Researchers Engineer Cyanobacteria to Secrete Fatty Acids; Potential to Lower the Production Cost of Biofuels
29 March 2010
| The recombinant strategy used to engineer the cyanobacterium. Source: Liu et al. Supplementary Materials. Click to enlarge. |
Researchers at the University of Arizona’s Biodesign Institute have engineered the cyanobacterium Synechocystis sp. PCC 6803 to secrete fatty acids (C10-C18), providing a mechanism to reduce some of the cost barriers that have hampered biofuel production. Their results were published online 29 March in the Proceedings of the National Academy of Sciences (PNAS).
The researchers had earlier modified these microbes to self-destruct and release their lipid contents. In the group’s latest effort however, the energy-rich fatty acids were extracted without killing the cells in the process. Lead author Xinyao Liu hypothesized that if cyanobacteria could overproduce fatty acids, the accumulation within the cells would eventually cause these fatty acids to leak out through the cell membrane. To accomplish this, Liu introduced genes for the expression of the enzyme thioesterase into the cyanobacteria.
Thioesterase clips the bonds associating the fatty acids with more complex molecules. This use of modified thioesterases to cause secretion of fatty acids was first described for Escherichia coli by John Cronan of the University of Illinois more than a decade ago.
A second series of modifications enhances the secretion process, by genetically deleting or modifying two key layers of the cellular envelop—the S and peptidoglycan layers—allowing fatty acids to more easily escape outside the cell, where their low water solubility causes them to precipitate out of solution, forming a whitish residue on the surface. Study results showed a 3-fold increase in fatty acid yield, after genetic modification of the two membrane layers.
To improve the fatty acid production even further, the group added genes to cause overproduction of fatty acid precursors and removed some cellular pathways that were non-essential to the survival of cyanobacteria. Such modifications ensure that the microbe’s resources are devoted to basic survival and lipid production.
Liu et al. made five successive generations of genetic modifications into the photosynthetic microbe. The mutant strains were able to overproduce fatty acids and secrete them into the medium at an efficiency of up to 133 ± 12 mg/L of culture per day at a cell density of 1.5 × 108 cells/mL (0.23 g of dry weight/liter).
Although the fatty acid secreting strains had a long lag phase with many cells having damaged cell membranes when grown at low cell densities, these strains grew more rapidly in stationary phase and exhibited less cell damage than wild-type in a stationary culture. The results, the team wrote in their paper, suggest that fatty acid secreting cyanobacteria are a promising technology for renewable biofuel production.
The real costs involved in any biofuel production are harvesting the fuel precursors and turning them into fuel. By releasing their precious cargo outside the cell, we have optimized bacterial metabolic engineering to develop a truly green route to biofuel production.
—Roy Curtiss, director of the Biodesign Institute’s Center for Infectious Diseases and Vaccinology and professor in the School of Life Sciences
| Biodesign researcher Xinyao Liu discusses the process. |
The team, which includes researchers Daniel Brune and Wim Vermaas, is optimistic that significantly higher fatty acid yields will be obtainable, as research continues.
Resources
Xinyao Liu, Daniel Brune, Wim Vermaas, and Roy Curtiss III (2010) Production and secretion of fatty acids in genetically engineered cyanobacteria. PNAS 10.1073/pnas.1001946107
This sounds like the right direction. Whether the product is biodiesel or biobutanol, the most efficient pathway should continuously produce the hydrophobic fuel, allowing it to be skimmed off at regular intervales for continuous production.
Posted by: HealthyBreeze | 29 March 2010 at 02:01 PM
Good progress. It'd be interesting to see how long the process can continue before the cell breaks down.
Posted by: sulleny | 29 March 2010 at 02:28 PM