An international team of researchers from Bio Architecture Labs, a synthetic biology and enzyme design company focused on the production of biofuels and biochemicals from macroalgae (seaweed) (earlier post), reports the development of a synthetic yeast platform based on Saccharomyces cerevisiae that can efficiently produce ethanol from brown seaweed; the paper is published in the journal Nature.
In January 2012, BAL scientists reported the engineering a strain of Eschericia coli that could break down and then ferment alginate—one of the most abundant sugars in brown algae, but a sugar that industrial microbes can’t metabolize—into ethanol. That paper was featured on the cover of the journal Science. (Earlier post.)
Brown macroalgae exhibit several features of an ideal feedstock that can complement the increased global demand on energy and food production. The cultivation of this biomass does not require arable land, fresh water or fertiliser, circumventing adverse impacts on food supplies and resource availability. Large-scale cultivation of brown macroalgae is already being practiced in several countries, yielding over 70 million metric tons per year in 2006. Because brown macroalgae do not contain lignin, simple biorefinery processes such as milling, leaching and extraction can separate the sugars for conversion into biofuels and renewable chemicals.
… The most abundant sugars in brown macroalgae are alginate, mannitol and glucan (present as laminarin and cellulose). Conventional industrial microbes can use mannitol and hydrolysed glucan. However, the full potential of biofuel and renewable chemical production from brown macroalgae cannot be realized unless alginate is co-fermented. Alginate composes 30–60% of the total sugars in brown macroalgae, so the inability of industrial microbes to catabolise alginate results in a substantial loss of product yield. Additionally, the excess reducing equivalents produced by ethanol fermentation from mannitol can be redox-balanced by alginate catabolism, which would otherwise require electron shunts such as oxygen. Thus, enabling the co-fermentation of alginate and mannitol in an existing industrial microbe is a key criterion for the economic and efficient use of the sugars derived from brown macroalgae.—Enquist-Newman et al.
While the engineering of the E. coli system provided a “compelling proof-of-principle example” that may be suitable for higher value renewable chemicals, the team noted in the new Nature paper, S. cerevisiae “is a more amenable hos for commercial-scale fuel ethanol production and is the standard microbe in the bioethanol industry.”
Enabling ethanol production from the brown macroalgae using S. cerevisiae platform requires the engineering of both the mannitol and alginate catabolic pathways.
The BAL scientists discovered an alginate monomer (4-deoxy-L-erythro-5-hexoseulose uronate, or DEHU) transporter from the alginolytic eukaryote Asteromyces cruciatus.
They integrated and overexpressed the gene encoding this transporter into an S. cerevisiae strain, along with the necessary bacterial alginate and deregulated native mannitol catabolism genes. This engineering enabled the yeast strain efficiently to metabolize DEHU and mannitol.
Further adapted to grow on mannitol and DEHU under anaerobic conditions, the yeast was capable of ethanol fermentation from mannitol and DEHU, achieving titers of 4.6% (v/v) (36.2 g l−1) and yields up to 83% of the maximum theoretical yield from consumed sugars.
These results show that all major sugars in brown macroalgae can be used as feedstocks for biofuels and value-added renewable chemicals in a manner that is comparable to traditional arable-land-based feedstocks.—Enquist-Newman et al.
Brown seaweed biorefinery. In the Supplementary Information accompanying the paper, the BAL team outlines a “brown macroalgae biorefinery” process. The concept is similar to the corn wet mill process in which dry corn gluten feed, corn gluten meal, corn oil, sugars (starch, fructose (i.e., high-fructose corn syrup), and dextrose), ethanol, and other renewable chemicals are simultaneously produced.
Brown seaweed contains large quantities of potassium chloride (15-35% of total dry weight), which is a major ingredient of potash fertiliser (>97%), and proteins (7- 15% of the total dry weight), which is an important ingredient of animal feed. Both products have sufficient market sizes to accommodate co-products produced from a brown macroalgae biorefinery process at commercial scale.
BAL, in collaboration with Harris Group, Inc. and EcoShift Consulting performed a technoeconomic assessment and life cycle assessment following discussions with the US Environmental Protection Agency (EPA) and US Department of Energy (DOE) Advanced Research Projects Agency-Energy (ARPA-E) for a brown macroalgae biorefinery process.
These analyses showed that ethanol can be produced from brown macroalgae in a manner that is cost competitive with ethanol produced from alternative sources, and up to 63.6% of greenhouse gas (GHG) emissions could be reduced compared to conventional gasoline production and consumption. Therefore, the ethanol produced from brown macroalgae can meet the “advanced biofuel” standard based on the US EPA Renewable Fuel Standard.
Maria Enquist-Newman et al. (2013) “Efficient ethanol production from brown macroalgae sugars by a synthetic yeast platform,” Nature doi: 10.1038/nature12771