New technique uses most abundant gas on Earth to help create bioethanol

Zymomonas mobilis bacterium might be tricky to say, but this bioethanol-producing microbe could become a household name if Indiana University (IU) biologists have their way. The biologists claim to have found a quicker, cheaper, cleaner way to increase bioethanol production in this microorganism by using the most abundant element in the Earth’s atmosphere: nitrogen gas (N₂). By replacing chemical fertilizers with N₂, production costs could be slashed and cellulose ethanol derived from wood pulp made much more economically viable – so much so that the researchers believe it may compete with corn ethanol and gasoline on price.

Cellulose found in woody plants such as trees, grasses, and other inedible plant substances – like olive stones – is generally low in nitrogen, which makes cellulose all that much more difficult to convert given that nitrogen is a staple requirement for feeding ethanol-producing microbes. As a result, cellulosic ethanol makers spend many millions of dollars a year on nitrogen-rich fertilizers such as diammonium phosphate or corn-steep liquor.

In an unexpected outcome, an IU team led by biologist Dr James B. McKinlay inadvertently happened upon a possible solution to this problem when they experimented with the effects of nitrogen gas on the Zymomonas mobilis bacterium.

"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," said Doctor McKinlay. "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."

Quickly realizing that the bacterium was able to use N₂ without affecting the production of ethanol, the scientists reasoned that nitrogen gas may serve as a far cheaper replacement for nitrogen fertilizers used in the creation of cellulosic ethanol.

“Until recently, ethanol has been produced almost entirely from food crops, but last year there was a surge in cellulosic ethanol production as several commercial facilities opened,” said Dr McKinlay. “Cellulosic ethanol offers more favorable land use and lower carbon emissions than conventional ethanol production. Even so, cellulosic ethanol is struggling to be cost-competitive against corn ethanol and gasoline.”

The researchers also noted that while the largest costs in cellulosic ethanol production are the biomass and the enzymes required to breakdown the plant matter into sugars, the enormous amounts of nitrogen-rich chemical fertilizers used in the fermentation vats of biofuel producers to improve yield are also represent a significant and on-going cost.

As a result, the scientists estimate that using N₂ in place of solid nitrogen-enriched fertilizer could help save an average cellulosic ethanol production plant somewhere in the region of a million dollars a year. Added to the fact that the N₂ could easily be produced at the facility, there would also be a potential reduction in transportation costs, and a lowering of greenhouse gas emissions.

“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,” said Dr McKinlay.

The work has resulted in a provisional patent being filed with the United States Patent and Trademark Office. The research was published in the journal Proceedings of the National Academy of Sciences.

Source: Indiana University