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Figure 11.31 Dependence of microbial growth on temperature for a hypothetical mesophile.

These three values are referred to as the organism's cardinal temperatures (Figure 11.31). The minimum temperature for growth probably represents the point at which the cell's lipid-rich membranes become too rigid to be functional (somewhat like the increase in viscosity of an automobile's engine oil in winter), and/or reactions become too slow to maintain essential cell processes. The maximum temperature is reached when critical cell components are destroyed; in most cases this results from the denaturation (loss of three-dimensional structure) of proteins. The optimum usually is taken to be the temperature giving the highest (1 value, but it can also refer to a slightly different temperature (due to changes in yield or decay, for example) at which doubling time is shortest or biomass accumulation is most rapid.

The minimum and maximum temperatures for growth may form a narrow band or be widely separated. Similarly, growth rates may drop off rapidly as temperature departs from the optimum, or there may be a broad plateau of nearly optimal temperatures. However, the optimum temperature is always closer to the maximum than it is to the minimum. This is because increases in temperature will lead to increases in reaction rates, speeding growth, up to the point at which the enzymes or other cell constituents become unstable.

A common way to incorporate temperature effects into models is to use the following approximation, which is derived from work by von Arrhenius in the nineteenth century. Based on equation (5.48), the maximum specific growth rate at the temperature of interest (T, in °C) is estimated from the rate (120 at 20° C as where O is a dimensionless (no units) empirical coefficient, often taken to be 1.05 in the absence of an experimentally derived value. A related rule of thumb that is sometimes used is that reaction rates (and hence growth) will double for every 10° C increase in temperature. This is called the Qi0 rule and corresponds to an O value in equation (11.33) of about 1.07. However, both of these approximations are suitable for mesophilic organisms only at temperatures between their minimum and optimum; outside this range (such as can occur in composting) they can give grossly misleading results.

The other kinetic coefficients (Ks, Y, b) also are affected by temperature. Decay rates (b) probably increase with temperature because of increased reaction rates, but the relationships with the other parameters are not well established. For the relatively few times in which it has been determined, Ks values have been observed to increase with temperature in some cases, but to decrease in others.

pH Microbial growth occurs over a wide range of pH due to the existence of both acidic and alkaline extremophiles (Chapter 10). However, as with temperature, individual species have a much narrower range, with minimum, maximum, and optimum pH values. This stems from the effect that pH has on functional groups in essential microbial components, especially proteins. The pH may also affect the form (including shape and charge), and hence availability and toxicity, of organic and inorganic (e.g., NH3/NH4+, H2S/HS~) substrates. In fact, pH is so important that even extremophiles maintain nearly neutral pH conditions inside the cell.

The overall pH range for a species may be narrow or broad. Unlike the case of temperature, the optimum can be near either end or, more likely, in the middle of this range; also, it may be narrow or broad (several pH units). Since there is no general pattern, the effect on kinetic coefficients (especially p.) must be determined individually for each situation of interest.

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