## Threshold Concentration

In the previous section the substrate consumption kinetics qs was assumed as an irreversible Michaelis-Menten kinetics equation (equation 14a). From this it follows that qs becomes zero at substrate concentration Cs = 0. However, it is known that microbial metabolism stops at a certain concentration of the substrate. This is called the threshold concentration Cs thresh.

This threshold concentration is thus the substrate concentration where substrate consumption qs becomes zero. This should be distinguished from the minimal substrate concentration Cs,min, where the growth rate l becomes zero.

This threshold situation is achieved when the coupled system of catabolic reaction and the energy generating system of the organism [most probably the proton motive force (pmf) process] are in equilibrium. Because the pmf requires at least 15-20 kJ for each proton (6), and assuming that a full catabolic reaction is minimally coupled to the extrusion of one proton, it appears that there must be a minimal catabolic Gibbs energy release of about 15-20 kJ. The following examples seem to support this idea where threshold behavior has been reported for a number for growth systems (Example 10):

• Anaerobic metabolism of CH4 production from acetate (33)

• Aerobic metabolism of ferrous into ferric iron (12,13)

• Anaerobic production of H2 converting 1 ethanol into 2 H2 and 1 acetate and consumption of 4H2 to produce acetate, CH4, or H2S from 2HCO——, HCO—— or SO2— (34,35)

In all these situations of observed threshold concentration the actual DGCAT could be calculated to be in the range of — 10 to — 30 kJ per catabolic reaction. Example 10 shows how the actual (DG)cat follows from textbook thermody-namic calculations.

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