Cyanobacteria are a large group of oxygenic photoautotrophic bacteria and, like plants and algae, can capture CO2 via the Calvin-Benson-Bassham cycle and convert it to a suite of organic compounds. They are important primary producers of organic material and play significant roles in biogeochemical cycles of carbon, nitrogen, and oxygen [1, 2]. Through their photosynthetic capacity cyanobacteria have been tremendously important in shaping the course of evolution and ecological change throughout Earth's history, and they continue to contribute to a large share of the total photosynthetic harnessing of solar energy and assimilation of CO2 to organic compounds. For example cyanobacteria account for 30% of the annual oxygen production on Earth. Our oxygenic atmosphere was originally generated by numerous cyanobacteria during the Archaean and Proterozoic Eras. Many cyanobacteria are diazotrophs and can assimilate atmospheric N2 and convert it to organic matter. Cyanobacteria occupy a wide array of terrestrial, marine, and freshwater habitats, including extreme environments such as hot springs, deserts, bare rocks, and permafrost zones. In their natural environments, some cyanobacteria are often exposed to the highest rates of UV irradiance known on our globe. Cyanobacteria are Gram-negative bacteria but they combine properties of both Gram-negative and Gram-positive bacteria; they contain an outer membrane and lipopolysaccharides (LPS), defining characteristics of Gram-negative bacteria, and a thick, highly cross-linked peptidoglycan layer similar to Gram-positive bacteria.
Cyanobacteria and eukaryotic microalgae (often collectively referred to as “algae” or “microalgae”) hold particular interest in the biofuel sector since they can tolerate high CO2 levels such as flue gas streams [2], and may be superior to higher plants in energy efficiency, biomass and oil productivity, and land and water usage. Since a large number of cyanobacterial and microalgal species are halophilic they can be grown in seawater, saline drainage water, or brine from petroleum production and refining industry or CO2 injection sites, thereby sparing freshwater supplies. The use of potable water for cultivation can also be avoided by utilizing municipal wastewater as a nutrient source. Further, 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.
As opposed to eukaryotic microalgae that can accumulate large amounts of triacylglycerols (TAGs) as storage lipids, the cyanobacteria studied to date produce little or no TAGs but their fatty acids (FAs) are directly shuttled to membrane lipid synthesis (Fig. 1). On the other hand, cyanobacteria are well suited for approaches aimed at redirecting carbon flux in lipid metabolism to specific biofuel molecules. First, whereas in plants and algae, including microalgae, lipid metabolism involves several different cellular compartments, in cyanobacteria, all metabolism occurs via soluble or membrane-bound enzymes in the cytoplasm. Second, being bacteria, cyanobacteria are well suited for synthetic biology and metabolic engineering approaches for phototrophic production of various desirable biomolecules, including high-density liquid biofuels such as alkanes, alkylesters (Fig. 1) and isoprenoids.
The main objectives of the CyanoFuels project are to:
1. Design strains of cyanobacteria that can capture flue gas-concentrations of CO2 (~15% CO2) for photosynthetic conversion to high levels of advanced biofuels.
2. In collaboration with Cheryl Kerfeld (JGI) survey the vast diversity of cyanobacterial species for identification of novel gene clusters of interest for biofuel synthesis.
3. Demonstrate the applicability of the SR-FTIR/MS platform developed by Hoi-Ying Holman (ALS, ESD) for single-cell metabolic fingerprinting of engineered cyanobacteria in real time.
Our work to date has focused on the cyanobacterium Synechocystis6803 (S. 6803). We have now completed the construction of more than 20 engineered strains of S. 6803, designated the S. 6803-FUEL series, as part of generating S. 6803 strains for high-yield accumulation of defined alkanes. For example, by introduction of a FAD-FAR operon behind the stron S. 6803 PpsbA2 promoter we could demonstrate a 5-fold increase in C17 -alkane (heptadecane) production [3]. We could also demonstrate the applicability of SR-FTIR spectromicroscopy as a powerful means for metabolic screening and phenotyping of live individual cells
References
[1] Jansson, C. (2011) Metabolic engineering of cyanobacteria for direct conversion of CO2 to hydrocarbon biofuels. Progr. Bot. 73, 81-93.
[2] Jansson, C., and Northen, T. (2010) Calcifying cyanobacteria - the potential of biomineralization for carbon capture and storage. Curr. Opin. Biotechnol. 21, 365-371.
[3] Hu, P., Borglin, S., Kamennaya, N.A., Chen, L., Park, H., Mahoney, L., Kijac, A., Shan, G., Chavarría, K.L., Zhang, C., Quinn, N.W.T., Wemmer, D., Holman, H.Y., and Jansson, C. (2012) Metabolic phenotyping of the cyanobacterium Synechocystis 6803 engineered for production of alkanes and free fatty acids. Applied Energy, in press.