Microbes at the Interface

Pb isotopes reveal that elements migrate both from below and also from above the surface as they are incorporated into vegetation and soils. Redox reactions promote fractionation of the isotopes of several elements, with the heavier isotope generally favored in the more oxidized phase. Organisms, which rely on redox reactions for their survival, fractionate metal isotopes, even in ore deposits, which tend to be havens for microbial activity that promotes element mobility into the surrounding environment. 

Iron is a redox-active element that is extremely abundant in the geosphere and critical for organisms in the biosphere. Variations in the isotopic composition of Fe in Nature often reflect the equilibrium fractionation of 3%o in the ratios 

Isotopic compositions of Mo and Fe measured by MC-ICP-MS indicate that during assimilation of Mo and Fe via nitrogen fixation by Azotobacter vindandii, the lighter isotope of Mo ( Mo versus Mo) is preferentially assimilated by the bacteria with a fractionation factor between assimilated and dissolved Mo of0.9997 [2]. In contrast, the heavier isotope of Fe ( Fe versus Fe) was removed from solution by the bacteria with a fractionation factor of 1.0011. Fractionation of Mo isotopes results from kinetic effects involving metal-binding ligands, whereas Fe 

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