EDTA in the presence of an excess of calcium: an initial peak in cupric ion 
activity is measured by the electrode, and it takes about four hours to 
approach the equilibrium value. Such phenomena have to be taken into 
account when studying the toxicity of metals to any aquatic organism, as 
transient effects can lead to large overestimations of toxicity. 
The release of chelating metabolites has been widely assumed as a 
conditioning mechanism for culture media (39). As is the case for natural 
waters, most of the chemically quantitative work on this topic has focused on 
the synthesis and exudation of iron chelating agents, particularly hydroxamates 
(19, 30). What seems often overlooked is that hydroxamic acids do not chelate 
exclusively iron, and that their binding of other metals can result in sizable 
decrease of these metals’ activities (1). 
By direct potentiometric techniques, extracellular metabolites of algae have 
been characterized in terms of copper complexing capacity and affinity (48). 
According to this work, the ligand produced by the algae under the conditions 
of the experiments is characterized by a constant of approximately unity for 
the reaction: 
Cu" + + HY — = H + + CuY 
If one assumes the ligand to be copper specific, the effect of its release in Aquil 
and Aquil with Tris is shown in Figure 4-5. Note that a significant decrease in 
the cupric ion activity does not begin until the total ligand concentration 
reaches 10'^M, an upper limit for the measured ligand releases. Although 
[Cu“ + ] start decreasing at a slightly lower ligand concentration when the 
copper concentration is elevated, the release of such relatively weak 
complexing ligand has little overall effect on the cupric ion activity in a well 
chelated medium. Ligands, with higher affinity for copper, appear to be 
released by some blue green algae (22). 
In principle, phytoplankton could modify the trace metal chemistry of the 
medium by assimilating artificial chelating agents. However, this potential 
problem is avoided by using EDTA or NTA which have been shown not to be 
assimilated by algae (23). Although photodegradation of EDTA and NTA has 
been reported (40), the light intensities normally used for culturing 
phytoplankton are insufficient to promote it in the laboratory. 
PRECIPITATION 
According to the computations of Table 4-1, the precipitation of several 
solids is calculated to be thermodynamically favorable in typical culturing 
media. Visible precipitates are indeed a common observation of users of algal 
48 
