In the lobby of the Microbial Sciences building at the University of Wisconsin, leafcutter ants in a display colony hike back and forth. Improbably large leaf fragments wobble on their backs as the ants ferry them between a dwindling pile of oak leaves and a garden of fungus studded with leaves in assorted states of decay.
Made up of a single species of fungus and a handful of bacterial strains, the fungus garden breaks down the ants’ leafy harvest through an efficient natural process. It’s a process that researchers believe could be a model for producing biofuel in a more sustainable way.
As we transition away from petroleum dependence, ethanol-based biofuel has risen to the forefront as one of the most accessible sources of renewable energy. It’s produced by fermenting plant sugars, which are strung together into long chains called polysaccharides. Before the fermentation process can begin, these chains have to be snipped apart, a process that varies in difficulty depending on the type of plant being used.
Polysaccharide chains found in corn kernels -- the primary biofuel crop in the U.S. -- are relatively simple to break up. But corn depletes the soil and guzzles water and fertilizer, and using it for fuel siphons calories from the food supply to gas tanks.
On the other hand, perennial grasses and agricultural “waste” like cornstalks offer a biofuel source that has a lighter impact on the environment. But these woodier fibers -- referred to as “cellulosic” biomass -- are a tangle of robust polysaccharides that are trickier to deconstruct. Further complicating this problem, the molecular structure of plant biomass isn’t uniform. What breaks down the polysaccharides near the surface of a cornstalk or blade of grass might not work at all on those buried more deeply.
But finding efficient ways to extract energy from plants and other forms of biomass is not a new problem. In fact, it’s a problem that Earth’s plant eaters solved millions of years ago. And according to University of Wisconsin researcher Frank Aylward, if you’re looking for a model system, you can’t do better than leafcutter ants.
They may not have the imposing mien of herbivores like giraffes or elephants, but in Central and South America, leafcutter ants dominate, munching through more of the region’s foliage than any other organism.
But the ants can’t digest leaves by themselves -- they have to rely on the garden’s microbes. “We sort of think of the fungus gardens as being an external gut,” Aylward explains. The garden digests biomass and reconstitutes its molecules in little nutrient packets holding a cocktail of carbohydrates, lipids, and proteins.
“The ants are essentially doing what we want to do with biofuel,” says Aylward. “They’re taking all of this recalcitrant plant biomass that’s full of all of these really complicated polymers and they’re degrading it and converting it into energy.” The transformation from leafy greens to energy source is mediated by hundreds of enzymes produced by the fungus garden’s microbes. If these enzymes chow down so efficiently on the leaves of Central America, Aylward and his coworkers wondered, could they be just as effective at breaking apart the sugars of cellulosic biomass in an industrial setting?
One model for a commercial biofuel process patterned after the fungus garden could entail splicing the genetic codes for the garden’s most effective enzymes into other microbes, prompting them to churn out biomass-digesting proteins.
But first, scientists needed to identify which enzymes the garden uses to digest leaves for the ants and which microbial residents produce them. By sequencing the genomes of the fungus and bacteria and comparing that data to the garden’s enzyme soup, Aylward and his coworkers were able to identify a fungus called Leucoagaricus gongylophorus as the garden’s biomass-degrading workhorse.
They also found that the fungus calibrates its enzyme cocktail for different stages of leaf decay. The biomass profile changes at each level in the garden -- the freshest leaves sit near the top and the mostly decomposed waste material at the bottom. And Aylward found that the garden’s enzymes changed, too. That insight could provide the biofuel industry with some clues about which enzymes might excel early in the polysaccharide-decomposition process and which ones to apply later on.
Incidentally, this division of labor also reveals which enzymes the garden deploys together at each level. This is a huge boon to anyone designing industrial applications, since enzymes tend to work much better in specific combinations -- and the garden has had 50 million years of symbiosis with the ants to find the most efficient combinations.
Aylward has already been approached by companies interested in synthesizing some of the garden’s enzymes and using them in biofuel production.
“It's difficult to think that we can actually find a process that improves on nature,” Aylward points out, “so it probably makes sense to learn from it.”