Iron Bacteria - Habitat

Habitat

Iron bacteria colonize the transition zone where de-oxygenated water from an anaerobic environment flows into an aerobic environment. Groundwater containing dissolved organic material may be de-oxygenated by microorganisms feeding on that dissolved organic material. Where concentrations of organic material exceed the concentration of dissolved oxygen required for complete oxidation, microbial populations with specialized enzymes can reduce insoluble ferric oxide in aquifer soils to soluble ferrous hydroxide and use the oxygen released by that change to oxidize some of the remaining organic material:

H₂O + Fe₂O₃ → 2Fe(OH)₂ + O₂

(water) + (Iron oxide) → (Iron hydroxide) + (oxygen)

When the de-oxygenated water reaches a source of oxygen, iron bacteria use that oxygen to convert the soluble ferrous iron back into an insoluble reddish precipitate of ferric iron:

2Fe(OH)₂ + O₂ → H₂O + Fe₂O₃

(Iron hydroxide) + (oxygen) → (water) + (Iron oxide)

Since the latter reaction is the normal equilibrium in our oxygen atmosphere while the first requires biological coupling with a simultaneous oxidation of carbon, organic material dissolved in water is often the underlying cause of an iron bacteria population. Groundwater may be naturally de-oxygenated by decaying vegetation in swamps; and useful mineral deposits of bog iron ore have formed where that groundwater has historically emerged to be exposed to atmospheric oxygen. Anthropogenic sources like landfill leachate, septic drain fields, or leakage of light petroleum fuels like gasoline are other possible sources of organic materials allowing soil microbes to de-oxygenate groundwater.

A similar reversible reaction may form black deposits of manganese dioxide from dissolved manganese, but is less common because of the relative abundance of iron (5.4 percent) in comparison to manganese (0.1 percent) in average soils. Other conditions associated with iron bacteria result from the anaerobic aqueous environment rather than the iron bacteria visibly colonizing that habitat. Corrosion of pipes is another source of soluble iron for the first reaction above and the sulfurous smell of rot or decay results from enzymatic conversion of soil sulfates to volatile hydrogen sulfide as an alternative source of oxygen in anaerobic environments.