April 2011


Producing and Extracting Biofuel Precursors from Cyanobacteria

Cyanobacteria were genetically engineered to overproduce fatty acids, which were readily extracted.
This post was chosen as an Editor's Selection for ResearchBlogging.org Biofuels, fuels derived from plants or other living material, may be a reasonable short-term transition from conventional fuel to long-term energy needs (e.g. nuclear fusion, which is probably still decades away). They're also definitely an attractive renewable starting material to carbon-based products (this need isn't going anywhere).

As they currently stand, biofuels clearly have many limitations. For example, depending on the source (e.g. corn), they can be far more costly to health and the environment than gasoline.

Growing corn for biofuels, thereby partially replacing soybeans as a crop in the United States midwest, may hinder natural pest control. Furthermore, burning plants for power is still polluting and inefficient.

Switchgrass may overcome the limitations of other plants, especially with the assistance of genetic engineering. However, land-use conflicts may still arise.

Furthermore, switchgrass isn't readily grown in all regions of the world. Alternatively, biofuels may also be produced from algae and other microbes, eliminating (or greatly reducing) land-use conflicts.

Roy Curtiss III (Arizona State University, United States) and coworkers have produced and recovered fatty acids (biofuel precursors) from genetically-engineered cyanobacteria. Their process eliminates many of the costly processes typically seen in biofuel production.

Fatty acid production.

The scientists inserted fatty acid synthesis genes from Escherichia coli into the DNA of Synechocystis microbes, which they termed "Sun Devil" strains. Cyanobacteria genes were deleted if they competed energetically or synthetically with fatty acid production, and inserted if they increased fatty acid production or chain length (thereby improving fuel quality).

These genetically-engineered cells were fragile during lag phase (the growth period prior to cell division), increasing the length of this phase, which would reduce fatty acid yields in an industrial bioreactor. However, the genetic modifications imparted stability during stationary phase (in which cell growth is balanced by cell death, and the phase in which many cells are grown in the biotechnology industry).

What's important is how much of this fatty acid content is extractable from the cells after production. This is described next.

Fatty acid recovery.

The scientists have developed a comparatively environmentally-friendly and cost-effective means of isolating the fatty acids from the cells. By rational genetic engineering, the designed their cells such that removing carbon dioxide from the cell culture promoted the production of genes that destroyed the cell membranes (45% damage per day), facilitating isolation of their constituent fatty acids.

Cell growth conditions are under optimization to speed up membrane damage (and thus fatty acid isolation). As it stands now, fatty acid recovery is approximately 40%, at a total of approximately 21 milligrams per liter of cell culture.

The scientists' fatty acid recovery method of simply removing carbon dioxide from the cells' air supply is simple and is clearly advantageous to other costly methods common in the field, such as cell concentration. Their approach should also be applicable to other microbes (e.g. algae) commonly used in industry.

Other molecules beyond fatty acids (e.g. alkanes) may be isolated from other genetically-engineered cells by the scientists' recovery system. However, the yields of these molecules to date have been lower than that seen for fatty acids.

Final comments.

Maybe it's because I'm not active in this line of research, but it seems to me that the total recovered fatty acid yield is still rather low. A 2500 liter bioreactor volume would be required to produce 52.5 kilograms of fatty acids (my own calculations based on the scientists' reported data).

Continually processing such huge volumes seems to me to be a huge engineering challenge, and would require a lot of water. I'd welcome any comments on how this could be (or is) done.

NOTE: The scientists' research was funded by Arizona State University, British Petroleum and Science Foundation Arizona, and the Department of Energy Advanced Research Projects Agency.

Liu, X., Sheng, J., & Curtiss III, R. (2011). Fatty acid production in genetically modified cyanobacteria Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1103014108

Liu, X., Fallon, S., Sheng, J., & Curtiss, R. (2011). CO2-limitation-inducible Green Recovery of fatty acids from cyanobacterial biomass Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1103016108