Methane and hydrogen production in a termite-symbiont system

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  • Methane and hydrogen production in a te

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Abstract

<jats:p>Methane and hydrogen emission rates and the δ<jats:sup>13</jats:sup>C of CH<jats:sub>4</jats:sub> were observed for various termites in Australia, Thailand and Japan. Combined with the already reported emission rates of CH<jats:sub>4</jats:sub> in the literature, the phylogenetic trend was examined. Emission rates of the observed termites were categorized into five groups: group I with high CH<jats:sub>4</jats:sub> and low H<jats:sub>2</jats:sub> emission rates with a CH<jats:sub>4</jats:sub>/H<jats:sub>2</jats:sub> ratio of typically 10/1; group II with high CH<jats:sub>4</jats:sub> and high H<jats:sub>2</jats:sub> emissions with a CH<jats:sub>4</jats:sub>/H<jats:sub>2</jats:sub> ratio of 4/1–1/2; group III with low emission rates of CH<jats:sub>4</jats:sub> and H<jats:sub>2</jats:sub>; group IV with high H<jats:sub>2</jats:sub> and insignificant CH<jats:sub>4</jats:sub> emissions; and group V with insignificant emissions for both CH<jats:sub>4</jats:sub> and H<jats:sub>2</jats:sub>. In lower termites, there are both colonies infected and uninfected with methanogens even in the same species, and no specific trend in CH<jats:sub>4</jats:sub> and H<jats:sub>2</jats:sub> 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 <jats:italic>Dicuspiditermes</jats:italic> in Thailand and <jats:italic>Amitermes</jats:italic> in Australia were classified into groups with high H<jats:sub>2</jats:sub> 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 CH<jats:sub>4</jats:sub> emission rate during dietary evolution from wood‐ to soil‐feeding, and two types of the system with different efficiencies of interspecies transfer of H<jats:sub>2</jats:sub> have been formed. The δ<jats:sup>13</jats:sup>C of CH<jats:sub>4</jats:sub> was discernible with a difference in the decomposition process in the termite–symbiont system among lower termites, fungus‐growing termites and other higher termites.</jats:p>

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