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Metal ion binding to the bacterial cell wall is the first step in the interactions of a metal with a bacterium. Cadmium and zinc ion binding to isolated cell walls of Rhodococcus erythropolis A177 has been studied for a wide range of proton and calcium concentrations. The release of calcium ions and protons during metal ion binding is an indication of the competitive nature of the binding. Calculations, based on the metal binding data and the pH-dependent charging behavior, reveal that the cell wall becomes positively charged at high coverage with bivalent ions. A cooperative effect of the presence of calcium on metal ion binding was observed. Apparently, the cell wall structure is altered in the presence of bivalent ions. Since calcium is a dominant bivalent cation in most natural systems, one may regard the calcium as a "structure determining ion". On the basis of a qualitative interpretation of the data, the NICA model was selected for a quantitative description of the data.

Abstract-Speciation and bioavailability are the keywords in the relation between the total metal content of the system and the resulting effects for biota. The metal ion binding to the biotic surface is pH dependent, as is metal ion binding to soils. The binding of a metal ion to the biotic surface of an organism when present in soil can decrease with increasing pH, whereas the binding behavior of the biotic surface as such will always increase with increasing pH. The metal toxicity for plants often increases with increasing pH for water culture experiments, in which the opposite effect is observed for plants growing in soils. These seemingly contradictive observations can be explained by considering the interaction between an organism and metal ions present in soil to be the result of competition for that metal ion by all components (including the organism) present in the system. This concept is illustrated on the basis of model calculations concerning pH-dependent copper binding to maize root cell walls, fungal mycelia, and yeast cells present in a sandy soil and concerning cadmium binding to a bacterium present in a clay and a sandy soil as influenced by pH and calcium concentration. In addition, the concept is applied for calculating the impact of algal bloom on the copper speciation in an aquatic system. The concept might be a valuable tool in predicting quantitatively the metal ion sorption to biota present in a complex system and to predict the relative change in availability due to environmental changes.