research team from the Biodesign Institute at Arizona State University has developed a process that reprograms photosynthetic microbes to secrete lipids, making byproduct recovery and conversion to biofuels easier and potentially more commercially viable.
Roy Curtiss, of the Institute’s Center for Infectious Diseases and Vaccinology, and professor in the School of Life Sciences, is part of the large, multidisciplinary ASU team that has been focusing on optimizing cyanobacteria as a renewable source of biofuels. “The real costs involved in any biofuel production are harvesting the goodies and turning them into fuel,” said Curtiss. “This whole system that we have developed is a means to a green recovery of materials not requiring energy dependent physical or chemical processes.”
To get cyanobacteria to more readily release lipids, Curtiss and postdoctoral researcher Xinyao Liu, placed a suite of genes into photosynthetic bacteria that produced enzymes to degrade membrane lipids, poking holes in the membranes to release free fatty acids into the water. By genetically reprogramming the cells, the enzymes are only produced when carbon dioxide is removed from their environment.
“We first freed up fatty acids by triggering self-destruction of the bacteria by adding nickel,” Liu said, “but this is not so good for the environment. So, this time we did it by stopping carbon dioxide supply. The strategy of adding nothing for recovering fuels from biomass is designed to drastically reduce processing costs.”
“We have created a very flexible system that we can finely control,” added Liu. “After teaching cyanobacteria to excrete fuels, we don’t want to waste the useful lipids in the photosynthetic membranes, so we developed a greener way to recycle the remaining value of the biofactory.”
The team tested fat-degrading enzymes, called lipases, from bacterial, fungal and guinea pig sources to see which would work best. These lipases are able work like molecular scissors, clipping off the fatty acids from the photosynthetic membranes. They also worked to optimize the growth conditions of their green recovery method, testing variables such as the cell culture density of the microbes, light intensity and agitation of the cultures.
“Due to rapid DNA sequencing and public gene databases, we can now use this vast and ever-increasing store of gene sequences with powerful computer search methods to identify the best genes and proteins with optimal functions and capabilities independent of their origin in microbes, plants and animals,” said Curtiss.
Next, the group will test their results in large-scale photobioreactors, which are being designed by engineers in the institute’s Swette Center for Environmental Biotechnology to optimally capture the free fatty acids. Ultimately, the team hopes to achieve development of a new, economical and environmentally friendly, carbon neutral source of biofuels. “We are optimistic that we can make the system even better, leading to the commercialization of our green recovery method bundled with other technologies,” said Liu.
The project has been part of the state of Arizona’s strategic research investments to spur new innovation that may help foster future green and local industries. Other key contributors were Biodesign colleagues Sarah Fallon and Jie Sheng.
For more information: The results were published in the early online edition of the Proceedings of the National Academy of Sciences http://www.pnas.org/content/108/17/6905.full?sid=6d498fcf-5e5c-43f4-8d8a-c54c60a2e029; Print version: Proc. Natl. Acad. Sci. USA. 108:6905-6908.