Regulation of Zn Accumulation by a Freshwater Gram-Positive Bacterium (Rhodococcus opacus)

Zn uptake by an aquatic Gram-positive bacterium was not predicted by the activity of Zn 2+ in the bulk solution. The failure of the simple Free Ion Activity Model did not appear to be due to a mass transfer limitation of the uptake process. Instead, Rhodococcus opacus is shown to produce a small (<3 kDa), complexing ligand capable of rapidly reducing [Zn 2+ ] concentrations in the external medium. Furthermore, organism-mediated modifications of the Zn efflux rate and modifications of the internalization rate suggest a metal uptake system that is highly responsive to trace metal concentrations in the medium. Purely chemical explanations were not sufficient to predict Zn uptake fluxes, instead resistance mechanisms activated by the bacteria in the presence of Zn appeared to be in large part responsible for an important biological control of the Zn accumulation.