Facultative endosymbiosis between cellulolytic protists and methanogenic archaea in the gut of the Formosan termite .

Anaerobic protists frequently harbour methanogenic archaea, which apparently contribute to the hosts' fermentative metabolism by consuming excess H. However, the ecological properties of endosymbiotic methanogens remain elusive in many cases. Here we investigated the ecology and genome of the endosymbiotic methanogen of the protists in the hindgut of the termite . Microscopic and 16S rRNA amplicon sequencing analyses revealed that a single species, designated here " Methanobrevibacter cononymphae", is associated with both and and that its infection rate in cells varied from 0.0% to 99.8% among termite colonies. Fine-scale network analysis indicated that multiple 16S rRNA sequence variants coexisted within a single host cell and that identical variants were present in both species and also on the gut wall. Thus, " Methanobrevibacter cononymphae" is a facultative endosymbiont, transmitted vertically with frequent exchanges with the gut environment. Indeed, transmission electron microscopy showed escape or uptake of methanogens from/by a cell. The genome of ". Methanobrevibacter cononymphae" showed features consistent with its facultative lifestyle: i.e., the genome size (2.7 Mbp) comparable to those of free-living relatives; the pseudogenization of the formate dehydrogenase gene , unnecessary within the non-formate-producing host cell; the dependence on abundant acetate in the host cell as an essential carbon source; and the presence of a catalase gene, required for colonization on the microoxic gut wall. Our study revealed a versatile endosymbiosis between the methanogen and protists, which may be a strategy responding to changing conditions in the termite gut.