Fractionation of Fe isotopes by soil microbes and organic acids

Small natural variations in Fe isotopes have been attributed to biological cycling. However, without understanding the mechanisms of fractionation, it is impossible to interpret such variations. Here we show that the delta Fe -56 of Fe dissolved from a silicate soil mineral by siderophore-producing b acteria is as much as 0.8 parts per thousand lighter than bulk Fe in the mi neral. A smaller isotopic shift is observed for Fe released abiotically by two chelates, and the magnitude of the shift increases with affinity of the ligand for Fe, consistent with a kinetic isotope effect during hydrolysis of Fe at the mineral surface. Fe dissolved abiotically without chelates sho ws no isotopic shift. The delta Fe-56 of the exchange fraction on soil grai ns is also lighter by similar to0.6 parts per thousand -1 parts per thousan d than Fe from both hornblende and iron oxyhydroxides, The kinetic isotope effect is therefore preserved in open systems such as soils. When recorded in the rock record, Fe isotopic fractionation could document Fe transport b y organic molecules or by microbes where such entities were present in the geologic past.