Evidence for non-methanogenic metabolisms in globally distributed archaeal clades basal to the Methanomassiliicoccales
Recent discoveries of mcr and mcr-like complexes in genomes from diverse archaeal lineages suggest that methane (and more broadly alkane) metabolism is an ancient pathway with complicated evolutionary histories. The conventional view is that methanogenesis is an ancestral metabolism of the archaeal class Thermoplasmata. Through comparative genomic analysis of 12 Thermoplasmata metagenome-assembled genomes (MAGs), we show that these microorganisms do not encode the genes required for methanogenesis, which suggests that this metabolism may have been laterally acquired by an ancestor of the order Methanomassiliicoccales. These MAGs include representatives from four orders basal to the Methanomassiliicoccales, including a high-quality MAG (95% complete) that likely represents a new order, Ca. Lunaplasma lacustris ord. nov. sp. nov. These MAGs are predicted to use diverse energy conservation pathways, such as heterotrophy, sulfur and hydrogen metabolism, denitrification, and fermentation. Two of these lineages are globally widespread among anoxic, sedimentary environments, with the exception of Ca. Lunaplasma lacustris, which has thus far only been detected in alpine caves and subarctic lake sediments. These findings advance our understanding of the metabolic potential, ecology, and global distribution of the Thermoplasmata and provide new insights into the evolutionary history of methanogenesis within the Thermoplasmata.