The uptake kinetics of Cd and Zn, as influenced by metal interaction and metal pre-exposure, was examined in the gram-positive marine bacterium Bacillus firmus over a wide range of ambient free-Cd and -Zn concentrations. Bacteria were exposed to experimental media with different concentrations of Cd and Zn over a short, 15-min period. Zinc was found to be an effective competitive inhibitor of Cd uptake when the Zn(2+) concentration ([Zn(2+)]) was increased to 10(-8) M, whereas the Cd concentration (ranging from 10(-9) to 10(-6) M) did not affect Zn uptake. Inhibition of Cd uptake was dependent on [Zn(2+)] instead of the [Zn(2+)] to Cd(2+) concentration ratio. Cadmium uptake at different [Zn(2+)] was significantly inhibited by a sulfur ligand (SH) blocker (N-ethylmaleimide) and a Ca-channel blocker (lanthanum), suggesting that competition between Cd and Zn most likely occurred via binding to the same transport sites. Cadmium efflux also was determined in the presence of different [Zn(2+)]. A biphasic depuration of Cd was found when [Zn(2+)] was greater than 10(-8) M, whereas the calculated Cd efflux rate was independent of [Zn(2+)]. We further exposed B. firmus at different Cd or Zn concentrations for 24 h, then determined the metal uptake and efflux kinetics as well as the metallothionein (MT) induction. Both the Cd and Zn cellular concentrations increased with greater exposed metal concentration, but the MT levels and efflux were little affected by the elevated metal concentration. To some extent, however, the Cd uptake was reduced with an elevated intracellular Zn concentration, suggesting that at high Cd concentrations, intracellular Zn can suppress the Cd uptake in B. firmus. These results help to understand the interactions of metals in the marine environments.