Nitrogen-Fed Bacteria Could Power Our Future

Breah LaSarre, Indiana University. These Zymomonas mobilis don't look like much, but they could represent a major step forward in clean transport fuels

A bacterium that doesn't need its nitrogen preprocessed could breathe new life into environmentally friendly ethanol production, turning mowing the lawn from a chore into an income source.

Most ethanol is currently produced from corn or sugar, competing with food for the raw materials while also combating rising pricesamong other problems. If the transportation systems of the future are to be powered by low carbon biofuels, we need something better.

Cellulosic ethanol is widely promoted as the great hope. Vast quantities of cellulose and lignin produced by grasses rot around the world every day. Ethanol made from cellulose and lignin would, in theory, emit no more carbon dioxide than is released naturally, and no more than the plants draw from the atmosphere to grow in the first place. In the real world, inefficiencies creep in, but even so, cellulosic ethanol is likely to have about one-sixth of the Greenhouse impact of fossil fuels for the same amount of energy. 

So far, however, cellulosic production has proven to be frustratingly slow and expensive. The recent collapse in oil prices has left biofuels even further from being cost-competitive, so something big is needed.

The bacteria or yeasts used to turn sugars from cellulose, hemicelluse and lingin into ethanol need nitrogen to grow, but the switchgrass or lawn clippings to feed them are lacking in this regard. Current producers add nitrogen-rich chemicals to drive the process, adding millions of dollars a year to the cost of the currently small amount of ethanol produced in this way, and forming a major obstacle to growth.

However, Indiana University's Dr. James McKinlay has announced in the Proceedings of the National Academy of Sciences that this isn't always necessary. Zymomonas mobilis is an ethanol-producing bacterium that can use nitrogen gas (N2) instead. As the most common component in the atmosphere, nitrogen is hardly in short supply.

Most living things can't get their nitrogen directly from the atmosphere, instead depending on symbiotic prokaryotes to “fix” it for them. While McKinlay hadn't expected his first finding, the next observation surprised him even more.

 “When we discovered that Z. mobilis could use N2 we expected that it would make less ethanol. N2 utilization and ethanol production demand similar resources within the bacterial cell so we expected resources to be pulled away from ethanol production to allow the bacteria to grow with N2,” McKinlay said. “To our surprise the ethanol yield was unchanged when the bacteria used N2. In fact, under certain conditions, the bacteria converted sugars to ethanol much faster when they were fed N2.”

The university has patented the process for using nitrogen-fed Z. mobilis to convert sugars from cellulose into ethanol. But this is not the big breakthrough cellulosic ethanol needs. The corn steep liquor and diammonium phosphate currently used are less expensive than the raw plant material and the enzymes that turn it into sugar, “But we recognize nitrogen fertilizers as a smaller, yet considerable, cost contributor that could potentially be more readily addressed,” said McKinlay. 

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