Fractionation studies

Stable isotope fractionation studies have been traditionally used in geodynamics and material sciences. These techniques are being increasingly used in biological sciences, but few attempts to search for and exploit variations in the abundance of Fe stable isotopes in biological materials have been made. This is due in part to some analytical difficulties. Elements with intermediate masses such as Fe have a lower degree of natural stable isotope fractionation, compared to the larger fractionation occurring in small mass stable isotopes, such as C, O, N and S. Improvements made recently in mass spectrometry techniques for the analysis of Fe isotopes (Crews et al., 1994; Polyakov, 1997; Belshaw et al., 2000) have allowed to find a significant degree of Fe isotope fractionation associated to biological processes (Beard et al, 1999; Brantley et al, 2001; Anbar, 2004).

Plants may produce a measurable Fe-isotopic fractionation because Fe trafficking involves a number of steps, such as reduction by enzymes, transport across membranes, chelation by small organic compounds or proteins, etc. (Marschner, 1995), and these mechanisms may have different rates with different Fe isotopes. Experiments with dissimilatory Fe-reducing bacteria of the genus Shewanella algae grown on a ferrihydrite substrate indicate that the 856Fe of Fe(II) in solution was isotopically lighter than the ferrihydrite substrate by 1.3%o (Beard et al., 1999). The authors indicated that the negative S56Fe values found in sedimentary rocks could be associated with the biogenic fractionation, and suggested that the isotopic Fe signature may be used to trace the distribution of microorganisms in modern and ancient Earth. More recently, Brantley et al. (2001) explained variations in S56Fe as the consequence of the mechanisms used by bacteria to dissolve Fe from minerals. In order to scavenge Fe, bacteria and plants produce small organic molecules (siderophores and phytosiderophores, respectively) that have large association constants for Fe(III) (von Wiren et al., 2000; Boukhalfa and Crumbliss, 2002). The S56Fe of Fe dissolved from a silicate soil mineral by siderophore-producing bacteria was 0.8%o lighter than the bulk Fe in the mineral (Brantley et al., 2001). These data support that bacteria and plants have an isotopically lighter Fe pool than that of Fe-minerals, due to the biological Fe dissolution and uptake processes. A smaller isotopic shift was also observed for the abiotic release of Fe from the silicate soil mineral by two chelates, and the magnitude of the shift increased with the affinity of the ligand for Fe, supporting a kinetic isotope effect during Fe hydrolysis at the mineral surface. Iron isotopic fractionation could therefore document Fe transport by organic molecules or by microorganisms and plants where such entities were present in the geologic past (Brantley et al., 2001).

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