11.1.2: Indirect impacts

Microbes catalyze reactions directly to obtain energy and nutrients. But those reactions can in turn drive other reactions. In this way, microbes mediate many reactions indirectly. For example, consider the following glucose fermentation reaction: \[\text{glucose} \longleftrightarrow \frac{4}{3} \text{ propionate} + \frac{2}{3} \text{ acetate} + \frac{2}{3} \text{ CO}_{2} + 2 \ \text{H}^{+} + \frac{2}{3} \text{ H}_{2} \text{O}\]

The reaction produces carbon dioxide and hydrogen ions, which would both work to lower the pH of the environment. The hydrogen ions do so directly, and the carbon dioxide can lower pH by forming carbonic acid: \[\text{CO}_{2} \ (aq) + \text{H}_{2} \text{O} \longleftrightarrow \text{H}_{2} \text{CO}_{3} \longleftrightarrow \text{H}^{+} + \text{HCO}_{3}^{-}\]

By lowering the pH of the environment through acid production, these reactions would have a broad range of consequences. The pH of a solution is a major control on the chemical speciation of its solutes as well as the mineral surfaces it contacts. Both tend to become increasingly protonated as acid is added to a solution and pH decreases. Moreover, the acid can drive mineral dissolution reactions, which we discuss in the section on microbial weathering below. Carbon dioxide generated by organic matter degradation and plant respiration (i.e., root respiration) is a major driver of mineral weathering in soils and the subsurface (Chorover et al., 2020; Olshansky et al., 2019; Sanchez-Canete et al., 2018).

The bottom line is that microbes directly or indirectly mediate many of the chemical reactions that occur at Earth’s surface and in the upper 5 km of the subsurface. These reactions significantly affect the compositions of our soil, sediment, rocks, water, and atmosphere.

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