Ants may provide cellulosic solution

By Craig A. Johnson | October 06, 2009
Report posted on Nov. 5, 2009, at 4:25 p.m. CST

At the Great Lakes Bioenergy Research Center in Madison, Wis., researchers are looking to leafcutter ants for new enzymatic processes that will further progress efforts to commercialize cellulosic ethanol. Leafcutter ants, which are found in tropical climates and live in enormous colonies that can number in the millions, have evolved several features over time that make their particular cocktail of enzymes attractive to researchers.

"Our lab is an evolution and ecology lab, and we're very interested in natural systems that take advantage of lignocellulolytic biomass and use microbes to break down [cellulosic] feedstocks," said Garret Suen, a post doctoral research fellow at the GLBRC. "If we go to a system that is specialized to produce exactly what it is we're looking for, we may find something of use."

Converting plant cell walls into simple sugars, the basic premise for cellulosic ethanol, is a major challenge for scientists. Leafcutter ants, which tend massive fungal gardens of their decaying byproducts, may present a worthwhile solution.

Before receiving a U.S. DOE grant, the GLBRC was already studying the symbiotic relationship between the ants and their fungal gardens. Over 50 million years, the ants and the fungus have evolved to the degree that if the ants were to die, or are removed from the system, the fungus dies as well, and vice versa. "The fungus-growing ant system is obligate, and one of the most complex symbioses that's described in nature," Suen said.

"A mature leaf cutter colony can defoliate a tree overnight," Suen said. With colonies ranging from a few thousand members to several million members, leafcutters are a major pest in the tropics. The ants are especially dangerous for agricultural operations such as orange and papaya plantations. But it is this ability to process huge volumes of biomass that has researchers fascinated.

Traditional corn processing in an ethanol plant involves pre-treating with acid, steam or ammonia to break down the bonds of lignin, hemicellulose and cellulose. After that, a cellulase is added to catalyze the cellulolysis of cellulose. Researchers at the GLBRC are studying multiple enzymes used by the ants in the fungal gardens to craft the perfect cocktail for ethanol production. According to Suen, "That cocktail is probably a mixture of about six or seven enzymes. One is definitely a cellulase that does the majority of the breaking of the beta -1,4 glycocitic link between the glucose and cellulose chain. But, then there are other things such as cellulose-binding modules, a protein which binds the cellulose itself, and then allows the cellulase to do its job."

Using several different enzymes may allow existing ethanol plants to become multi-feedstock facilities--if they can capture the enzymes they need. One difficulty with enzymatic research is that bacteria are exceedingly difficult to grow in a lab. "Right now we can only cultivate about 1 percent of bacteria in the lab," Suen said. The bacteria that are more difficult to cultivate are typically less aggressive bugs in the growing media and are pushed out by stronger bacteria before they can get a foothold. "[And] if there are 10 enzymes involved in a really efficient breakdown of biomass, is it going to be as efficient if we can only get eight of them?" Suen said.

Research at the GLBRC is providing a new look at some very old progress made in the ants' natural communities. With the leafcutter ants, the mixture of enzymes works in balance, but that nuanced formula would be nearly impossible to synthesize in a lab. "There is a renaissance, and researchers are going back to natural communities—an area called natural products. These are novel compounds in that no one has ever seen them before," Suen said.