Termite gut bacteria produce a wealth of wood-degrading enzymes.
When Jared Leadbetter travelled to Costa Rica in May 2005, it wasn't for the orange and blue poison arrow frogs, the butterflies with wings the size of saucers or the squirrel monkeys. Leadbetter, an environmental microbiologist at the California Institute of Technology in Pasadena, went for the termites.
One morning, he and some colleagues hiked into the Bosque Lluvioso forest near the town of Guápiles and quickly found what they were looking for — a basketball-sized termite nest attached to a tree trunk. Using a machete, Leadbetter took a slice of the nest and transported it and its inhabitants, the tropical Nasutitermes termites, to Costa Rica's National Biodiversity Institute. There he dissected almost 200 termites until well into the night.
Termites are famous for their wood-degrading abilities, and are aided in this effort by a gut-dwelling community of microorganisms. The microbes' activities are key to environmental-carbon turnover, and such microbes have recently been identified as a potential source of biochemical catalysts for turning wood into biofuels. Although termites have been studied for a century, information about the specific roles the host and the microbiota have in the degradation of plant biomass is scarce.
Leadbetter, who has been studying wood-feeding termites and their gut microbes since the early 1990s, travelled to Costa Rica in search of a termite unlike those he typically collects from fallen ponderosa pines in Los Angeles. The gut communities of Californian termites contain protozoa, whereas the guts of most tropical termites are rich in bacteria. Leadbetter wanted to avoid protozoa-rich termites, because he planned to sequence the genomes of termite hindgut microbiota. “Protozoa have huge genomes, and much of their DNA doesn't encode the genetic information used to make enzyme catalysts,” he explains.
The tropical quarry proved a successful model. Leadbetter and his collaborators' metagenomic analysis of an area of the termite hindgut known as the P3 section (see page 560) revealed a large set of genes encoding enzymes that break down cellulose and xylan, two of the main polysaccharides found in wood. The researchers found that more than 250 species of bacteria live in this one-microlitre environment.
“For the first time, we have shown that all those bacteria in the gut encode a diversity of genes and enzymes that may have a role in the initial dismantling of wood,” says Leadbetter. “The story is far from being over, but this is a real exclamation point. There aren't just a few, but hundreds of genes involved.”
Among the organisms Leadbetter found in the termite's hindgut was a high concentration of spirochetes — helical bacteria that are usually associated with disease. The high density of beneficial spirochetes seen in these termites is rare, if not unheard of, in other environments, and until now little was known about the breadth of their roles in symbiosis. Leadbetter's research implicates spirochetes in the initial hydrolysis of wood polysaccharides, making them a crucial part of the termite's digestive machinery.
“There are different compartments in the gut that will have different microbial communities and may have a whole different story to tell,” says Leadbetter. “Biology remains on the frontier. When you think about how much we still have to discover it can either be imposing or exciting. I find it to be both.”