Kinetics of competitive cometabolism under aerobic conditions

Abstract

Commonly observed competitive substrate inhibition in cometabolism of organic contaminants is used as rate- and reducing-power-determining factors to develop a kinetic model of the competitive cometabolism. Analogous to the well-known theory of Leudeking-Piret kinetics where the product formation demands reducing power, cometabolism is modeled as a reducing power demanding process that also competes with microbial growth for the available reducing power from the degradation of energy-yielding primary substrate. The model further incorporates other growth-associated phenomena such as substrate inhibition and multiple growth/nongrowth substrate interactions that may occur during cometabolic transformation processes. The kinetic model is used successfully to predict a variety of degradation patterns of growth/nongrowth substrates, displayed by microbial cultures when exposed to different concentration ratios of growth to nongrowth substrate: a complete degradation of nongrowth substrates that coincides with the simultaneous depletion of a growth substrate and, in some other cases, an incomplete degradation of a nongrowth substrate following the complete depletion of a growth substrate. These distinct patterns of substrate degradation are attributed to intrinsic specificities of enzymes for cometabolism and lack of reducing power available from the growth substrate degradation. The efficacy of cometabolic capabilities of actively growing microbial cultures and pre-cultured resting cells is discussed in terms of reducing power available in such systems.