Tough Little Bug Smashes Down Wall Between Us and Sustainable Biofuel

The biofuel fuel vs. food debate has really been getting a workout this year, especially in the U.S. where a historic, devastating drought has put the squeeze on corn ethanol. More sustainable biofuel crops like perennial grasses and shrub willow are under development, but the sugars in plants like these are locked away behind tough, woody cell walls, and getting at them can be a costly process. Now researchers at Brown University have found a bacteria called Streptomyces, which could be deployed as a microscopic “biorefinery” to get the job done.

Biofuel from Bacteria

With soft biomass such as cow manure, a microbe-based biofuel process can be relatively quick and inexpensive, and it is already being deployed commercially with a push from the Department of Agriculture’s AgStar program. Microorganisms simply chew away at the organic matter, releasing methane gas as they go. The gas can be captured and used to power generators, or used directly to heat boilers.

Getting liquid biofuel or biofuel precursors from woody biomass is a whole different ballgame. Very few microorganisms can digest lignin, the polymer that puts the wood in woody (or grassy) biomass.

The Key to Biofuel from Woody Biomass

Streptomyces is one of those rare, wood-loving microorganisms, but knowing what it does is one thing. Getting it to work on a commercial scale is another thing entirely, kind of like the microbial version of herding cats, and that depends on a detailed understanding of the process.

The Brown team, headed by chemistry professor Jason Sello and biology professor Rebecca Page, had previously identified the genes in Streptomyces that encode enzymes, which the bacteria deploys to break down lignin.

Normally, the genes in question are dormant, but the team found that the they would switch on when the bacteria was exposed to a lignin-enriched environment, using a compound called protocatechuate. Exploring this phenomenon further, the team discovered that the “finger” on the switch was a protein called PcaV.

As described by Brown writer Kevin Stacey, the latest step in the research shows that PcaV really does work like a microscopic finger. Normally, PcaV binds itself to DNA and physically prevents the lignin-degrading genes from going into action. Once exposed to lignin, PcaV loses interest in DNA, exposing the genes, which then go to work on expressing the enzymes.

Once the enzymes break lignin into simpler carbon compounds, the rest is pretty clear sailing. Some of the carbon goes to sustain the bacteria, and the rest gets converted into triglycerides, which could then be processed into biodiesel or other products.

As to exactly why PcaV disengages from DNA in the presence of lignin, the team got into that level of detail, too. They identified an amino acid in PcaV called arginine-15, which plays a key role in keeping PcaV bound to DNA. In the presence of lignin, arginine-15 acts like a “molecular switch” that breaks the bond.

About that Willow Biofuel

Streptomyces has a long (long) way to go before it’s ready for its commercial biofuel close-up, but in the meantime woody biomass is already emerging as a more sustainable biofuel crop, particularly shrub willow.

As a perennial, willow resolves energy issues related to annual plantings. It can be grown on land that is too marginal for other crops, providing rural property owners with a long term, sustainable alternative to more destructive forms of revenue such as natural gas fracking.

Willow is also a hardy, drought-resistant plant. In fact, stress might be good for it, at least in terms of biofuel production: researchers in the U.K. are finding that windy conditions trigger a gene in willow that boosts sugar production, making it significantly more amenable to biofuel conversion.

Image: Streptomyces courtesy of Brown University

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