Methanogens in the Digestive Tract of Termites

Abstract

Methanogenesis in termite guts is a product of symbiotic digestion, fueled by hydrogen and reduced one-carbon compounds that are formed during the fermentative breakdown of plant fiber and humus. Methanogens are restricted to the hindgut region and can be found in several distinct microhabitats. In lower termites, the methanogens belong almost exclusively to the genus Methanobrevibacter. They are either endosymbionts of flagellate protists or colonize the periphery of the hindgut, a habitat that is not fully anoxic. The oxygen-reducing capacities of the few isolates available so far indicate that they are well adapted to the continuous influx of oxygen across the gut wall. In higher termites, which lack gut flagellates, the hindgut is highly compartmented and characterized by strong differences in pH, redox potential, and other microenvironmental conditions. Here, the archaeal communities differ strongly between compartments and comprise not only Methanobacteriales, but also Methanosarcinales, Methanomicrobiales, and the recently discovered Methanomassiliicoccales. All methanogens in termite guts belong to distinct phylogenetic clusters that are restricted to the intestinal tracts of insects and millipedes. Only few representatives have been isolated in pure culture. The high methane emissions of termites, together with their enormous biomass in the tropics, make them a significant natural source of this important greenhouse gas.