ABSTRACT: Archaea of the order Methanomassiliicoccales use methylated amines such as trimethylamine as the substrates for methanogenesis. They form two large phylogenetic clades and reside in diverse environments, from soil to the human gut. Two genera, one from each clade, inhabit the human gut: Methanomassiliicoccus, which has one cultured representative, and "Candidatus Methanomethylophilus," which has none. Questions remain regarding their distribution across biomes and human populations, their association with other taxa in the gut, and whether host genetics correlate with their abundance. To gain insight into the Methanomassiliicoccales clade, particularly its human-associated members, we performed a genomic comparison of 72 Methanomassiliicoccales genomes and assessed their presence in metagenomes derived from the human gut (n = 4,472, representing 22 populations), nonhuman animal gut (n = 145), and nonhost environments (n = 160). Our analyses showed that all taxa are generalists; they were detected in animal gut and environmental samples. We confirmed two large clades, one enriched in the gut and the other enriched in the environment, with notable exceptions. Genomic adaptations to the gut include genome reduction and genes involved in the shikimate pathway and bile resistance. Genomic adaptations differed by clade, not habitat preference, indicating convergent evolution between the clades. In the human gut, the relative abundance of Methanomassiliicoccales spp. correlated with trimethylamine-producing bacteria and was unrelated to host genotype. Our results shed light on the microbial ecology of this group and may help guide Methanomassiliicoccales-based strategies for trimethylamine mitigation in cardiovascular disease.IMPORTANCE Methanomassiliicoccales are less-known members of the human gut archaeome. Members of this order use methylated amines, including trimethylamine, in methane production. This group has only one cultured representative; how its members adapted to inhabit the mammalian gut and how they interact with other microbes is largely unknown. Using bioinformatics methods applied to DNA from a wide range of samples, we profiled the abundances of these Archaea spp. in environmental and host-associated microbial communities. We observed two groups of Methanomassiliicoccales, one largely host associated and one largely found in environmental samples, with some exceptions. When host associated, these Archaea have smaller genomes and possess genes related to bile resistance and aromatic amino acid precursors. We did not detect Methanomassiliicoccales in all human populations tested, but when present, they were correlated with bacteria known to produce trimethylamine. Due to their metabolism of trimethylamine, these intriguing Archaea may form the basis of novel therapies for cardiovascular disease.
