We humans are great at making ethanol from grains. We’ve been doing it for thousands of years to make beer and liquor, and our expertise is one reason that corn ethanol has been the biofuel of choice so far. But the biofuels of the future, experts say, will come not from the starch in corn but from the cellulose in grasses and other abundant green plants. There’s just one problem: We’re not good at breaking down the tough structure of cellulose to get at the sugars inside.
But cows are.
Cows, like termites and leafcutter ants, love to eat tough plant material, and host bacteria with the molecular machinery to do so in their guts. Scientists, in their attempts to get better at breaking down cellulose, have tried to copy nature by studying the enzymes that allow those grass-eating animals to do their thing. And now researchers say they have found a treasure trove of new microbe-produced enzymes inside a cow that could help them in their quest.
In a study published Thursday in the peer-reviewed journal Science, researchers described how they incubated bags of switchgrass inside cow rumens and from that found 27,755 “candidate genes” with the potential for efficiently breaking down plant cellulose into usable sugar that can then become ethanol. [MSNBC]
Eddy Rubin and his team executed this chemical excursion by surgically opening a hole into the first of the cow’s four stomachs.
The researchers employed a cow with a surgically placed tube, called a fistula, which allowed them direct access to the rumen. As Rubin explains, “Over millions of years, in exchange for housing in the cows, these organisms have gotten good at paying their rent by providing the host with broken down cellulose—sugars the cow can use as an energy substrate.” [Scientific American]
Into the cow went nylon bags full of switchgrass, one of the prime candidates for a cellulosic biofuel crop. Doing the experiment inside the cow was critical, because many of the microorganisms that help cows break down cellulose don’t survive in lab conditions. In the course of three days, those grass digesters attached themselves to the researchers’ switchgrass. When the team pulled the grass back out, they set to work untangling all the DNA they’d collected.
Rubin’s team used metagenomics, a gene-sequencing approach that maps the DNA of a community of organisms instead of one single creature or plant. Metagenomics are being used to explore the microbes living in and on people, as well as by genome entrepreneur Craig Venter to catalog all the tiniest creatures of the sea. [Reuters]
That metagemoic analysis produced the 27,000-plus candidate genes. The next step was to test some of them, so Rubin and company expressed 90 of the genes and set the enzymes they produced to work on grasses.
More than half of this subset showed the capacity to degrade at least one of the feedstocks, which the study authors say suggests the larger pool of candidates is “highly enriched” with enzymes whose activity could be useful in biofuel production. [Scientific American]
It’ll take some time to sort through this new chemical body of knowledge. But in one fell swoop, Rubin’s team added volumes to the library of known enzymes for biofuel production. The key is to find a mix that someday allow for cheap, or at least cheaper, cellulose breakdown on the industrial scale. Switchgrass and its ilk have a few potential advantages: They don’t reduce the food supply the way that corn grown for ethanol can, and they don’t require the same kind of extensive fertilization. But if all the sugar stays locked up in cellulose, it’s of no matter.
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Image: Jonas Lovaas Gjerstad ; Wikimedia Commons