The glucose concentration allows growth at only 83% of the maximum (limiting by the stated criterion), whereas the ammonium-N concentration allows growth at 95% of the maximum (not limiting, even though it is at a lower concentration than the glucose).
Equation (11.20) for microbial growth has the same form as equation (5.38), which described enzyme kinetics for two substrates. However, whereas the enzymatic equation is mechanistic (derived from enzyme kinetics), the growth equation is again empirical.
A somewhat different problem is the presence of multiple substrates fulfilling the same nutritional need. In laboratory systems, microbiologists are able to study the uptake of a single substrate by a pure culture of bacteria. However, in most environmental systems, there are likely to be several utilizable organic substances present, for example, and perhaps several electron acceptors and nitrogen sources as well. Will an organism use only one at a time, or two or more simultaneously?
This turns out to be a very complex question, and the answer depends on the specific substrates and organism involved. On a practical level, this is often addressed by using some single general measure of organic matter, such as BOD or COD (see Section 13.1.3), rather than trying to account for the actual mechanisms involved. Electron acceptors are more likely to be used sequentially, with oxygen preferred when it is available. However, the ability to use a specific electron acceptor depends on the particular organism.
Of course, there are usually many different microorganisms present as well, not a pure culture of a single species. This, similarly, is commonly handled by using a general measure of biomass that lumps all or many different species together. Although these simplifications severely compromise any mechanistic basis for the models used, they have still been found in practice to provide useful information.
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