Iron is an abundant element with a very electropositive midpoint potential, making it an excellent electron acceptor (see the location of the Fe half-reaction in Figure 2). Like sulfur, iron is not a scarce element - iron is the fourth most abundant element found in the Earth's crust (sulfur is the tenth most abundant element). Iron exists naturally in two dominant oxidation states, ferrous (Fe2+) and ferric (Fe3+).
In aerobic aqueous and soil environments, ferrous iron (Fe2+) is thermodynamically unstable - it acts as an electron donor, and O2 acts an electron acceptor. This reaction occurs chemically and biologically (manifest as rust). At pH less than 5, the chemical reaction between ferrous iron and O2 is very slow; thus, Fe2+can be a stable-enough resource to be used as an electron donor by ferrous-iron-oxidizing bacteria (e.g., Thiobacillus fer-rooxidans) which grow by coupling Fe2+ oxidation to aerobic respiration. This ability of prokaryotes to oxidize Fe(II) is an important component of iron cycling in habitats featuring geochemical circumstances that hamper the spontaneous chemical reaction between Fe2+ and O2 (e.g., low pH, low O2, or high temperature).
The oxidized form of iron (ferric iron, Fe3+) is a useful terminal electron acceptor for anaerobic microorganisms. Fe3+ cannot be found in nature as a naked cation - it is highly reactive and spontaneously forms insoluble hydroxide and oxyhydroxide minerals. The susceptibility of these minerals to microbial reduction is inversely proportional to the degree of crystallinity of iron minerals -amorphous Fe hydroxides are the best substrates for iron-reducing microorganisms. Ferric iron-reducing bacteria couple the oxidation of a variety of organic and inorganic compounds to the reduction of ferric iron. Respiratory electrons pass through an electron-transport chain, which may terminate with Fe3+-specific reduc-tases. The energy released during electron transfer is coupled to the generation of a proton motive force and ATP. Iron reduction has many ecological and biogeo-chemical implications. Key microorganisms considered active in anaerobic iron respiration include those related to Geobacter and Shewanella.
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