(635c) Model of Microbial Fatty Acid Production Under Different Growth Regimes

J. Tyler Youngquist, Chemical and Biological Engineering, University of Wisconsin, Madison, WI, Wesley D. Marner II, University of Wisconsin, Madison, WI, and Brian F. Pfleger, Chemical and Biological Engineering, University of Wisconsin, Madison, WI

Microbial fatty acids are promising precursors for producing drop-in biofuels such as alkanes, olefins, and esters.  However, for this process to become economically viable, engineered microorganisms must be able to approach the maximum theoretical yield of fatty acids from a given renewable carbon feedstock.  While significant effort has focused on modifying the organisms, little published work has described efforts to understand the cultivation conditions that maximize fatty acid production.  Here, we present a mathematical model for microbial fatty acid production in an engineered strain of Escherichia coli based on changes in controlled growth rate and nutrient composition in the media.  To estimate parameter values for the model, growth and production experiments were performed in a lab scale chemostat under numerous growth regimes and media compositions.  To facilitate this effort, a strain containing three chromosomally located copies of a gene encoding a medium chain thioesterase under the control of the P_trc promoter was constructed.  The resulting strain could be cultivated in the absence of antibiotics with fatty acid overproduction induced with IPTG. Three independent chemostat experiments were maintained over a period of one to two weeks during which time multiple dilution rates and media compositions were tested. Fatty acid titers, sugar concentrations, and key nutrient concentrations were measured at each steady state.  Higher titers and yields were observed under low dilution rates, however productivity was diminished.  Using the data obtained from the chemostat cultures, the model was able to identify an optimal growth regime that predicts the highest yield of fatty acids on a single carbon substrate for our current overproducing strain.  Such a model will prove useful for scaling up production of fatty acid products from new engineered microorganisms.