Diesel from Cyanobacteria

The diminishing oil reserves and increasing concern over CO2 emissions from fossil fuels has lead to a lot of research in finding a feasible alternative. One possible alternative are biofuels. Biofuels can be produced from plants such as corn and sunflowers by fermenting them into alcohol to go into biodiesel. One reason why this has become increasingly popular is that the CO2 released from combustion is theoretically offset by the CO2 fixated by growing more crops for fuel. It has attracted criticism however over its competition with crop production for food and its limited seasonal growth.

A different option for biofuel production is by using photosynthetic microbes. Many species of algae can convert sunlight and atmospheric CO2 into highly reduced molecules like triacylglycerols and other energy storage compounds. The benefits of using algae over plants are they do not compete with the food industry and grow far more efficiently. Until recently the method used for harvesting lipids from these cells is by killing them, drying them out and extracting lipids with solvents. This is termed biomass recovery and is sadly expensive, accounting for 70-80% of production costs.

Liu et al (2011) developed a different method of harvesting lipids from microbes. Instead of algae, genetically modified cyanobacteria were developed that could continuously secrete fatty acids into the culture medium. Although cyanobacteria do not store fatty acids to the same level as certain algae they are easier to genetically manipulate. Liu et al (2011) used Synechocystis sp. to create a new strain by introducing a mutated acyl-ACP thioesterase gene from E. coli into its genome. They appropriately called this strain Sun Devil (SD). During fatty acid synthesis for cell membrane production, acyl-ACP is a protein with a growing fatty acid (acyl) chain attached to it. As concentration of acyl-ACP increases it has a feedback inhibition effect on fatty acid synthesis to prevent over production. Acyl-ACP thioesterase is an enzyme that removes the growing fatty acid from ACP. The mutated acyl-ACP thioesterase in SD is expressed at very levels and not only causes a large number of fatty acids to be released from ACP but there is also no feedback inhibition on further fatty acid synthesis. The overall effect of this is an over production and accumulation of fatty acid within the cell. Further modification of these cells was to weaken their cell wall to allow fatty acid release into the media.

The method from this study proved to be very productive. With a cell density of 1.0 X 109 cells/ml, fatty acid secretion yield was as high as 200 mg/L. The authors used a photograph in their paper which showed an 800ml flask with SD growing in culture medium. Around the surface of the medium fatty acids have precipitated out of solution to form what looked like cream floating on top. Cream that looked like it could just be skimmed off the top. A negative point however was the fact that at lower cell densities there was a lot of cell death, possibly from lysis of cells from their weakened cell walls.

Although this particular species of cyanobacteria is a fresh water species I think it does show a real opportunity for exploiting cyanobacteria for the production of fuel. As there are so many species of marine cyanobacteria I think it is very likely that this will be a major area of research in the future. Reading through the literature it appears that most of the algae and cyanobacteria studied in this field so far have been marine organisms. I am also very happy to see that realistic alternatives to fossil fuels are being studied as I think it is going to be a real issue in the not too distant future.

A Review of:
Liu X, Sheng J and Curtiss R (2011) Fatty acid production in genetically modified cyanobacteria. PNAS. 108: 6899-6904.