Researchers engineer E. Coli to produce ethanol directly from brown seaweed with yield equivalent to ~80% of the theoretical maximum
|Design of the microbial platform for production of biofuels and renewable commodity chemical compounds from macroalgae. Wargacki et al. Click to enlarge.|
A team of scientists from Bio Architecture Lab (BAL) in Berkeley, California, (earlier post) has engineered a strain of Eschericia coli to break down and then to ferment alginate—one of the most abundant sugars in brown algae, but a sugar that industrial microbes can’t metabolize—into ethanol. Their paper is featured on the cover of the 20 January issue of the journal Science.
When engineered for ethanol synthesis, the BAL microbial platform produced bioethanol directly from macroalgae via a consolidated process (CBP), achieving a titer of 4.7% volume/volume and a yield of 0.281 weight ethanol/weight dry macroalgae (equivalent to ~80% of the maximum theoretical yield from the sugar composition in macroalgae).
Macroalgae (seaweed) offers a number of attractive attributes as a feedstock for renewable fuels and chemicals (high sugar content, no requirements for arable land, fresh water, and fertilizer and no food vs. fuel issues. Furthermore, the team notes in their paper, because brown macroalgae does not contain lignin, sugars can be released by simple operations such as milling or crushing. About 60% of the dry biomass of the seaweed are sugars, with more than half of that being alginate, said Daniel Trunfio, Chief Executive Officer at Bio Architecture Lab.
This bio-architectural feature gives macroalgae a distinct advantage over lignocellulosic biomass, facilitates higher yields, and averts the need for energy-intensive pretreatment and hydrolysis processes before fermentation. An analysis prepared for the US Department of Energy (DOE) reports a macroalgae productivity of 59 dry metric tons/ha/year and an ideal ethanol yield from macroalgae of 0.254 weight (wt) ethanol/wt dry macroalgae. Based on these numbers, an optimum bio-ethanol productivity of 19,000 liters/ha/year is estimated. This value is approximately two times higher than the ethanol productivity from sugarcane and 5 times higher than the ethanol productivity from corn.
The most abundant sugars in brown macroalgae are alginate, mannitol, and glucan (glucose polymers in the form of laminarin or cellulose). Ethanol production from glucan and mannitol yields approximately 0.08 to 0.12 wt ethanol/wt dry macroalgae. However, the full potential of ethanol production from macroalgae cannot currently be realized because of the inability of industrial microbes to metabolize the alginate component.—Wargacki et al.
The BAL team developed a pathway to metabolize the alginate. Core to the work is the discovery of a DNA fragment from the bacterium Vibrio splendidus encoding enzymes for alginate transport and metabolism. The BAL team integrated this with other elements to generate a microbial platform that can simultaneously degrade, uptake, and metabolize alginate.
In the new platform, the alginate polymer is first degraded into oligomers (short fragments) by an alginate lyase secreted by the cells, the gene for which came from the bacterium Pseudoalteromonas sp. The oligomers are then transported into the E. coli using capabilities endowed by some of the V. splendidus fragment. Other V. splendidus genes contributed to metabolizing the alginate oligomers and allowed them to be converted into chemical building blocks. A pathway from Zymomonas mobilis turns that into ethanol.
BAL earlier was a beneficiary of the US Department of Energy’s Advanced Research Projects Agency - Energy (ARPA-E) awarded to DuPont, for the development of a process to convert sugars from seaweed into isobutanol.
BAL’s technology to ferment a seaweed feedstock to renewable fuels and chemicals has created an entirely new pathway for biofuels development, one that is no longer constrained to terrestrial sources. When fully developed and deployed, large scale seaweed cultivation combined with BAL’s technology promises to produce renewable fuels and chemicals without forcing a tradeoff with conventional food crops such as corn or sugarcane.—ARPA-E Program Director Dr. Jonathan Burbaum
In addition to work for DuPont, the development of BAL’s technology is also supported by the Concurso Nacional Grant provided by InnovaChile CORFO and Statoil. In 2010, BAL and Statoil formed a strategic partnership for the production of ethanol derived from macroalgae grown off the coast of Norway. (Earlier post.)
Adam J. Wargacki, Effendi Leonard, Maung Nyan Win, Drew D. Regitsky, Christine Nicole S. Santos, Peter B. Kim, Susan R. Cooper, Ryan M. Raisner, Asael Herman, Alicia B. Sivitz, Arun Lakshmanaswamy, Yuki Kashiyama, David Baker, and Yasuo Yoshikuni (2012) An Engineered Microbial Platform for Direct Biofuel Production from Brown Macroalgae. Science 335 (6066), 308-313. doi: 10.1126/science.1214547
Erik Stokstad (2012) Engineered Superbugs Boost Hopes of Turning Seaweed Into Fuel. Science335 (6066), 273. doi: 10.1126/science.335.6066.273