3.0 METABOLIC CLASSIFICATION

The three nutritional requirements that are quantitatively the most important for microbes are a carbon substrate, an energy source and an electron donor. These together with the electron acceptor are governed by the enzymes in the cell. The carbon source available to the microbes can either be organic or inorganic. those microbes that require organic compounds as either their sole or principal carbon substrates are classified as heterotrophs, whilst those that require inorganic carbonaceous compounds are classified as autotrophs. Microbes that utilize light as their energy source are described as phototrophs and those that obtain energy form the oxidation of either organic or inorganic compounds are described as chemotrophs. Microbes that utilize organic compounds as their source of electrons are described as organotrophs, whilst those employing inorganic electron sources are described as lithotrophs.

As far as microbial processes are concerned, the effect of temperature on overall reaction rates has received inadequate attention with respect to both process research and process operation. For chemical reactions an increase in temperature usually enhances the reaction rate and, by analogy, it is generally assumed that a similar effect occurs in the case of microbially mediated reactions. As far as microbes are concerned, they exhibit maximum critical temperatures above which growth ceases within two or three degrees.

It has been fruitful to analyze microbial systems that resemble a CSTR (chemical engineering jargon for a Continuous Stirred Tank Reactor). The usual assumptions are:

  • a pure monoculture growing in accordance with Monod type kinetics,
  • discretely dispersed cells in a defined liquid growth medium,
  • a completely mixed continuous flow reactor with no wall effects,
  • a single carbon energy substrate that in turn limits growth.
    Experimental evidence suggests that, within a specific temperature range, a temperature increase will enhance the maximum specific growth rate constant, the rate of endogenous metabolism and the affinity of the microorganisms for the growth-limiting substrate, but that effects on the death and lysis rate constants occur primarily at temperatures above the critical value for growth, i.e., outside the optimum range for growth. The overall effect of increasing temperature is, within limits, is to increase the maximum potential growth rate of an ideal culture, but with a concomitant decrease in the yield coefficient for the conversion of the limiting carbon energy substrate into microbial mass.
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