(like that of copper on EDTA in Figure 4-3) where the cupric ion activity 

 increases rapidly with increasing copper concentration. Exact quantitification 

 of this phenomenon awaits a better mathematical description of adsorption 

 processes on hydrous iron oxides in seawater. Despite great recent advances in 

 the modeling of adsorption in aqueous systems (50, 42, 15), it is still the least 

 quantifiable chemical process in thermodynamic calculations. The presence of 

 precipitates in a culture medium modifies its global trace metal chemistry to an 

 unpredictable degree. This creates the most common difficulty in interpreting 

 experiments on toxicity of metals to a variety of organisms. Note that 

 adsorption on the walls of a glass culture vessel is equally hard to predict. 

 Choice of container material which minimizes adsorption of solutes is critical 

 to the design of trace metal toxicity experiments. 



Adsorption on tlie surface of algal cells can also be important for the trace 

 metal chemistry of the medium in dense cultures. There is, however, no 

 practical way to distinguish it from intracellular uptake. The effects of cellular 

 uptake processes including adsorption on the cell surface, have been discussed 

 earlier. 



INDIRECT CHEMICAL EFFECTS 



The general principles of coordination, precipitation, and adsorption which 

 have been discussed heretofore, are readily understood and their importance in 

 toxicity studies is usually recognized. Wliat is less often perceived is the global 

 interdependency of the chemistry of culture media, the indirect interactions 

 (43, 27). For example, upon variations in tlie total copper concentrations, it is 

 natural to relate the observed effects to changes in the cupric ion activity. 

 However, as illustrated in Figure 4-8, activities of the zinc and ferric ions are 

 also increased when the total copper is augmented in Aqtiil. Conceivably, any 

 or all of these increased activities could be responsible for the observed effects. 

 It is then a difficult choice to either maintain all metal activities constant by 

 adhoc modification of all analytical concentrations — a method which 

 multiplies the wor kfor medium preparation and can create other interpretive 

 ambiguities — or to perform the multitude of necessary controls on an al- 

 ready arduous series of experiments. Table 4-3 shows how the total metal 

 concentrations have to be varied concomitantly with that of copper, to vary 

 exclusively the cupric ion activity in Aquil with two EDTA concentrations 

 (35). 



The indirect interactions illustrated in Figure 4-8 are almost exclusively 

 mediated by EDTA, which chelates all the interdependent metals. In principle, 

 a convenient way to avoid the complications created by these interactions is to 

 reduce them to a minimum. Tliis can be achieved by uncoupling tlie system 

 using more specific complexing agents. Figure 4-8 shows how the metal 



53 



