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 ac tivity of Zn2+ in the bulk solution. The failure of the simple Free ion Act ivity Model did not appear to be due to a mass transfer limitation of the u ptake process. Instead, Rhodococcus opacus is shown to produce a small (<3 kDa), complexing ligand capable of rapidly reducing [Zn2+] concentrations i n the external medium. Furthermore, organism-mediated modifications of the Zn efflux rate and modifications of the internalization rate suggest a meta l uptake system that is highly responsive to trace metal concentrations in the medium. Purely chemical explanations we re 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 importan t biological control of the Zn accumulation.