8.3: Impacts of changes in product and reactant activities

As noted above, changes in the chemical composition of an environment can alter the free energy yield of microbial reactions. We can illustrate this relationship by extending the example calculation above to consider environments with different pH values and activities of ferrous iron. For convenience, the microbial reaction we are considering is repeated here: \[\text{H}_{2} \ (aq) + \text{Fe}_{2} \text{O}_{3} \ (s) + 4 \ \text{H}^{+} \longleftrightarrow 3 \ \text{H}_{2} \text{O} \ (l) + 2 \ \text{Fe}^{2+}\]

First, let’s consider the impact of variation in pH. As we do, we will assume concentrations identical to those in Table \(8.1\), except allow pH to increase from \(6\) to \(8\). As that happens, the Gibb’s free energy yield of the reaction changes from \(-31.4\) to \(14.0 \ \text{kJ}/\text{mol}\) (Figure \(8.2\text{A}\)). Thus, as pH increases, the reaction becomes endergonic and not useful as a source of energy for chemotrophs.

We can anticipate this change in reaction energy yield based on the reaction itself, which includes a large number of hydrogen ions as reactants. As we increase pH, or in other words, decrease the activity of hydrogen ions, we are removing a reactant from the system and thus making it less favorable for the reaction to move forward.

Next, we will consider the impact of variation in ferrous iron activity. Holding pH at \(6\), if ferrous iron concentration increases from \(100 \ \mu \text{M}\) to \(1 \ \text{mM}\), the Gibb’s free energy yield of the reaction changes from \(-31.4\) to \(-20.1 \ \text{kJ}/\text{mol}\) (Figure \(8.2\text{B}\)). Thus, as ferrous iron activity increases, the reaction becomes less thermodynamically favorable. However, it remains exergonic and thus has the potential to continue to serve as an energy source across the range of ferrous iron activities considered.

Again, the impact of increasing ferrous iron activity can be anticipated based on the reaction, which includes ferrous iron as a product. Accumulation of a product causes a reaction to become less thermodynamically favorable. Taken together with our observations about pH above, we can generally say that reactant removal and product accumulation can cause reactions to become less thermodynamically favorable, whereas reactant addition and product removal cause reactions to become more favorable.

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