Copper isotope fractionation during surface adsorption and intracellular incorporation by bacteria

Copper isotopes may prove to be a useful tool for investigating bacteria-metal interactions recorded in natural waters, soils, and rocks. However, experimental data that constrain Cu isotope fractionation in biologic systems are limited and unclear. In this study we utilized Cu isotopes (δ 65Cu) as a tool to investigate Cu-bacteria interactions, including surface adsorption and intracellular incorporation. Experiments were conducted with individual Gram-positive (Bacillus subtilis) and Gramnegative (Escherichia coli) bacterial species as well as with bacterial consortia from several natural environments. Adsorption experiments were conducted with live or dead cells over the pH range 2.5 to 6. Surface adsorption of Cu onto live bacteria cells resulted in apparent separation factors (Δ65Cu solution-solid = δ65Cusolution - δ 65Cusolid) ranging from +0.3‰ to +1.4‰ for B. subtilis and +0.2‰ to +2.6‰ for E. coli. The preference of the lighter Cu isotope by the cells appears to be metabolically-driven, as heat-killed bacterial cells did not significantly fractionate Cu isotopes. For the intracellular incorporation experiments, all bacteria and consortia were grown in a basal media amended with Cu(II)-citrate. The bacteria and consortia preferentially incorporated the lighter Cu isotope with an apparent Δ65Cusolution-solid ranging from ∼ +1.0‰ to +4.4‰. Our results indicate that live bacterial cells preferentially sequester the lighter Cu isotope regardless of the experimental conditions. The mechanisms involved are likely related to the active cellular transport and regulation of Cu. Hence, Cu isotopes may prove to be a powerful chemical tool for probing molecular-scale bacteria-Cu interactions. Cu isotopes in natural systems may also be used to distinguish microbial activity from abiotic geochemical reactions