A KINETIC-MODEL FOR SUBSTRATE AND ENERGY-CONSUMPTION OF MICROBIAL-GROWTH UNDER SUBSTRATE-SUFFICIENT CONDITIONS

The growth of heterotrophic microorganisms can be classified into substrate-limited and substrate-sufficient growth according to the relative availability of the substrate (carbon and energy source) and other nutrients. It is generally observed that the consumption rates of substrate and energy (ATP) are higher under substrate-sufficient conditions than under conditions of substrate limitation. The excess substrate and ATP consumption is often influenced by the residual concentration of substrate in a relatively wide range. To account for these effects, a kinetic model is proposed to describe substrate and ATP consumption rates of microbial growth under substrate-sufficient conditions. According to the model, the specific substrate consumption rate of a substrate-sufficient culture can be expressed as the sum of the substrate consumption rate under substrate-limited conditions at the corresponding specific growth rate and an additional consumption rate due to excess substrate. The same kinetic form also applies to the specific ATP consumption rate and to the specific oxygen consumption rate of anaerobic culture, respectively. The linear equations for substrate and ATP consumption rates of Pirt and of Stouthamer and Bettenhausen can be used for substrate-limited growth. The excess of substrate and ATP consumption rates at carbon surplus can be described in a form similar to that of Michaelis-Menten kinetics. The proposed kinetic model has been verified with experimental data from three continuous cultures representing both anaerobic and aerobic microbial growth on substrates with low and high degrees of reductance. Using this model, the parameters maximum growth yield and maintenance requirement (both in terms of substrate and ATP) of a culture under different growth limitations can be better defined and quantified. The range of residual substrate concentrations in which the specific rates of substrate and ATP consumption are affected can also be assessed. This information should be helpful in designing medium and reactor operating conditions and in interpreting experimental results obtained under different limiting conditions.