Arizona State University researchers have reengineered the genetics of cyanobacteria, producing mutant strains that continuously secrete fatty acids through their cell walls. The reprogramming has essentially turned the microbes into tiny biofuel production facilities.

By Introducing an enzyme called thioesterase into cyanobacteria researchers were able to coax them into overproducing fatty acids. Accumulation within the cells eventually caused these fatty acids to leak out through the cell membrane, through the process of diffusion.

The enzyme is able to uncouple fatty acids from complex carrier proteins, freeing them within the cell where they accumulate, until the cell secretes them. Thioesterase acts to efficiently clip 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.

Photosynthetic microbes called cyanobacteria offer attractive advantages over the use of plants like corn or switchgrass, producing many times the energy yield with energy input from the sun and without the necessity of taking arable cropland out of production.

(Photo Credit: Biodesign Institute, Arizona State University)

A second series of modifications enhances the secretion process by genetically deleting or modifying two key layers of the cellular envelope—known as 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. The Study results show 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.

"The results to date are encouraging and we are confident of making further improvements to achieve enhanced productivity in strains currently under construction and development, says Professor Roy Curtiss. "In addition, optimizing growth conditions associated with scale-up will also improve productivity."