While most iron is oxidized as a result of interaction with atmospheric oxygen or oxygenated waters, oxidation by bacteria is an active process in anoxic environments and in oxygenated, low pH (Fe values between 2 and 3‰ were measured. However, a Rayleigh trend with a fractionation factor of α ~ 1.0022 was observed, which is smaller than the fractionation factor in the abiotic control experiments (α ~ 1.0034), which has been inferred to reflect a biological isotope effect. Using iron isotopes, an improvement in the understanding of the metabolic processes controlling iron oxidation and energy production in these organisms can be developed.
Photoautrophic bacteria, which oxidize Fe(II) under anaerobic conditions, have also been studied. The Thiodictyon bacteria precipitate poorly crystalline hydrous ferric oxide when they oxidize iron. The precipitate was enriched in the Fe relative to Fe(II), with a δFe value of +1.5 ± 0.2‰.