-H5 does not possess genes required for the first six steps of hydrogenotrophic methanogenesis. Comparison between the genomes of different members of the order Methanomassiliicoccales revealed strong conservation in energy metabolism, particularly in genes of the methylotrophic methanogenesis pathway, as well as in the biosynthesis and use of pyrrolysine. Unlike members of Methanomassiliicoccales from human sources, ISO4-H5 does not encode the ability to produce coenzyme M, and so likely requires external coenzyme M to survive.