5.3.2: Ferrous iron oxidation

Microorganisms can oxidize ferrous iron to ferric iron a few different ways. Chemolithotrophic microorganisms can couple ferrous iron oxidation with reduction of oxygen or nitrate, as shown in the following reactions: \[\begin{align} & \text{Fe}^{2+} + \text{H}^{+} + 0.25 \ \text{O}_{2} \longleftrightarrow \text{Fe}^{3+} + 0.5 \ \text{H}_{2} \text{O} \\ & \text{Fe}^{2+} + 1.2 \ \text{H}^{+} + 0.2 \ \text{NO}_{3}^{-} \longleftrightarrow \text{Fe}^{3+} + 0.1 \ \text{N}_{2} + 0.6 \ \text{H}_{2} \text{O} \end{align}\]

Iron oxidation coupled with oxygen reduction (reaction \((\PageIndex{1})\)) is not always biologically catalyzed. The reaction can occur at a rapid pace in solutions with relatively high oxygen concentrations and near-neutral pH. Thus, use of that reaction by microorganisms is largely limited to environments that contain water with low oxygen content and/or acidic pH (Bird et al., 2011; Emerson, 2012; Sobolev and Roden, 2001). Similarly, ferrous iron can also abiotically react with nitrite produced during nitrate reduction (Klueglein Nicole et al., 2014).

In addition to chemolithotrophs, anoxic phototrophs can oxidize ferrous iron to provide electrons for carbon fixation, as discussed in Chapter 4. An example reaction with biomass represented as \(\text{CH}_{2} \text{O}\) follows: \[\text{HCO}_{3}^{-} + 4 \ \text{Fe}^{2+} + 5 \ \text{H}^{+} \longleftrightarrow \text{CH}_{2} \text{O} + 4 \ \text{Fe}^{3+} + 2 \ \text{H}_{2} \text{O}\]

Many microorganisms are capable of driving these reactions forward. Some chemolithotrophic species capable of iron oxidation include Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans, Gallionella ferruginea. Phototrophic iron oxidation has been documented in purple nonsulfur, purple sulfur, and green sulfur bacteria (Bird et al., 2011). For more details, Bryce et al. (2018) and Emerson et al. (2010) provide recent reviews of the ecology of iron oxidizers.

In solutions with pH greater than 3, the ferric iron these iron-oxidizing microorganisms generate is largely insoluble and forms iron (oxyhdr)oxide minerals, as discussed in Chapter 12 and mentioned in Section 5.3.1. For this reason, iron oxidation may often be catalyzed outside of the cell to avoid clogging as well as acidifying the cytoplasm (Bird et al., 2011; Cosmidis and Benzerara, 2022). Also, in environments where ferrous iron oxidation occurs, the upper surface of the solution often has an oily sheen (Fig. \(5.6\)). That sheen is actually a film of nanocrystalline (oxyhydr)oxide minerals, which is known as schwimmeisen.

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