Bioenergetic interpretation on the S0/X0 ratio in substrate-sufficient batch culture

A batch culture of microorganisms can be classified to substrate-limited and substrate-sufficient growth according to the relative availability of the substrate to biomass, i.e. the S0/X0 ratio. It is generally observed that the growth yields are greater at lower S0/X0 ratios than at higher S0/X0 ratios. The excess substrate should cause serious uncoupling between anabolism and catabolism leading to energy spilling. Based on a balance on substrate reaction, the total quantity of substrate consumed by microorganisms is taken to be the sums of the quantity of substrate utilization for growth, the quantity of substrate utilization for maintenance and that due to energy spilling, a kinetic model is proposed to describe the dependence of the observed growth yield on the S0/X0 ratio for substrate-sufficient batch culture. It is demonstrated that decline in the observed growth yield with increasing the S0/X0 ratio is mainly due to energy spilling under substrate-sufficient conditions. The proposed model describes the literature data satisfactorily.