WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- Propulsion researchers have pinpointed algae as a promising wellspring for producing new alternative fuels, the military certification of which ranks high among Air Force priorities. In evaluating the capacity of non-petroleum-based products to meet operational performance demands while effecting comparable costs and a smaller carbon footprint, AFRL has progressed beyond initial research involving Fischer-Tropsch fuels made from coal, biomass, and natural gas to more recent investigations of hydrotreated renewable jet fuel, a hydrocarbon aviation fuel rendered from animal fat or plant oils (i.e., triglycerides) via hydroprocessing. The ensuing discovery of algae as a viable feedstock for generating the large amounts of oil needed to make HRJ fuel is thus a major milestone towards realizing a high-performance, cost-effective, environmentally sound fuel product.

Any alternative fuel approved for military use must equal or surpass conventional products in terms of capability, cost, and conservation (ecological sustainability). Various forms of algae have demonstrated this potential, specifically in their capacity to produce oil in the quantities required for effective hydrotreating. Despite their relatively high oil output per acre--a process that consumes carbon dioxide and does not compete with the food market--algae are not without drawbacks, one of which is the tendency for variable lipid accumulation between species, meaning that oil accumulation must sometimes be induced.

Accordingly, the goal of this project was to demonstrate the feasible use of chloroplast genetic engineering to alter lipid metabolism in the alga Chlamydomonas reinhardtii, thereby increasing fatty acid biosynthesis and lipid accumulation. This system could potentially be applied to different algae species. Lipid synthesis in green algae is a tightly regulated process in the chloroplast that is controlled at the acetyl-CoA [coenzyme A] carboxylase (ACCase) enzyme level. Over-expression of ACCase can increase fatty acid and lipid accumulation in algae and plants. Another limiting factor is the lack of acetyl-CoA in chloroplasts.

Under AF sponsorship, Dr. Oscar Ruiz, from the Inter American University of Puerto Rico-Bayamon, developed a heterologous algae system by genetically engineering the chloroplast genome with the acc1 and acs1 genes, which code for acetyl-CoA carboxylase and acetyl-CoA synthetase, respectively. These key enzymes then underwent production free of cellular control, which enabled continuous biosynthesis of fatty acids and lipid accumulation. Subsequent gas chromatography analyses demonstrated that the modified algae lines had up to 5.9% more lipids by weight than the native Chlamydomonas cells. This system could potentially decrease algae-derived fuel costs by maximizing oil production in algae species with naturally high oil content or those that are well adapted to different environments. Dr. Ruiz was also able to increase the production of medium-chain fatty acids and the accumulation of lipids in tobacco plants by using chloroplast genetic engineering. Because medium-chain carbon molecules are the main constituent of jet fuel, a genetically engineered plant that produces medium-chain fatty acids would potentially help reduce the cost of jet fuel production by reducing the processing required to convert lipids into fuel.