6.6: Interactions between controls

We have looked at some environmental controls on an individual basis in this chapter and will consider additional controls in the chapters ahead. One thing to bear in mind as you learn about these controls is that they can interact. An interaction effect occurs when the effect of one variable on a system depends on the state of a second variable.

As an example, it has long been known that the effect of pressure on microbial growth for some species depends on the temperature of the environment. In early research on this topic, Zobell and Johnson (1949) studied the effects of pressure and temperature on several species of microorganisms isolated from marine and terrestrial environments. Relationships varied between isolates, but for many, they found that the isolates were better able to grow at higher pressures if temperature was also higher. Thus, the response to high pressure depended on temperature for those isolates.

As a second example, research has shown that the activities of enzymes used by soil microorganisms to degrade organic matter are sensitive to temperature. Min et al. (2014) investigated how that sensitivity varies with the pH of the environment for two enzymes, one that soil microbes use to obtain carbon (BGase; liberates glucose carbon from cellulose) and one used for nitrogen (NAGase; liberates nitrogen-acetylglucosamine from chitin). They found that the temperature sensitivity of BGase differed significantly with pH whereas that of NAGase did not vary with pH. As a result, an increase in temperature can cause changes in flows of carbon and nitrogen during organic matter decay but the nature of those changes is dependent on the pH of the environment. Their results have implications for changes in nutrient availability in soils in response to climate change (Min et al., 2014; Souza and Billings, 2022). We will more broadly consider nutrient availability as a control on microorganisms in the next chapter (Chapter 7).

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