Microbial processes in soil-feeding insects
Little is known about the digestive processes of soil- and humus-feeding arthropods and the involvement of their microbial symbionts. Humivorous insects are hot spots of carbon and nitrogen mineralization particularly in tropical soils and globally significant sources of the greenhouse gas methane (Brune, 2010a). In the hindgut of humivorous scarab beetle larvae (Pachnoda spp.), the presence of many insect-specific lineages of uncultivated Clostridiales and Bacteroidales suggests that soil organic matter is metabolized through fermentation (Andert et al., 2010). Fermenting bacteria that use humic substances as external electron acceptor seem to be responsible also for the efficient reduction of Fe(III) minerals during gut passage (Hobbie et al., 2012).
We have followed the microbial mineralization and transformation of nitrogenous soil components in the gut of soil-feeding termites using 15N tracers (Ngugi et al., 2011; Ngugi and Brune, 2012). Both denitrification and nitrate ammonification are important processes in the gut of soil-feeding termites, and the digestion of peptides and the subsequent fermentation of amino acids give rise to an enormous accumulation of ammonia, which is partially oxidized and denitrified by a hitherto unknown process. In a collaborative study, the group of Rong Ji at Nanjing University extended our findings of a preferential mineralization of peptidic soil components also to the gut of humivorous earthworms (Shan et al., 2010).