Researchers have investigated the possible use of algae for biofuel production for many years, dating back at least to efforts by the U.S. Department of Energy in the 1970s. These early efforts failed to result in production technologies that were economically competitive with petroleum-derived fuels, and so the field stagnated. However, recent years have seen an increased interest in the use of algae to produce biodiesel and other fuels. Within the past couple of years, most of this renaissance was fueled by news of several big business deals involving algal biofuel firms, particularly the research partnership between ExxonMobil and Synthetic Genomics for development of improved algal strains using synthetic biology. With this renewed interest has come increased activity in the application of biotechnology to improve the strains of algae that might be used in biofuel production, thus providing process improvements that could make such methods economically competitive. I’ll briefly summarize some of the biotechnology strategies that are being used or that have been proposed, again with the caveat that this is cannot be a comprehensive scientific review. Among published reviews are Rosenberg et al. 2008, Li et al. 2008; Angermayr et al. 2009 and Mayfield (undated).
Of the three classes of organism that are amenable to improvement for biofuel purposes, algae are the laggards with regard to the development of technologies to enable reliable, stable genetic transformation. A number of algal strains can be genetically engineered, and in fact, engineered algae have long been proposed for use in industrial production of pharmaceuticals and other high value products. But according to Rosenberg et al, “routine transformation has only been achieved in a few algal species” in spite of significant and ongoing progress in extending such techniques to other species. Techniques exist, or are under development, for targeting genetic changes either to nuclear DNA or chloroplast DNA of microalgae species.
The biofuel strategy most often contemplated for algae is the large-scale production of biodiesel, jet fuel, and other petroleum-derived fuels. Biodiesel is usually composed of a mixture of fatty acid methyl esters, which can be produced from any biologically-derived source of fatty acids, with the final product created by a transesterification reaction. Mixtures of these esters can mimic the composition of petroleum-derived fuels such as diesel, jet fuel and others, which are complex mixtures of linear and branched hydrocarbons and cyclic alkanes. So, most genetic engineering strategies contemplated for algae involve enhancing or altering natural lipid biosynthetic pathways, to create a mixture of fatty acids suitable for conversion to biodiesel or other hydrocarbon-based fuels. One example (described in Rosenberg et al) is to transform an algal strain to express one or more enzymes involved in lipid biosynthesis – the example given in Rosenberg et al. is the enzyme acetyl-CoA carboxylase, which in early experiments was engineered into an algal strain, albeit with little impact on lipid biosynthesis. It has been reported that Aurora Biofuels is pursuing a similar approach to the enhancement of lipid synthesis in algae.
Other approaches to engineering algae are described in the table below. Such ideas include finding ways to enhance the efficiency of photosynthesis in algal production strains (e.g. by engineering the strains to synthesize large amounts of photoreceptor molecules), or to enable algae to utilize alternative food sources, particularly the ability to metabolize and ferment sugars. Solazyme has conducted research in these fields, and has pending patent applications relating to engineering light utilization and to alternate feedstocks for industrial algae strains. One unique approach that has been taken by Algenol Biofuels is to engineer cyanobacteria to express the enzymes pyruvate decarboxylase and alcohol dehydrogenase, allowing the algae to convert the common Krebs cycle molecule pyruvate first to acetaldehyde and then to ethanol, a strategy which the company is beginning to commercialize. Finally, it is also possible that genetic strategies might be used to attack what is a significant problem facing the use of algae to produce biodiesel: the need to separate the lipid products of the organisms from the aqueous medium in which the algae are grown. This often requires costly, energy-intensive physical processes, so that finding a biological method for secretion and sequestration of lipids from microalgae could be a very important development for the biofuels industry. Synthetic Genomics is reportedly trying to accomplish this goal using its synthetic biology expertise.
Genetic Engineering Strategies for Algae.
- Enhance algal growth rate.
- Enhance or alter lipid biosynthesis.
- Enhance photosynthesis.
- Enable use of alternate food sources.
- Enable secretion of lipids to aid oil/water separation.
The renewed interest in algal biofuels over the past several years has led to a lot of activity in the field, with many companies operating, building, or announcing plans to build pilot or demonstration plants, many of which might use algae modified using some of these approaches. I’ll discuss the companies in this sector of the industry, their technologies and commercial plans in later installments of this blog.
D. Glass Associates, Inc. is a consulting company specializing in several fields of biotechnology. David Glass, Ph.D. is a veteran of nearly thirty years in the biotech industry, with expertise in patents, technology licensing, industrial biotechnology regulatory affairs, and market and technology assessments. This blog provides back-up and expanded content to complement a presentation Dr. Glass made at the EUEC 2010 conference on February 2, 2010 entitled “Prospects for the Use of Genetic Engineering in Biofuel Production.” The slides from that presentation are available at www.slideshare.net/djglass99.