The Incidence of Oscillatory Behavior in the Continuous Fermentation of Zymomonas mobilis

The incidence of oscillatory behavior in the continuous culture of Zymomonas mobilis has been examined using a combination of experimental investigations and a predictive model. The tendency to oscillatory behavior was assessed by perturbing the feed substrate concentration and dilution rate in a continuous fermentation starting from a number of distinct initial conditions. The entire range of qualitative dynamic behavior was observed: overdamped, underdamped, and sustained oscillatory responses. The predictive capabilities of a model previously proposed by our research group were confirmed over this range of operation. A key component of this model is the inclusion of a dynamic specific growth rate term which accounts for the inhibition associated with historical ethanol concentration change rate. Parameters and relationships estimated for the model were used to identify key characteristics leading to oscillatory behavior. In particular, differences in the sensitivities of ethanol production rate versus specific growth rate to ethanol concentration and its rate of change dictate whether sustained oscillations will occur. Experimental evidence indicates that a change in morphology is associated with oscillatory behavior. The change in morphology to a more filamentous form may explain the change in specific growth and product formation characteristics.