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). Wliat 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: 



Cu2+ + 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 Hmit for the measured ligand releases. Although 

 [Cu"-"*"] start decreasing at a sliglitly 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). Althougli 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 



