Termites digest wood with the help of their intestinal microorganisms. The first metagenomic analysis of the inhabitants of a termite gut provides insight into this feat of biomass-to-energy conversion.
The gut of wood-feeding termites is a tiny but astonishingly efficient bioreactor, in which microbes catalyse the conversion of lignified plant cell walls to fermentation products that drive the metabolism of their host. Molecular phylogenetic data have revealed the presence of hundreds of microbial species in this microlitre-sized environment, but little is known about their functional diversity. On page 560 of this issue, Warnecke et al.1 report an analysis of the metagenome of bacteria in the largest part of the gut — the hindgut paunch — of a termite species of the genus Nasutitermes (Fig. 1).
Metagenomics is the burgeoning study of the entire genetic material from particular environments, and allows investigation of microorganisms that cannot be cultured in the laboratory. In this case, Warnecke et al. compiled an enormous amount of (unavoidably highly fragmented) sequence data from the total genomic DNA of the inhabitants of the termite hindgut paunch, and compared those data with known gene sequences in public databases. Such sequence comparisons offer a wealth of information, but nevertheless can be deceptive. So, crucially, the authors also identified the major proteins secreted into the hindgut fluid, and went further by confirming the catalytic properties of selected gene products by expressing the genes in the bacterium Escherichia coli.
The main structural polysaccharides of wood (lignocellulose) are cellulose and various hemicelluloses: both are efficiently digested by termites. In lower (more primitive) termites, hydrolysis of cellulose and hemicelluloses is catalysed by flagellate protozoa housed in the hindgut paunch; bacteria are also present. Higher termites, which include Nasutitermes and most termite species worldwide, lack flagellates. Instead, their hindgut is host to a largely bacterial community, although the involvement of the bacteria in cellulose and hemicellulose digestion has not been clear2.
The metagenomic analysis1 of the Nasutitermes hindgut reveals a rich diversity of bacterial genes encoding hitherto unknown glycosyl hydrolases. These enzymes constitute more than 100 families of proteins that cleave the glycosidic bond between carbohydrates, or between a carbohydrate and non-carbohydrate entity. The authors' sequence analysis suggests that many of the gene products fall within the glycosyl hydrolase families specializing in the degradation of cellulose and hemicelluloses. For several gene products, they also demonstrate cellulase activity in vitro. So far, bacteria with such cellulolytic power have not been isolated from the hindgut of higher termites; until recently, even their activity had remained undetected because the enzymes are not soluble but are associated with the particulate fraction of the gut content3.
Many of the genes identified by Warnecke et al.1 could be assigned to one of two groups of bacteria — the fibrobacters and spirochetes — known to be abundant in the hindgut of Nasutitermes species. The fibrobacters from termite guts have not been cultured in the lab, but are known to be distant relatives of the fibre-degrading bacteria found in the intestines of cows and other ruminant animals4. The implication that members of the spirochetes are involved in cellulose digestion, however, was unexpected. This result is of similar importance to the earlier, equally surprising discovery that termite-gut spirochetes can carry out reductive acetogenesis5 (a mode of energy metabolism that results in the reduction of carbon dioxide to acetate). This finding answered the long-standing question regarding the identity of the organism responsible for one of the key metabolic activities in termite guts6.
Warnecke et al.1 show that almost all of the genes for an operational (acetyl-CoA) pathway for reductive acetogenesis seem to be present in the spirochetes of the Nasutitermes hindgut. This complements evidence for the predominance of spirochetes among the bacteria expressing this pathway in lower termite species. In Nasutitermes, however, the pathway seems to be peculiar, because the enzyme necessary for the production of formate is severely under-represented in the metagenome. It remains to be seen whether hydrogen — the central free intermediate during lignocellulose degradation in lower termites7 — is equally important in higher termites and, if so, whether the putatively spirochetal iron-only hydrogenases in the Nasutitermes hindgut are involved in hydrogen production or consumption. Other puzzling aspects of the metagenome are the presence of genes apparently encoding sensory hydrogenases, and an abundance of other genes putatively involved in signal transduction and chemotaxis in the hindgut community.
The gut microbiota of termites can theoretically convert a sheet of A4 paper into two litres of hydrogen. Figures such as this have focused attention on termite guts as a source of microorganisms and enzyme systems for the production of biofuels. The biotechnological challenges in this process are getting a grip on the initial step — the breakdown of the highly stable polymers of lignocellulose to microbial substrates — and redirecting the carbon and electron flow in the metabolic fermentations to useful products (such as ethanol or hydrogen). The largely unexplored biodiversity and biochemistry of the termite gut is a promising source of novel catalytic capacities. For example, it is intriguing that termites efficiently digest wood in the apparent absence of any classical lignin-degrading enzymes. The gut microbiota also holds the answers to puzzles such as the unknown mechanisms that allow the release of reducing equivalents, formed during glycolysis, as free hydrogen against a high partial pressure.
The data set provided by Warnecke et al.1 is a treasure trove for researchers with different interests. But it is just a beginning. For example, the presumably anaerobic microbiota of the hindgut paunch differs from that of the hindgut wall8 — a microhabitat characterized by the continuous influx of oxygen9. Microorganisms with different specializations are probably present in other gut regions, especially in the extremely alkaline (pH 10–12) hindgut compartments characteristic of many higher termites2. And functionally distinct microbial communities are to be expected in termites with other feeding habits, such as the soil-feeding species (for which nitrogen-rich soil peptides are important dietary components) or fungus-cultivating species (which thrive on material that has been partially delignified and is enriched in fungal biomass)2.
Finally, another item for the research agenda is the diverse but specific association of bacteria and flagellate protozoa in the hindguts of lower termites10. These are not only a fruitful source of microbial biodiversity and functional novelties, but also hold the key to understanding the metabolic basis and evolutionary origin of a highly successful, tripartite symbiosis.
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Journal of Industrial Microbiology & Biotechnology (2012)