text: Relatively little is known about isotopic fractionation of zinc by biological processes, but several studies have elucidated that Zn isotopes fractionate during surface adsorption, intracellular uptake processes and speciation. Many organisms, including certain species of fish, plants and marine phytoplankton, have both high- and low-affinity Zn transport systems, which appear to fractionate Zn isotopes differently. A study by John et al. observed apparent isotope effects associated with Zn uptake by the marine diatom Thalassiosira oceanica of -0.2‰ for high-affinity uptake (at low Zn concentrations) and -0.8‰ for low-affinity uptake (at high Zn concentrations). Additionally, in this study, unwashed cells were enriched in Zn, indicating preferential adsorption of Zn to the extracellular surfaces of T. oceanica. Results from John et al. demonstrating apparent discrimination against the heavy isotope (Zn) during uptake conflict with results by Gélabert et al. in which marine phytoplankton and freshwater periphytic organisms preferentially uptook Zn from solution. The latter authors explained these results as due to a preferential partitioning of Zn into a tetrahedrally coordinated structure (i.e., with carboxylate, amine or silanol groups on or inside the cell) over an octahedral coordination with six water molecules in the aqueous phase, consistent with quantum mechanical predictions. Kafantaris and Borrok grew model organisms B. subtilis, P. mendocina and E. coli, as well as a natural bacterial consortium collected from soil, on high and low concentrations of Zn. In the high [Zn] condition, the average fractionation of Zn isotopes imparted by cellular surface adsorption was +0.46‰ (i.e., Zn was preferentially adsorbed), while fractionation upon intracellular incorporation varied from -0.2 to +0.5‰ depending on the bacterial species and growth phase. Empirical models of the low [Zn] condition estimated larger Zn isotope fractionation factors for surface adsorption ranging from +2 to +3‰. Overall, Zn isotope ratios in microbes appear to be driven by a number of complex factors including surface interactions, bacterial metal metabolism and metal speciation, but by understanding the relative contributions of these factors to Zn isotope signals, one can use Zn isotopes to investigate metal-binding pathways operating in natural communities of microbes.