10.2.4: Interactions among iron and sulfur cycling microorganisms

The simultaneous occurrence of iron reduction and sulfate reduction can lead to a variety of microbial interactions. Microbes that catalyze iron reduction and sulfate reduction often appear to compete with one another. However, abiotic reactions may provide a basis for mutualistic interactions between iron reducers and sulfate reducers and create opportunities for growth of microorganisms that can catabolize intermediate sulfur compounds.

As discussed in Section 5.3.1, dissimilatory iron reduction occurs when microorganisms catalyze iron reduction directly and use the reaction as a source of energy. Ferric iron can also be reduced abiotically by reacting with the sulfide produced by sulfate-reducing microorganisms (Fig. \(10.3\)). Where that is the case, the products of the reaction will include ferrous iron and sulfur compounds with intermediate oxidation states. Those sulfur compounds can then fuel microorganisms that grow using sulfur disproportionation reactions. They can also fuel microorganisms that reduce intermediate sulfur compounds back to sulfide. Thus, sulfate reducers create the opportunity for growth of other sulfur-cycling microorganisms where the sulfide they produce abiotically reduces ferric iron.

Where iron reduction by sulfide oxidation occurs, the relationship between sulfate reducers and other sulfur cyclers may be a form of commensalism. Specifically, microbes that reduce or disproportionate sulfur can exist because sulfate reduction and abiotic iron reduction create chemically favorable conditions for their growth. However, that characterization may be an oversimplification. For one reason, there may be overlap in the groups that use some of these reactions. Specifically, some microorganisms that can reduce sulfur or use sulfur disproportionation for energy can also use sulfate reduction. Secondly, if different species are involved, microorganisms using sulfur reduction may compete with sulfate reducers for electron donors.

Regardless of how ferric iron is reduced (abiotic or biotic), where iron reduction and sulfate reduction both occur, mackinawite \((\text{FeS})\) can precipitate: \[\text{Fe}^{2+} + \text{HS}^{-} \longleftrightarrow \text{FeS} \ (s) + \text{H}^{+}\]

Bethke et al. (2011, 2008) hypothesized that mackinawite precipitation can provide a basis for mutualism between iron reducers and sulfate reducers because it limits the accumulation of their metabolic products, ferrous iron and sulfide. Thus, both groups can benefit from mackinawite precipitation where their activities coexist. Consistent with this possibility, multiple studies have observed iron reduction and sulfate reduction falling into a \(1:1\) molar ratio \((\text{Fe} : \text{S})\), matching the stoichiometry of mackinawite (Bethke et al., 2011; Kirk et al., 2013; Paper et al., 2021). More details about mackinawite precipitation are provided in Chapter 11.

It remains unclear what factors determine whether iron reduction is a dissimilatory reaction or abiotic. Paper et al. (2021) hypothesized that the pH of the environment is one of the major controls based on results from laboratory experiments. Recall from Section 8.3 that the energy yield of iron reduction decreases as pH increases. At acidic pH, where energy available for iron reduction is often high, Paper et al. hypothesized that interactions between iron reducers and sulfate reducers is mainly competitive. However, at basic pH, where energy available for iron reduction can be low, iron reducers may become increasingly reliant on the benefits of mackinawite precipitation, thus encouraging mutualistic interactions with sulfate reducers. Alternatively, at basic pH, iron reduction may become increasingly driven abiotically by sulfur cycling. Additional research is needed to fully test these hypotheses.

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