mechanisms of metal toxicity are similar in many phyla, this framework should 
be useful for toxicity studies with many aquatic organisms, and can serve as the 
general basis for design and interpretation of such studies. 
Although the importance of both organic complexing agents and trace 
metals in phytoplankton cultures has been recognized for some time, the 
critical role played by the speciation of trace metals in controlling their 
toxicity and availability to algae has just begun to be understood. It has now 
been established that it is the activity of the free ions, rather than the total 
metal concentrations, which determine the toxicity of metals to 
phytoplankton (46, 2). The study of the chemical processes which govern the 
activity of a given trace metal in culture media becomes then a prime area of 
concern to phycologists interested in metal toxicity experiments. An 
enumeration of these processes includes inorganic complexation, chelation, 
precipitation and adsorption. In addition, indirect chemical effects involving 
several interacting chemical species in the medium can influence trace metal 
activities in unobvious ways. In this study, these basic principles of Aquatic 
Chemistry (43) that apply to metals in phytoplankton cultures will be 
discussed systematically. 
There are but a few free metal activities that can be experimentally 
measured in the range and under the conditions of interest. There are also few 
metallic complexes which can be analytically determined in chemical systems 
as complex as culturing media. Henceforth, theoretical equilibrium calculations 
will be used throughout this paper to assess metal speciation and activities. The 
assumption of equilibrium is a reasonable one when the proper precautions are 
taken during medium preparation. Thermodynamic calculations provide, then, 
a convenient means of illustrating the critical chemical principles, even if they 
have inherent uncertainties. Complications introduced in the chemistry of the 
system by kinetic phenomena, or by the influence of the algae, will be 
discussed for each of the examined processes. 
Copper has been the metal of choice in studies of metal toxicity of 
phytoplankton because it has been postulated that cupric ion toxicity might 
play a role in the ecology of phytoplankton in some natural waters (6, 37, 7, 
10). In keeping with this situation, this paper will focus, albeit not exclusively, 
on copper which provides a rather good example for metal speciation and 
toxicity in phytoplankton cultures. 
EQUILIBRIUM SPECIATION OF METALS IN CULTURING 
MEDIA 
Before studying in detail the role of chemical processes in controlling metal 
speciation, it seems useful to examine what metal species are expected to be 
important in typical culturing media. 
39 
