Binding of Cu2+ and Zn2+ to live cells of Shewanella putrefaciens was measured at pH 4, 5.5, and 7 for dissolved metal concentrations ranging from 0.1 to 100 microM. Release of organic compounds by the cells resulted in concentrations of dissolved organic carbon (DOC) between 0.5 and 1.6 mM. A discrete site, nonelectrostatic model was used to describe Cu2+ and Zn2+ binding to the cells. Binding of Zn2+, which increased with increasing pH over the entire range of dissolved Zn2+ concentration, could be explained by invoking two types of cell wall binding sites: acidic and neutral functional groups. Binding of Cu2+ exhibited a more complex pH dependence: at dissolved metal concentrations below 1 microM, binding to the cells actually increased with decreasing pH. This behavior could be reproduced by (1) assuming the existence of a small fraction of high-affinity binding sites in the cell wall (approximately 5%) and (2) including metal complexation by dissolved organic ligands. The latter compete with the neutral cell wall groups and decrease Zn2+ and Cu2+ binding at pH 5.5 and 7. The observed isotherms implied that binding of the metals was only weakly affected by cell wall charging. Model parameters derived from the single-metal binding isotherms were able to account for the observed competition of Zn2+ and Cu2+ for cell wall sites when both metals were present.