(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 the 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. What is less often perceived is the global 
interdependency of the chemistry of culture media, the indirect interactions 
(43, 27). For example, upon variations in the 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 Aquil. 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 work for medium preparation and can create other interpretative 
ambiguities — or to perform the multitude of necesary controls on an already 
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. This can be achieved by uncoupling the system 
using more specific complexing agents. Figure 4-8 shows how the metal 
53 
