Isotope fractionation during microbial metal uptake measured by MC-ICP-MS

High-precision isotopic analyses by MC-ICP-MS were used to investigate the mass-dependent fractionation of Mo and Fe isotopes during bacterial metal assimilation in experiments with Azotobacter vinelandii. A. vinelandii is a diazotroph with high demand for both Mo and Fe during nitrogen fixation. Our results demonstrate that the growth medium became progressively enriched in heavier isotopes of Mo during bacterial growth, indicating preferential assimilation of lighter isotopes. In contrast, for Fe, the medium become isotopically lighter as Fe was removed from solution. The experimental data can be interpreted in terms of Rayleigh fractionation, yielding fractionation factors of 0.9997 and 1.0011 for Mo and Fe, respectively. Hence, we infer Δ^97/95Mo_cells-medium = -0.3‰ and Δ^56/54Fe_cells-medium = 1.1‰. Fractionation of Mo isotopes could result from simple kinetic effects during assimilation, but may also be affected by complexation with high-affinity metal binding ligands. Kinetic effects cannot easily account for the sense of Fe isotope fractionation, and so equilibrium effects, possibly between different Fe complexes, are implied. Adsorption of Mo and Fe onto cell surfaces may also play a role and requires further examination. Isotope fractionation studies using MC-ICP-MS may provide new constraints on the processes by which microbes extract metals from their surroundings, ultimately yielding insights into the mechanisms of metal assimilation into the metallome.