2014 was a banner year for making automotive fuel from nonfood crops, with a series of major new production plants opening in the United States. However, producing this so-called cellulosic ethanol remains considerably more expensive than gasoline. So researchers are always on the lookout for new ways to trim costs. Now they have a new lead, a microbe that can use abundant nitrogen gas as the fertilizer it needs to produce ethanol from plants.
The discovery is “a major commercial accomplishment for biofuel production,” says Steven Ricke, a microbiologist and editor of a textbook on biofuel production at the University of Arkansas, Fayetteville, who was not involved in the study.
Scientists have long eyed biofuels as a cleaner and more sustainable alternative to traditional fossil fuels. Instead of pumping oil from the ground, researchers harvest plants like cassava and sugarcane, grind them up, add enzymes to break down the plant matter, and sprinkle in yeast. The microbe ferments sugars in the plants to produce ethanol, a form of alcohol, which is now commonly mixed with gasoline and used in cars and buses around the world.
But biofuels are controversial. The majority are derived from food crops, like corn. Critics say the increased demand for these crops could increase food prices. And although direct emissions of carbon dioxide from burning biofuels are less than those from traditional fuels, some scientists now argue that once indirect emissions from land use changes and producing the crop are considered, the overall emissions from some biofuels can actually be higher.
So in recent years, researchers have turned to nonfood crops—like trees and bamboo—for biofuel production. These crops need less fertilizer than traditional biofuel crops, and they often have less detrimental impact on the land. In an ideal world, biofuels would be produced only from plant materials that cannot be eaten, such as trees and parts of plants that are left in fields after harvest, like straw.
But there are problems. The enzymes needed to break down plants’ primary structural components—cellulose and hemicellulose—into simple sugars are expensive. To ferment the simple sugars, the microbes also need nitrogen to grow and divide. So researchers add fertilizer to their fermentation vats to boost the ethanol yields. It is estimated that an ethanol production plant may be spending more than $1 million on this a year.
The new study may offer a solution to this latter problem. Microbiologists at Indiana University, Bloomington, started with miscanthus, a type of tall, woody tropical grass that grows quickly in many places where food will not grow. But instead of using yeast to ferment their plants into fuel, they turned to Zymomonas mobilis, a bacterium also capable of doing the job. The bacteria need high levels of nitrogen to thrive, something miscanthus can’t offer.
So the researchers looked at the amount of ethanol that the microbe could produce with and without additional nitrogen fertilizer being supplied and found that it did better without it. This shows that the microbe has an unusual ability—it can use (or “fix”) nitrogen from the atmosphere.
The study, published online today in the Proceedings of the National Academy of Sciences, even showed that the bacterium produces ethanol more quickly and uses more of the plant material when it uses nitrogen gas than when it is fed nitrogen in fertilizer. If the same holds true in a production plant, this could reduce biofuel production costs, the authors say. The process is also more environmentally friendly, they add, because there are greenhouse gas emissions associated with producing nitrogen fertilizer.
However, questions remain about how well this process will work in a large biofuel plant. Whereas using Z. mobilis might make it cheaper for producers to use inexpensive, nonfood crops, there could also be added costs and problems, says Yong-Su Jin, a molecular biotechnologist at the University of Illinois, Urbana-Champaign, who was not involved in this study. For example, it might be necessary to pump in purified nitrogen gas for the bacterium to use, which would raise costs. He said that there was a possibility that it could introduce contamination.
The overall environmental benefits may also be slim. Even if nitrogen fertilizers are not used in the fermentation process, they might still be needed to grow the crops. And the new advance doesn’t address other environmental impacts from biofuels, such as the greenhouse gas emissions from growing, harvesting, and transporting the plants. According to Fengqi You, a chemical engineer at Northwestern University in Evanston, Illinois, further studies would be needed to consider all the environmental and economic costs and benefits of doing this on an industrial scale so that it can be compared with existing systems.