Affinity Constant of Electron Donor Ks

The constant Ks is an affinity constant in the substrate-uptake kinetics. It is known that reported Ks values can vary over a wide range (10— 3 to 10 —6 mol/L). Even for the same organism, for example, E. coli, many different Ks values are found (30). Much of this wide spread is probably due to systematic errors in the determination of the Ks parameter. These errors can be due to ill-defined conditions of the microorganisms (adaptation), statistically unsound procedures to evaluate Ks from experimental data, neglect of mass-transfer limitation in the case of microorganisms growing as flocs, and analysis/sampling problems in determining the substrate concentration (see Ref. 30 for an extensive discussion). Moreover, Ks is a typical kinetic parameter, determined by the kinetic properties of the first step in substrate transport into the microorganism. For example, for the anaerobic acetate conversion into CH4, the Methanosarcina type of organism has a high Ks value of 3 mM acetate and its substrate uptake is passive (non-energy linked). However, for the Methanotrix type of organism, where acetate is transported into the cell utilizing energy, Ks ~ 0.5mM (33).

Due to these factors, it is not possible to give, from a thermodynamic point of view, a generalization about the value of Ks for different microbial growth systems.

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