Methane and hydrogen production in a termite-symbiont system

Methane and hydrogen emission rates and the ¹³C of CH₄ were observed for various termites in Australia, Thailand and Japan. Combined with the already reported emission rates of CH₄ in the literature, the phylogenetic trend was examined. Emission rates of the observed termites were categorized into five groups: group I with high CH₄ and low H₂ emission rates with a CH₄/H₂ ratio of typically 10/1; group II with high CH₄ and high H₂ emissions with a CH₄/H₂ ratio of 4/1–1/2; group III with low emission rates of CH₄ and H₂; group IV with high H₂ and insignificant CH₄ emissions; and group V with insignificant emissions for both CH₄ and H₂. In lower termites, there are both colonies infected and uninfected with methanogens even in the same species, and no specific trend in CH₄ and H₂ emissions was observed within a genus. Whether protozoa in the hindgut of termites are infected with methanogens or not and the differences in species compositions of protozoa are possibly responsible for the inter-colonial variations. The proportions of infected colonies were possibly small for the family Kalotermitidae (dry wood feeders), and relatively large for families of wet or damp wood feeders. The hydrogen emission rate possibly depends on the locality of methanogens: namely, whether they are intracellular symbionts of protozoa or whether they are attached to the hindgut wall. Emission rates of higher termites were classified into groups according to genera and the diet. Most species of soil or wood/soil interface feeders classified into group I, while the soil feeders Dicuspiditermes in Thailand and Amitermes in Australia were classified into groups with high H₂ emission rates. Typical wood-feeding termites and fungus-growing termites were classified into group III. The results indicate that higher termites tend to increase the CH₄ emission rate during dietary evolution from wood- to soil-feeding, and two types of the system with different efficiencies of interspecies transfer of H₂ have been formed. The ¹³C of CH₄ was discernible with a difference in the decomposition process in the termite–symbiont system among lower termites, fungus-growing termites and other higher termites.