Biofuels represent a significant potential sustainable energy source, yet land-based biofuel crops pose land use challenges, using large stretches of arable land that could otherwise be utilized to produce food. Scientists are now turning to an ocean-based alternative: seaweed. Unlike other raw plant materials, brown seaweed does not contain a lignin structure. Biodegradation of lignin is a requirement for processing biofuel from raw plant materials. The sugars in brown seaweed can be released by simply milling or crushing the macroalgae, eliminating the need for a pretreatment and hydrolysis process before fermentation and lowering the potnential cost of bioconversion of the raw plant material into ethanol. Brown seaweed contains four types of sugars—alginate, laminarin, mannitol, and cellulose. Of these, alginate is most abundant. Previous attempts to convert macroalgae into biofuels were limited by the availability of microorganisms that could metabolize alginate polysaccharides. Wargacki et al. (2012) have engineered a microbial platform for direct ethanol synthesis of brown seaweed. The newly designed microbial platform can simultaneously degrade, uptake, and metabolize alginate to produce ethanol. The strain could potentially be engineered to produce a wide range of chemical and fuels. This project is a significant step in realizing production of renewable fuels from sustainable and scalable biomass sources. —Meredith Reisfield
Wargacki, A., Leonard, E., Win, M., Regitsky, D., Santos, C., Kim, P., Cooper, S., Raisner, R., Herman, A., Sivitz, A., Lakshmanaswamy, A., Kashiyama, Y., Baker, D., Yoshikuni, Y., 2012. An Engineered Microbial Platform for Direct Biofuel Production from Brown Macroalgae. Science 20, 308–313.
Wargacki et al. Engineered the bacterium Escherichia coli to digest brown seaweed and ferment ethanol. They isolated the genes of a marine microbe, Vibrio splendidus, that were responsible for breaking down alginate into simple sugars like pyruvate, and inserted the genes into an E. coli strain. This modification of E. coli allowed the bacterium to take up alginate oligomers which carried a stretch of DNA bearing genes for alginate-degrading enzymes. The strain was further engineered to convert sugars into ethanol and enable a single-step manufacture of ethanol from brown seaweed. The researchers added a chemical pathway borrowed from Zymomonas mobilis, a bacterium isolated from cane juice, to turn the pyruvate into ethanol.
When the new strain was fed crushed kombu (a common brown seaweed), the cells fermented a concentration of 5% ethanol, which is comparable to the benchmark for bioconversion of woody biomass. The U.S. Department of Energy reported a macroalgae productivity of 59 dry metric tons/ha/year, and an ideal ethanol yield from macroalgae of 0.254 weight ethanol / weight dry macroalgae. These numbers estimate an peak bio-ethanol productivity of 19,000 liters/ha/year, approximately twice the ethanol productivity from sugarcane and 5 times the ethanol productivity from corn. Initial evaluations of the E. coli platform in ethanol production achieved over 80% of the maximum theoretical yield of ethanol from sugars in seaweed. The engineered strain successfully processed alginate and glucose in addition to increasing the rate of mannitol fermentation. These results support the team’s hypothesis that alginate pathways play a significant role in balancing intracellular redox reactions by consuming excess-reducing equivalents generation in the fermentation of mannitol.
Brown seaweed seems to be an ideal source for the production of biofuels since it avoids the economic concerns associated with land management and the adverse effect on global food supplies, eliminating the “food versus fuel” concerns presented by the production of ethanol using sugar cane and corn. A few barriers remain to be conquered before we can take advantage of biofuels from beneath the waves. Seaweeds are currently harvested for use in human consumption, animal feeds, and agricultural fertilizers and are not currently grown at a sufficient scale for wide use as fuel. Although brown seaweed holds several advantages over currently existing methods to produce ethanol via microbial fermentation of biomass, the question remains whether seaweed can be produced at a scale that would have a significant impact on the global energy economy. The platform engineered by Wargacki, et al. is a significant step toward scalable and diverse feedstocks that would enable sustainable use of biomass technologies.