Free fatty acid production in Escherichia coli under phosphate-limited conditions

Microbially synthesized fatty acids are an attractive platform for producing renewable alternatives to petrochemically derived transportation fuels and oleochemicals. Free fatty acids (FFA) are a direct precursor to many high-value compounds that can be made via biochemical and ex vivo catalytic pathways. To be competitive with current petrochemicals, flux through these pathways must be optimized to approach theoretical yields. Using a plasmid-free, FFA-producing strain of Escherichia coli, a set of chemostat experiments were conducted to gather data for FFA production under phosphate limitation. A prior study focused on carbon-limited conditions strongly implicated non-carbon limitations as a preferred media formulation for maximizing FFA yield. Here, additional data were collected to expand an established kinetic model of FFA production and identify targets for further metabolic engineering. The updated model was able to successfully predict the strain's behavior and FFA production in a batch culture. The highest yield observed under phosphate-limiting conditions (0.1 g FFA/g glucose) was obtained at a dilution rate of 0.1 h(-1), and the highest biomass-specific productivity (0.068 g FFA/gDCW/h) was observed at a dilution rate of 0.25 h(-1). Phosphate limitation increased yield (∼45 %) and biomass-specific productivity (∼300 %) relative to carbon-limited cultivations using the same strain. FFA production under phosphate limitation also led to a cellular maintenance energy ∼400 % higher (0.28 g/gDCW/h) than that seen under carbon limitation.