11.1.1: Direct impacts

Microorganisms impact the chemistry of their environments by driving their reactions forward. This mechanism is obvious, but important to confront here because it is a fundamental part of the interactions between microorganisms and their environments. Microbial reactions can increase concentrations of reaction products and decrease concentrations of reactants. For example, consider the following overall reaction for nitrification: \[\text{NH}_{4}^{+} + 2 \ \text{O}_{2} \longleftrightarrow 2 \ \text{H}^{+} + \text{NO}_{3}^{-} + \text{H}_{2}\text{O}\]

The reaction produces nitrate and hydrogen ions and therefore can cause nitrate concentration to increase and pH to decrease where it occurs. In contrast, the reaction consumes ammonium and molecular oxygen and can cause their concentrations to decrease.

We can take advantage of these effects to calculate rates of microbial reactions. For example, imagine that you wanted to know how rapidly ammonium was being consumed in a culture of nitrifying microorganisms. If the initial ammonium concentration was \(1 \ \text{mM}\) and the concentration after four days was \(0.1 \ \text{mM}\), then overall rate of ammonium oxidation during that time is calculated as: \[rate = \frac{\left(\text{NH}_{4}^{+}\right)^{initial} - \left(\text{NH}_{4}^{+}\right)^{final}}{time} = \frac{1.00 \ \text{mM} - 0.10 \ \text{mM}}{4 \ \text{days}} = 0.23 \ \frac{\text{mM}}{\text{day}}\]

This simple calculation can be helpful, but it has its limitations. The value calculated represents an overall rate for the time period between measurements. The instantaneous rate could vary considerably during that time. Also, the microbial reaction of interest may not be the only control on reactant and product concentrations. If so, it would be necessary to assess the contributions of those other controls in order to accurately quantify the reaction rate.

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