6.4: Environment stability

Disturbances, events that disrupt the stability of an environment, have the potential to be a major environmental control on microbial populations. Disturbances can directly alter a community but they may also alter the environment in a way that indirectly affects the community (Shade et al., 2012). Specifically, if the physical and/or chemical properties of a system change as a result of a disturbance, those changes can in turn alter the biological activity within the system.

As an example, consider the impact of water level fluctuations in wetlands (Fig. \(6.3\text{B}\)). Anaerobic metabolisms, such as microbial iron reduction, may be favorable within water-saturated anoxic wetland soil. Some of the ferrous iron produced by iron reducers in such an environment could accumulate within the pore water. It could also adsorb to mineral surfaces and precipitate as ferrous iron minerals. However, if the environment dries and the soil becomes oxic, microorganisms may be able to oxidize the ferrous iron back to ferric iron and thus complete the iron cycle. Changes in conditions within this hypothetical environment, therefore, would directly affect the availability of oxygen and indirectly affect which components of the iron cycle were being catalyzed.

Disturbances may have a greater influence on microbial communities at or near the surface than they do deep in the subsurface. Earth’s surface is dynamic. Conditions change over the course of a day and with the seasons because of the rotation of the Earth, the tilt of its axis, and its orbit around the Sun. Conditions can change in hours or less in response to changing weather, floods, fires, landslides, volcanic eruptions, and other events. Therefore, there are many sources of potential disturbances at the surface. These events would also impact the shallow subsurface, but their effects would attenuate with depth. As such, the deep subsurface is likely much more stable than the surface and shallow subsurface.

The stability of a microbial community is characterized by its resistance, the degree to which the community is insensitive to the disturbance, and its resilience, the rate at which a community returns to pre-disturbance condition (Shade et al., 2012). A community’s stability in the face of disturbance can be measured in terms of whether the function of the community changes and/or whether the composition of the community changes. Communities with greater functional redundancy (i.e., the coexistence of multiple species that carry out the same metabolic function) may see compositional changes without functional changes (Allison and Martiny, 2008; Konopka, 2009).

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