Seaweed study fuels bioenergy enthusiasm

Engineered E. coli can convert cell wall component into ethanol

Seaweed has long made biofuel prospectors drool, but they hadn’t figured out how to efficiently chew through the stuff — until now. Researchers have engineered a bacterium that can break down and digest seaweed’s gummy cell walls to yield ethanol and other useful compounds. If scientists can make the process work at larger scales, seaweed could soon be a serious contender as a source of renewable fuel.

OFFSHORE ENERGY Scientists have engineered a bacterium that breaks down seaweed, such as this Macrocystis species, to yield fuels such as ethanol. Bio Architecture Lab Inc.

The new research “makes a pretty large leap forward,” says metabolic engineer Hal Alper of the University of Texas at Austin. Unlike corn and many other biofuel feedstocks, seaweed doesn’t need arable land, fertilizer or freshwater. If seaweed can be efficiently munched into ethanol, it broadens the biofuel horizon, says Alper, who was not involved in the research. Seaweed, he says, may be “that new source for unconventional carbon that everyone’s been looking for.”

Scientists from Bio Architecture Lab, a biofuel and renewable chemicals company headquartered in Berkeley, Calif., were interested in creating a biofuel bacterium that is a one-stop shop: They wanted a microbe that could efficiently digest the seaweed cellular building block alginate without pretreatment with chemicals or heat. Alginate is commonly used in ice creams and some textiles, but has proved difficult to break down and metabolize into fuel.

So, led by synthetic biologist Yasuo Yoshikuni, the team took the workhorse bacterium E. coli and within it, patched together the genes and other parts needed for superior seaweed-to-fuel conversion. From the marine bacterium Pseudoalteromonas, the scientists used genes for an enzyme that cuts alginate into smaller molecular bits. The researchers rigged these genes to a cellular transport system already found in E. coli, so the bacterium would secrete the alginate-slicing enzyme into its environment.

Then the team scoured scientific literature and databases for a microbe with serious alginate-digesting machinery. They hit upon the marine microbe Vibrio splendidus, and took a hefty chunk of the Vibrio DNA for use in the E. coli. When the team fed alginate to their engineered E. coli, the microbes pumped out ethanol, the researchers report in the Jan. 20 Science. The system yields 80 percent of the theoretical maximum amount of ethanol for a given amount of biomass, the scientists noted, and with further tweaking will probably be even more efficient.

Part of the beauty of the system is its flexibility, says Yoshikuni. Because the alginate-degrading enzyme is released into the environment, initial breakdown products can easily be harvested for creating useful compounds such as precursors to nylon or plastics. And when E. coli consume the broken-down alginate the bacteria generate a lot of pyruvate, a chemical intermediate useful for making fuels such as butanol or biodiesel. 

Seaweed is already harvested at commercial scale in several countries for other uses, and Bio Architecture Lab is working on a pilot plant in Chile to convert seaweed into fuel, says company CEO Daniel Trunfio. Also, any seaweed will do, he notes. “We like to say we’re seaweed agnostic — we can process any brown algae.”

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