Fractionation of stable iron isotopes in higher plants

Although the fractionation of stable iron isotopes by biological processes in the environment is currently a matter of intense debate, the isotope fractionation associated with the growth of higher plants has, to date, not been characterized. We show that iron isotope fractionation induced by higher plants is substantial and also generates systematic plant-specific patterns. We suggest a hypothesis in which these patterns mirror the two different strategies that plants have developed to incorporate iron from the soil: reduction of Fe(III) in soils by strategy I plants results in the uptake of iron, which is depleted in 56Fe by up to 1.6 per mil relative to 54Fe when compared to the available Fe in soils; complexation with siderophores by strategy II plants results in the uptake of iron that is 0.2 per mil heavier than that in soils. Furthermore, younger parts of strategy I plants get increasingly depleted in heavy isotopes as the plant grows, while strategy II plants incorporate nearly the same isotope composition throughout. This points to entirely different translocation mechanisms between strategy I and II plants. Such presumably redox-related differences in translocation have been under debate up to now. We conclude that plant metabolism represents an important cause of isotopic variation in the biogeochemical cycling of Fe. Therefore, heavy stable metal isotope systems now start to be viable indicators of geosphere-biosphere metal transfer processes.