New Mode of Energy Metabolism in the Seventh Order of Methanogens as Revealed by Comparative Genome Analysis of “Candidatus Methanoplasma termitum”

TLDR

Methanomassiliicoccales are a recently defined seventh order of methanogens that are obligate hydrogen‑dependent methylotrophs lacking the CO₂ reduction pathway and produce methane from methanol or methylamines. The study proposes that the Fpo‑like complex in Methanomassiliicoccales directly interacts with subunit D of the heterodisulfide reductase to form an energy‑converting ferredoxin:heterodisulfide oxidoreductase. By sequencing the complete genome of *Candidatus Methanoplasma termitum* and comparing it with three human‑gut strains, the authors identified a novel Fpo‑like complex lacking an F420‑oxidizing module that may mediate this interaction. These results reveal a new mode of energy metabolism in Methanomassiliicoccales, where the heterodisulfide functions both in electron bifurcation and as a terminal acceptor in a membrane‑associated redox process.

Abstract

The recently discovered seventh order of methanogens, the Methanomassiliicoccales (previously referred to as “Methanoplasmatales”), so far consists exclusively of obligately hydrogen-dependent methylotrophs. We sequenced the complete genome of “Candidatus Methanoplasma termitum” from a highly enriched culture obtained from the intestinal tract of termites and compared it with the previously published genomes of three other strains from the human gut, including the first isolate of the order. Like all other strains, “Ca. Methanoplasma termitum” lacks the entire pathway for CO2 reduction to methyl coenzyme Mand produces methane by hydrogen-dependent reduction of methanol or methylamines, which is consistent with additional physiological data. However, the shared absence of cytochromes and an energy-converting hydrogenase for the reoxidation of the ferredoxin produced by the soluble heterodisulfide reductase indicates that Methanomassiliicoccales employ a new mode of energy metabolism, which differs from that proposed for the obligately methylotrophic Methanosphaera stadtmanae. Instead, all strains possess a novel complex that is related to the F420:methanophenazine oxidoreductase (Fpo) of Methanosarcinales butlacks an F420-oxidizing module, resembling the apparently ferredoxin-dependent Fpo-like homolog in Methanosaeta thermophila. Since all Methanomassiliicoccales also lack the subunit E of the membrane-bound heterodisulfide reductase (HdrDE), wepropose that the Fpo-like complex interacts directly with subunit D, forming an energy-converting ferredoxin: heterodisulfideoxidoreductase. The dual function of heterodisulfide in Methanomassiliicoccales, which serves both in electron bifurcation and as terminal acceptor in a membrane-associated redox process, may be a unique characteristic of the novel order.