Science News - August 1, 2009

lulosic fuel production, Lynd and several other researchers conclude in a February 2008 commentary in Nature Biotechnology.

Lignin is typically removed after pretreatment and then burned in the refinery’s boiler, replacing some fossil fuel use. The remaining plant matter is then broken into simple sugars, typically by a cocktail of microbial enzymes known as cellulases. Other microbes are then called in to ferment the sugars into ethanol.

Breaking down cellulose with enzymes is usually a separate step from fermentation — and a very costly one. But recent attempts to combine the conversion of cellulose to sugars with the conversion of sugars to fuel — called consolidated bioprocessing — have been successful. A strain of the soil-dwelling bacterium Clostridium phytofermentans will happily munch biomass such as wood pulp waste and will ferment it into ethanol. That discovery, by microbiologist Susan Leschine of the University of Massachusetts Amherst, led to the development of Qteros, a cellulosic-ethanol start-up in Marlborough, Mass. And in May, Mascoma researchers reported the engineering of a yeast and the bacterium Clostridium thermocellum to produce cellulases and ethanol in a single step.

At the San Francisco conference, posters reported on investigations of even more enzymes from various sources: bacteria that live in the deep sea, penicillin, diseased sea squirts, the bread mold Neurospora, a yeast that grows on wood-boring beetles and soil microbes from a Puerto Rican rainforest. Scientists are also fighting recalcitrance from the inside out by breeding lines of low-lignin plants.

Of course, getting a lot of ethanol in a benchtop flask is one thing. Scaling up to a silo-sized bioreactor is another. Industrial models exist — such as wringing pulp from trees for the paper industry or mass-producing cornstarch. “But we haven’t done it with cellulose yet,” says McMillan.

More than a dozen pilot plants for producing cellulosic ethanol are under construction and a handful are operat-

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ing, with 2011 seen as the year for cellulosic technologies to walk the walk. The group at Idaho National Lab hopes to be able to demonstrate a system from field to refinery by autumn of 2010.

Environmental cost

Yet concerns remain that the environmental side of the biofuels equation is still not worked out. Some argue that the numbers are too fuzzy to proceed with confidence that environmental burdens and benefits have been fully considered.

“There are people who say we don’t have enough knowledge to move forward—to some extent that is true,” says Michigan State University’s Philip Robertson, coauthor of the Science policy paper. “But we do know a lot about sustainability — enough to implement logical science-based standards.” This includes things like the strategic use of cover crops, fertilizer and tilling.

There is also the consideration of land-use changes — if forests are cleared for biofuels production, far more carbon will be released than is saved by the nonpetroleum fuels, several studies suggest. Such findings have led to scrutiny that has stung many in the industry who argue that biofuels are being held to a much higher standard than fossil fuels. If the petroleum isn’t “charged” for the greenhouse gas emissions of the U.S. military keeping supply lanes open in the Persian Gulf, why should emissions from cleared forests be included in the biofuels ledger? asks Bruce Dale of Michigan State University in a recent editorial in the journal Biofuels, Bioprod-ucts & Biorefining.

Congress is now considering legislation that may determine whether indirect land use can or cannot be a mark on the ruler used by the U. S. Environmental Protection Agency to measure biofuels’ impacts. Eventually, many researchers hope, a more detailed picture will emerge of the benefits and costs across all stages of the life cycles of fossil and next-generation fuels.

“Some really interesting services are going to emerge from these crops,” says Muth, of the Idaho National Lab. Some

biofuel plants help sequester carbon in the soil, for example. A 2002 analysis reported that by the second or third planting year, switchgrass plots experience far less soil erosion than annual crops such as corn. Species that do well near wetlands can act as filters, preventing nitrates and phosphates from getting into the water, Muth says. “If there is a value on carbon sequestration ... a value on clean water, there may be economic benefits for a lot of these crops.”

Robertson adds, “If certain practices were being promoted with incentives, it would ensure that we have a biofuels industry that is sustainable with a net benefit, not a cost. We don’t have that yet — I say ‘yet’ hopefully.”

With appropriate carrots and sticks, biofuels could play a big role in the energy portfolio of the future. There may even be a day when, Back to the Future style, garbage can be thrown into a personal-sized bioreactor that yields fuel. ( Trash biomass in the form of sugar beet pulp, tomato pomace, cashew apple, grape pomace, sweet gum and coffee pulp are all being investigated.) Several lines of research are investigating biofuel “coproducts,” high-value molecules that can be extracted during processing, such as proteins for animal feed or aromatics for perfumes and drugs. These products will also bring the net costs of these fuels down, one of several variables that can help the biofuels math add up to success as a fossil fuel substitute.

“It’s difficult to compare the costs of not changing with the costs of changing,” Lynd said at the May meeting in San Francisco. “Asking is this or that realistic is well-intentioned, but all solutions involve changes — we don’t have an option. Business as usual? Well, we think of it as a baseline, but it is a fantasy — even if you don’t care about carbon — just as a supply issue. Fossil fuels will all be gone. They’ll all be gone.” s