Wood and straw are among the waste products our society produces in the greatest abundance, so it would be great to find a way to turn them into a clean-burning liquid fuel. There's a fungus that breaks down plant fiber into sugar, but it doesn't do so all that efficiently. Could genetic engineering change that?
According to the Department of Energy, American farmers generate about 100 million tons of waste plant matter each year from growing grain crops alone. The timber, construction, and DIY sectors create a similar tonnage of wood waste each year, and then there's all the paper sent to landfills that for some reason isn't recyclable. All told, more than 300 million tons of wood and paper waste gets thrown out in the U.S. each year
All that former plant material is made of various grades of cellulose, hemicellulose, and lignin, which are complex carbon compounds made of polysaccharides, which are themselves made of simple sugars. Biofuels advocates have long eyed the prospect of breaking cellulose and hemicellulose down into sugars and ferment that sugar into alcohol fuels. The concept is cellulosic ethanol, which differs from ethanol made from starch (like the corn ethanol currently being added to gasoline) in that it has a much lower carbon footprint, doesn't compete with the world's hungry for edible starch, and could conceivably offer a way to reduce the amount of material we send to landfills.
All told, cellulosic ethanol offers the potential of supplying a third of our current fuel consumption, with a significant greenhouse gas reduction over gasoline or corn ethanol.
The problem is expense. Though cellulosic ethanol can be created by breaking down plant fibers with acid, the process consumes the acid, which must then be replaced. In theory, it's far more efficient to use enzymes to break those chains: enzymes, which are organic catalysts, aren't usually consumed by the chemical reactions they facilitate. A class of enzymes called "cellulases" break down cellulose and similar wood fiber compounds into sugars, which can then be turned into ethanol by yeasts or other organisms.
That sounds good on paper, but it turns out to be more expensive to turn that paper into ethanol than fuel buyers can afford. The enzymes that break down cellulose turn out to be about 20 times as pricey as those used to break down starch.
There are fungi that create cellulases naturally, unsurprising given that many fungi make their living by breaking down wood and other organic matter. One of these fungi, Trichoderma reesei, has been the object of close study by biofuels researchers: it's easy to grow in large quantities and makes a wide variety of cellulases. Some of those cellulases are really promising for potential industrial use. But Trichoderma reesei only makes those enzymes under certain conditions. If the fungus is exposed too too high a concentration of simple sugars, it stops enzyme production.
Which makes sense: it's making those enzymes in order to turn the cellulose into sugar, and if it''s got sugar, why waste the effort? Other wood-eating fungus species do the same thing. Past researchers have found that the Trichoderma fungus can be "tricked" into continued enzyme production even in the presence of sugar by exposing it to a "precursor chemical" called sophorose, but sophorose costs about $3,300 per gram -- more than 60 times the current price of gold. Not the kind of thing you want to have to shovel into fermenting vats.
So it's understandable that a recent study of a new strain of the Trichoderma fungus is attracting some attention. The study, led by Christian Derntl at the Vienna University of Technology in Austria, found that a "glucose-blind" strain of Trichoderma reesei -- which kept producing higher levels of enzymes even in the presence of sugar -- got that trait from a single difference in the fungus' genetic makeup.
What's more, the team noted that the gene "domain" responsible for the more consistent enzyme production is shared by other wood-degrading organisms. That means that splicing the "glucose-blind" gene into those other fungi might let them produce similarly high levels of the appropriate enzymes to break down cellulose into sugar, with potentially huge reductions in the cost of cellulosic alcohol/
That'll certainly raise the eyebrows of those of us who are flatly opposed to all use of Genetically Modified Organisms. But the possibility of using non-food plant material as a clean-burning, greenhouse-friendly fuel might sway some folks. And it's not just about using the hundreds of millions of tons of wood and crop waste currently being thrown away each year in the U.S. Cellulosic biofuel advocates talk about planting unused lands like freeway medians and temporarily vacant lots with switchgrass, a native grass species that grows profusely in much of the U.S. with no irrigation or fertilizing needed. If it gets cheap enough to spin straw into fuel, perhaps our roadways could fuel themselves.
TrackBack URL: http://www.kcet.org/cgi-bin/mt/mt-tb.cgi/19441