Sustainable liquid biofuels can be produced from lignocellulosic biomass such as wood and straw. These materials contain polysaccharides (polymers of sugars) that can be converted into simple sugars which can be fermented to produce liquid biofuels. Because lignocellulose is hard to digest, there is a need for new and improved enzymes to convert these materials into sugars. A number of animal species have evolved that live on a diet of woody materials, and these are of interest from the point of view of how they digest such a diet. One of the best studied groups is the termites that play important roles in the turnover of plant biomass in the tropics. Like most animals that eat lignocellulose, digestion in termites is achieved through the action of highly complex microbial communities in their digestive system, that work along with some of the termites own enzymes to break these materials down. In contrast, marine wood boring isopods, such as Limnoria quadripunctata, are highly unusual in that they live on a diet of wood, but have no resident microbes in their digestive system. Instead, the “gribble”, as these organisms are known colloquially, have to digest lignocellulose with their own enzymes.

Indeed the digestive system of L. quadripunctata (shown in figure 1) is effectively an enzyme reactor. The gut itself is a linear structure encased in a protective sheath possibly of chitin and usually packed full of wood particles. The digestive enzymes are produced in the hepatopancreas, which is plumbed into the gut, although no wood particles enter it. The enzymes produced in the hepatopancreas appear to be pumped into the gut by the contraction of the hepatopancreas, which similarly may withdraw liquid from the gut to absorb digested products released by the enzymes.

Figure 1The digestive system of L. quadripunctata

A, Diagram of the digestive system showing  the linear hindgut (hg) packed with wood particles, and the hepatopancreas (hp) that secretes digestive enzymes. B Electron micrograph of the digestive system showing wood particles (wp) packing the hind gut, and the hepatocancreas. C, detail of the hepatopancreas with secretory cells indicated with asterisks.

To understand the process by which gribble digest wood, we have sequenced the genes that are expressed in the hepatopancreas. We sequenced several hundred thousand expressed gene tags   from this organ (see figure 2) and found that more than ¼ of all these genes encoded glycosyl hydrolases (enzymes that digest polysaccharides). Amongst these glycosyl hydrolases were several types of apparent cellulase, some never previously reported in animals, which dominate the expression profiles. We are beginning to characterise the activity of recombinant versions of these glycosyl hydrolases, and are also trying to uncover the functions of a number of abundant but unknown proteins that may play a part in the digestive process. Our programme of work will help uncover the mechanisms of wood digestion in this animal, identify potentially valuable enzymes, and hopefully also help to explain why the gut is devoid of microbial organisms in these fascinating creatures.

Figure 2

Graphic representation of the major classes of genes identified amongst 360,000 expressed sequence tags representing genes expressed in the hepatopancreas of L. quadripunctata. GH family indicates a range of glycosyl hydrolase encoding genes that dominate the expression profile.

Programme lead

Associated programme members

  • University of Portsmouth
  • Syngenta Biomass Traits Group