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

Anaerobic protists frequently harbour methanogenic archaea, which apparently contribute to the hosts’ fermentative metabolism by consuming excess H2. 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 cellulolytic Cononympha protists in the hindgut of the Formosan termite Coptotermes formosanus . Microscopic and 16S rRNA amplicon sequencing analyses revealed that a single species, designated here ‘ Candidatus Methanobrevibacter cononymphae’, is associated with both Cononympha leidyi and Cononympha koidzumii and that its infection rate in Cononympha cells greatly 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 Cononympha species and also on the gut wall; ‘ Ca. Methanobrevibacter cononymphae’ is facultatively associated with Cononympha and horizontally taken up. Indeed, transmission electron microscopy showed escape or uptake of methanogens from/by a Cononympha cell. The genome of ‘ Ca. 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 fdhA , unnecessary within the non-formate-producing host cell; the dependence on acetate as an essential carbon source, which is abundant within the host cell; 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. ### Competing Interest Statement The authors have declared no competing interest.