FRACTIONATION OF ZINC ISOTOPES DURING SURFACE ADSORPTION AND INTRACELLULAR INCORPORATION BY BACTERIA

The stable isotopes of zinc (Zn) are measurably fractionated during biological processing; however, the directions and magnitudes of these isotopic changes are not well-constrained. To help address this problem, we performed a series of experiments that were designed to isolate Zn isotope fractionation during (1) cell-surface adsorption and (2) intracellular incorporation. Batch adsorption experiments were conducted as a function of pH, using three different bacterial species. Experiments were performed under conditions of low and high bacteria:Zn ratio. Under both sets of conditions, the amounts of Zn adsorbed to the bacteria increased as a function of increasing pH, forming an adsorption edge. The different bacteria tested adsorbed Zn to roughly similar extents. Moreover, the changes in Zn isotope ratio during surface adsorption were roughly similar for the different bacterial species. In the experiments with low bacteria:Zn ratio, we were able to define a “universal” equilibrium Zn isotope separation factor (Δ⁶⁶Zn_{adsorbed-solution}) of +0.46‰. Because of the much greater amounts of suspended bacteria present, experiments at higher bacteria:Zn ratios appear to have been complicated by the presence of dissolved organic exudates. We were able to infer that the dissolved Zn complexes had Zn isotopic signatures that were substantially heavier than adsorbed Zn. Intracellular incorporation experiments were conducted by growing bacteria in growth media doped with varying concentrations of Zn-citrate or free Zn. Three different bacterial species were tested. We found that when exposed to Zn-citrate the measured Δ⁶⁶Zn_{incorporated-solution} ranged from -0.25‰ to +0.25‰, depending upon the bacterial species and the growth phase. The presence of free Zn (as opposed to Zn-citrate) created a metal stress response in the tested Pseudomonas strain, which resulted in a positive Δ⁶⁶Zn_{incorporated-solution} of up to +2.0‰. In summary, we found that bacterial surface complexation reactions likely fractionate Zn to similar extents, but that differences in intracellular metal processing among bacterial species can lead to substantial differences in Zn isotope fractionation. The presence of dissolved Zn organic complexes can also complicate the interpretation of Zn isotopic signatures in natural systems.