The employment of cyanobacteria for biofuel production is in its infancy. Most of the attention in the algal biofuel space is devoted to eukaryotic microalgae, mainly because of their capacity to store large amounts of TAGs. However, the successful biosynthesis of FA ethylesters (FAEE; a biodiesel) and hydrocarbon fuels in E. coli (Kalscheuer et al. 2006; Beller et al. 2010; Schirmer et al. 2010; Steen et al. 2010) suggests that similar strategies in pathway engineering should prove achievable also in cyanobacteria, where sunshine and CO2, rather than organic feedstocks, will serve as energy and carbon source, respectively. Furthermore, cyanobacteria have previously been engineered to produce alcohol-based fuels such as ethanol and isobutanol (Deng and Coleman 1999; Atsumi et al. 2009).
The capacity of cyanobacteria to thrive in high CO2 concentrations makes them an attractive system for beneficial recycling of CO2 from point sources such as coal-fired power plants via biofuel synthesis. Since many cyanobacteria are halophilic, raceway ponds can be sited away from agricultural land, making use of seawater or various sources of saline wastewater. A conceivable future scenario where CO2 recycling is combined with utilization of brine produced from CO2 injections during geological carbon capture and storage (CCS) is shown in Fig. 4.
Although the photoautotrophic cyanobacteria and microalgae offer obvious advantages over heterotrophic microorganisms and plants in biofuel synthesis, they also present several challenges that need to be addressed. (1) For example,
photosynthesis occurs only in light, which begs the question of how day-night cycles will affect biofuel production and accumulation; (2) A related issue is the efficiency of light harvesting and how to avoid light limitation and photoinhibition; (3) Whereas cultivation in raceway ponds requires measures for crop protection, growing cells in more controlled environments such as photobioreactors is currently all but cost prohibitive. Thus, how to prevent or mitigate contamination and grazing in open pond systems becomes an important question; (4) A major cost in the algal biofuel industry is associated with harvesting and extraction, and strategies that facilitate, or obviate the need for, these steps need to be further developed. One solution is to use filamentous or self-flocculating strains to expedite harvesting. Another approach is to achieve release of the biofuel molecules to the medium, either through cell lysis or by secretion. An example of the former is a nickel-inducible lysis system reported for S. 6803 (Liu and Curtiss 2009). The feasibility of secretion was illustrated by the release of free FAs from S. 6803 and Synechococcus elongatus PCC 7942 cells to the medium after inactivation of the AAS gene (Kaczmarzyk and Fulda 2010).
Acknowledgments This work was supported in part by U. S. Department of Energy Contract DE-AC02-05CH11231 with Lawrence Berkeley National Laboratory. Funding from the DOE-LDRD grant CyanoAlkanes is acknowledged.
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