Californian researchers enlist landfill bacteria and yeast to make methyl halides
A CELLULOSE-eating microbe discovered in a French landfill site in the 1980s may soon become a profitable source of fuel and chemicals made from biomass.
Using the microbial strain Actinolea fermentans, combined with genetically modified yeast, researchers at the University of California San Francisco (UCSF) Department of Pharmaceutical Chemistry were able to produce methyl halides from biomass such as unprocessed switchgrass, corn stover, sugar cane bagasse and poplar.
Derived from petrochemicals, methyl halides are used as soil fumigants and as precursors that can be converted into gasoline, olefins, aromatics, alcohols and ethers.
Methyl halides are produced by naturally occurring plants and organisms, such as marine algae and fungi, but in very low yield, says UCSF associate professor Christopher Voigt. This is the first time methyl halides have been produced from biomass in higher yield. "You can make benzene, toluene, xylene, ethylene, propylene, methanol and dimethyl ether from biomass-based methyl halides and it could fit into the chemical supply chain," he says. "We want to create a building-block molecule that could easily connect with what already exists in petrochemicals rather than try to create new molecules and new pathways."
Using synthetic biology and metagenomics, the UCSF found an enzyme called methyl halide transferase in a plant called turtleweed, or saltwort. The enzyme, which converts acetate to methyl halides, was genetically engineered into yeast, a versatile and robust organism for the industrial production of chemicals and fuels.
To nourish the new yeast, the team turned to the bacterium A. fermentans. Virtually unique among cellulolytic bacteria, A. fermentans is able to digest biomass and convert most of the carbon to acetate.
Just as important, the bacterium was compatible with the yeast. Most cellulolytic microbes become efficient only at a relatively high temperature, but A. fermentans has optimal growth rates at 30˚C (86˚F) - the same as yeast, Voigt notes.
The two comprise a symbiotic pair. If the bacterium is cultured alone, it will be inhibited by the accumulating acetate, which is toxic. To the yeast, however, acetate is food, which it digests, producing a valuable stream of methyl halides. "We have demonstrated that methyl halides can be produced at high titer from different forms of biomass using industrially relevant organisms," says Voigt.
"This approach differs from other engineered examples of cellulose digestion such as corn ethanol, where its economy is stuck between just two commodities - corn as an input, and ethanol as an output. Our technology produces flexibility in biorefining in terms of feedstock and chemical products."
The next step for UCSF is to improve yields and rates by altering the yeast's genes.
Voigt says they are still about one-fifth of the way to where they need to be before getting an optimum methyl halide production. "There are also unique challenges in terms of the scalability of the fermentation and chemical catalysis of the methyl halide," he adds.
Compared with the current sugar-to-ethanol process being developed, the UCSF process is still said to be 40 times slower. Assuming its system could be as efficient, Voigt calculates that it could produce cheaper gasoline from methyl halides than from oil.
Although it is difficult to create reliable cost models, as there are no cellulosic fuel crops in production yet, Voigt roughly estimates that their system could produce gasoline from sugarcane bagasse at $1.65/gal, while other biomass products, such as poplar could be cheaper, at around $1.10-1.30/gal.
The project is currently funded by private foundations, although Voigt says they are turning to different agencies and companies for their next round of research. "We've received a lot of interest and we are looking forward to working with them," says Voigt.