FRACTIONATION OF CU ISOTOPES DURING ADSORPTION AND METABOLIC UPTAKE BY BACTERIA

The isotopic composition of transition metals such as Cu can vary substantially in natural waters, soils and rocks. Cu isotopes can be fractionated during various transformations, including adsorption, precipitation and redox reactions. Reactions involving organic and biological constituents are of particular importance in controlling the cycling, fate, and transport of Cu. Hence, defining isotopic fractionation factors for these reactions is essential for the interpretation of Cu isotopic signatures in natural systems. In this investigation, we develop isotopic fractionation factors for Cu-bacteria interactions, including reversible surface adsorption (which may be a proxy for complexation with dissolved organic molecules) and intracellular uptake. Batch adsorption, reversibility, and metal loading experiments were conducted with Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria over numerous Cu:bacteria ratios and over the pH range 2.5 to 6. Bacterial growth experiments designed to isolate intracellular Cu from surface adsorption reactions were also conducted. In these metabolic uptake experiments, surface bound Cu was removed from the bacteria and the remaining cells were digested for evaluation of intracellular Cu. Isotopic results indicate that reversible adsorption with bacterial surfaces result in a separation factor, Δ⁶⁵Cu_{solid-solution}, of about +0.8‰ for B. subtilis and about +1.3‰ for E. coli, while intracellular incorporation results in a separation factor, Δ⁶⁵Cu_{solid-solution}, around –1.0‰. These initial isotopic results demonstrate that substantial fractionation of the Cu isotopes occurs both during surface adsorption and intracellular uptake with bacteria, but that the direction of fractionation appears to be opposite for each process. We are currently expanding on these initial isotope measurements and will report our latest results for both adsorption and uptake reactions.