A massive genomic study of the microbes living within the termite gut has identified close to 1,000 possible enzymes that break down wood. The plethora of cellulose-digesting proteins could shed light on the insects’ renowned wood-eating capacity and suggest cheaper, more efficient methods for generating cellulosic ethanol.
“The hard part [in producing cellulosic ethanol] is obtaining the metabolic intermediates from things like wood, but that’s the problem the termites have solved,” says Frances Arnold, a scientist at Caltech in Pasadena who was not involved in the research. “This paper provides an explosion of information about the genes involved in wood degradation in the termite.”
Cellulose is a fibrous complex carbohydrate that makes up plant cell walls. Biofuels made from cellulosic biomass, including cornstalks, woodchips, perennial grasses, and weeds such as switchgrass, could provide an alternative to corn-derived ethanol, which requires a large amount of energy to produce. However, breaking down cellulose into simple sugars that can be fermented into ethanol is currently a complex, inefficient, and expensive process. Scientists are searching for new enzymes that can more efficiently break down the hardy molecule and allow the production process to compete with corn-based ethanol.
In the new study, Jared Leadbetter, a microbiologist at Caltech, and his colleagues collected Nasutitermes termites from Costa Rica and isolated DNA from the microbial contents of part of their digestive tract. Scientists had previously theorized that these termites’ wood-digesting powers come primarily from the microbes that live in their gut. Using a metagenomics approach (see Metagenomics Defined), researchers sequenced and analyzed the genomic material from many different types of bacteria, searching for particular sequences known from other studies to be linked to the ability to break down cellulose. They identified nearly 1,000 candidate genes for glycohydrolases–enzymes that break down complex plant carbohydrates, such as cellulose.
Leadbetter and others will use the findings to figure out how termites, which derive virtually all their nutrients from wood, break down the material so efficiently. “Termites have been turning wood into their own biofuel for 200 million years,” says Leadbetter. “How does the system dismantle and degrade wood? If there’s any hope of engineering a system to make products we want, we need a better understanding of the system and take the best components for what we want to do.”
The next step will be to figure out exactly what the different enzymes do. “The functional analysis is really important,” says Arnold. “Are these better cellulases than the ones we have? Are there mixtures that can be useful?” One of the biggest hurdles in developing an efficient method of cellulosic ethanol production, says Arnold, is getting access to the solid cellulose matrix, a process that requires specific mixtures of enzymes.
The project, whose paper was published today in the journal Nature, was a collaboration between Caltech, the Department of Energy’s Joint Genome Institute in Walnut Creek, CA, and Verenium, an industrial enzyme and biofuel company in Cambridge, MA. Verenium is currently testing the wood-digesting ability of some of the newly identified microbial enzymes, as well as searching for combinations of enzymes that work together synergistically. Geoff Hazlewood, senior vice president of research at the company, says the microbial genes from the termite gut already appear to have several enzymes from a class known to be particularly powerful, as well as a wide variety of accessory enzymes necessary for the digestion process.
The termite gut microbial genome is likely to contain more genomic bounty. The researchers found 34 groups of genes with unknown function, including one specific sequence identified in a dozen other cellulose-digesting bacteria. Says Arnold, “Maybe we haven’t even found the biggest surprise yet.”