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Abstract

The symbiotic gut microbiota of termites plays important roles in lignocellulose digestion and nitrogen metabolism. Termites possess a dual cellulolytic system: in lower termites the cellulases are contributed by both the insect and its gut flagellates, whereas in higher termites, host cellulases and hindgut bacteria participate in fiber digestion. Commonly, the microbial feeding chain is driven by the primary fermentations of carbohydrates. However in soil-feeding taxa, which exploit the peptidic component of soil organic matter as a dietary resource and show pronounced differences in physiochemical conditions along their highly compartmented intestinal tract, amino acids are an important substrate for the microbiota. Hydrogen appears to be the central intermediate in the hindgut fermentations in all termites. In wood-feeding taxa, it is efficiently recycled by homoacetogenic spirochetes, which prevail over methanogenic archaea probably because of their ability to colonize the bulk volume of the hindgut, whereas methanogens are restricted to particular microniches at the hindgut wall or within the gut flagellates. As a general rule, the spatial separation of microbial populations and metabolic activities gives rise to steep gradients of metabolites. The continuous influx of oxygen into the hindgut affects microbial metabolism in the microoxic periphery, and the anoxic status of the gut center is maintained only by the rapid reduction of oxygen by both aerobic and anaerobic microorganisms. Lignin is not significantly mineralized during gut passage, but modification of polyphenols by processes yet uncharacterized may increase the digestibility of both lignocellulose and humic substances. In wood-feeding termites, gut microbiota fix and upgrade nitrogen and recycle nitrogenous waste products. The microorganisms responsible for these reactions are mostly unknown, but recent studies have indicated that bacterial ectosymbionts and endosymbionts of the gut flagellates play a major role in the nitrogen metabolism of lower termites.