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IU researchers find Z. mobilis can use N2 gas in cellulosic ethanol production; potential major cost savings

Researchers at Indiana University have shown—for the first time to their knowledge—that the ethanol-producing bacterium, Zymomonas mobilis, can use nitrogen (N2) gas in lieu of traditional nitrogen supplements. The finding, reported in a paper in Proceedings of the National Academy of Sciences (PNAS), could make cellulosic ethanol more competitive with corn ethanol and gasoline.

The raw materials for cellulosic ethanol are low in nitrogen, a nutrient required for ethanol-producing microbes to grow, so cellulosic ethanol producers are estimated to spend millions of dollars annually on nitrogen fertilizers such as corn steep liquor and diammonium phosphate. The IU team led by biologist James B. McKinlay showed that Z. mobilis can use N2 as a nitrogen source, something that the more traditional ethanol-producer, baker’s yeast, cannot do.

Despite being an electron-intensive process, N2 fixation by Z. mobilis did not divert electrons away from ethanol production, as the ethanol yield was greater than 97% of the theoretical maximum. In a defined medium, Z. mobilis produced ethanol 50% faster per cell and generated half the unwanted biomass when supplied N2 instead of ammonium.

—Kremer et al.

Knowing the bacterium could use N2 without hindering ethanol production, the team reasoned that N2 gas could serve as an inexpensive substitute for nitrogen fertilizers during cellulosic ethanol production.

In a cellulosic feedstock-derived medium, Z. mobilis achieved a similar cell density and a slightly higher ethanol yield when supplied N2 instead of the industrial nitrogen supplement, corn steep liquor. We estimate that N2-utilizing Z. mobilis could save a cellulosic ethanol production facility more than $1 million/y.

—Kremer et al.

Using N­2 gas could also have environmental benefits such as avoiding carbon dioxide emissions associated with producing and transporting the industrial fertilizers.

More work needs to be done to assess how this approach can be integrated and optimized on an industrial scale, but all of the data we’ve collected thus far are very encouraging.

—James McKinlay

A provisional patent has also been filed in relation to the study with the United State Patent and Trademark Office, McKinlay said.

Funding for the research was provided by an Oak Ridge Associated Universities Ralph E. Powe Junior Faculty Enhancement Award, the US Department of Energy grant through the Office of Science Biological and Environmental Research program, and by the College of Arts and Sciences at IU.

Resources

  • Timothy A. Kremer, Breah LaSarre, Amanda L. Posto, and James B. McKinlay (2015) “N2 gas is an effective fertilizer for bioethanol production by Zymomonas mobilis”, PNAS doi: 10.1073/pnas.1420663112

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