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Direct Biofuel Production from Brown Macroalgae

30 January 2012

Prospecting macroalgae (seaweeds) as feedstocks for bioconversion into biofuels and commodity chemical compounds is limited primarily by the availability of tractable microorganisms that can metabolise alginate polysaccharides, according to a research paper featured in Science Magazine.

Volatile energy costs and pressure to conserve fossil fuel resources have ignited efforts to produce biofuels and renewable commodity chemical compounds via microbial fermentation of biomass. Pursuant to these goals, microbial engineering aims to increase product yields and bioconversion efficiencies.

Equally critical, development of scalable and diverse feedstocks will empower sustainable use of this technology and drive the widespread adoption of renewable bio-economies.

At present, corn and sugarcane are vetted industrial feedstocks, but “food versus fuel” concerns may preclude their long-term use. Inedible lignocellulosic plant materials are preferable feedstocks, but current microbial technologies for fermentation of the simple sugars in lignocellulose have yet to overcome the cost of the complex processes needed to release these sugars from recalcitrant polysaccharides. Therefore, distinct strategies are required to develop scalable and sustainable non-lignocellulosic biomass resources such as marine macroalgae (seaweeds) for use as next-generation feedstocks.

Brown macroalgae exhibit several key features of an ideal feedstock for production of biofuels and renewable commodity chemical compounds. Requiring no arable land, fertilizer, or fresh water resources, cultivation of these crops circumvents economic concerns associated with land management and avoids adverse impacts on food supplies.

Macroalgae are already grown for human consumption, but not as a staple crop. Large-scale cultivation is practiced in several countries, yielding 15 million metric tons per year; in these countries, macroalgae are also used as animal feeds, agricultural fertilizers, and sources of polymers.

Because brown macroalgae does not contain lignin, sugars can be released by simple operations such as milling or crushing. 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 U.S. 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.

For example, fermentation of glucan in Saccharina latissima by Saccharomyces cerevisiae produced ~0.45 per cent volume/volume (v/v) ethanol). Compared with glucose, the catabolism of mannitol generates excess reducing equivalents, causing an unbalanced reduction-oxidation (redox) environment under fermentative conditions.

Hence, ethanol production from mannitol is feasible only in the presence of electron shunts (such as micro-aerobic conditions). Semifermentative conditions enabled ethanol production from mannitol by Zymobacter palmae with a yield of 0.38 wt ethanol/wt mannitol.

Additionally, this strain cometabolized glucan and mannitol, producing ~1.6 per cent v/v from these sugar fractions in Saccharina hyperborea under micro-aerobic conditions.

In contrast to mannitol, each mole unit of alginate fermented to ethanol consumes two reducing equivalents. Therefore, the catabolic pathway of alginate provides both an additional source of sugars and a counterbalance to the excess-reducing equivalents produced by mannitol catabolism, enabling ethanol fermentation from all three sugar components in macroalgae simultaneously.

January 2012

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