Cyanobacterial Biodiesel: Tubes in the Desert

Corn-based ethanol or soy/palm oil–based biodiesel are inherently limited in terms of per-hectare yields and suitable land area for development. However, algae and photosynthetic bacteria overcome these constraints for biofuels by providing a roughly 100-fold advantage in yield (Table 1) and not requiring farmland for production (i.e., they do not compete with food crops). A major ongoing ASU effort, funded in its initial pilot stages by BP and Science Foundation Arizona, generates biodiesel from lipid produced by the photosynthetic cyanobacterium Synechocystis. Cyanobacteria are much more amenable to metabolic engineering to improve biofuel productivity than are eukaryotic algae. The genome of Synechocystis has been fully sequenced, and the microorganism provides a facile substrate for genetic modification of metabolic pathways to optimize yields of C-16 and C-18 lipids for biodiesel production. In fact, much progress has already been made at ASU in increasing the yields of these lipids through genetic engineering. The current projection is for a proof of concept demonstration of the industrial scale utility of this approach in Calendar Year 2008. Beyond this demonstration, a larger field test bed will be built to refine the approach and enable industrial pilot scale efforts in the 2010 timeframe.

This platform for renewable solar energy-to-biofuels conversion combines innovative metabolic engineering with state-of-the-art, large-scale bioprocess engineering, efficient cell harvesting, cost-effective conversion of lipid to biodiesel, and generation of other valuable byproducts. This is possible because Synechocystis is fast growing and robust in accommodating diverse environmental conditions. It can be cultivated over a wide range of salt and fixed-nitrogen concentrations and at CO2 levels up to 5 percent. The system also requires minimal water consumption. These traits make the microorganism well suited for growth using flue gas effluent from power plants as a carbon source (recapturing the carbon dioxide from the plant before release into the atmosphere) and using agricultural run-off water contaminated with nitrogenous fertilizer as a fixed-nitrogen source when it is available. When N-contaminated water is not used, fixed nitrogen can be recycled so little new nitrogen will need to be added.

This renewable solar energy-to-biofuels approach is very well suited to arid regions with high levels of sunlight, and Central Arizona is ideal for this purpose. Biofuel production from cyanobacterial photobioreactors should be scalable to a point where it represents a major source of carbon-neutral fuel for the United States, as well as high-quality employment and overall economic growth in the State.
In order to demonstrate the feasibility of biofuel production, our current project involves the production of laboratory–scale photobioreactors, while simultaneously designing and implementing a rooftop photobioreactor, where we will then apply mathematical modeling tools for systems analysis. We plan to address issues associated with bioreactor scale-up prior to introducing the improved strains and equipment into the large-scale field test bed bioreactor for final validation. The current plan involves scaling to a point where, in two years time, we will have designed and fabricated a field-scale bioreactor. This will allow our laboratory-scale organism optimization to be evaluated for suitability in larger scale bioprocess production under "real-world conditions." The test-bed photobioreactor will be located at an APS power plant close to the ASU-Tempe Campus. This location will provide a secure site and will enable engineering assessment of operating the photobioreactor system using flue gas and water recycling from the power plant for biomass production. The proposed research—with its coordination of genetic improvement, testing at the pilot scale, and industry partners—will create a unique setting in which dramatic advances can be realized in a relatively short time.