Natural environments contain many organic compounds that are used by microbes. These substrates can be used either by microorganisms simultaneously or, in some cases, preferentially as carbon source. If the concentrations of two substrates are different, that is, one high and one very low, then the substrate with high concentration is used first.
In some cases one substrate may both enhance and slow down the degradation of other substrates. Both effects have been observed. Sometimes the suppression by one compound of the metabolism of the second is sequential, that is, first substrate disappears only after the second is mostly destroyed. An example of this is sequential destruction of linear alkanes by microorganisms. The reasons for sequential degradation of substrate in natural environments are not known.
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