Through uptake of carbon dioxide for photosynthesis, algae can modify the 
inorganic species of metals by decreasing the total concentration of carbonate 
in the system and increasing the pH. As pH increases, the hydroxyl ion activity 
increases and so does the importance of metal hydroxide complexes. The e ect 
on the carbonate ion activity and on the metal carbonate complexes is less 
straightforward, and depends on the original pH of the medium. In seawater 
media (pH 8), and in freshwater media around neutral pH, the carbonate 
complexes will increase with C0 2 uptake due to the predominance of the 
resulting pH increase over the total carbonate decrease. Such variations in 
metal chemistry can be alleviated by bubbling air in the cultures, thus insuring 
a steady concentration of carbonate in the medium. 
CHELATION 
The history of the development of artificial culturing media for algae is in 
part that of the replacement ot “growth factors” and soil extracts by 
chelating agents (17). The exact role of these chelating agents in promoting 
algal growth has been a subject of some controversy (6, 14). It is now well 
established that they do control the toxicity of various heavy metals - copper 
in particular (46). Whether they also increase the availability of some metallic 
nutrients — chiefly iron — is yet unproven. Figure 4-2 shows the percentage of 
chelated metal and the metal activities in Aquil (with contaminant metals) as a 
function of the concentration of EDTA, by far the most widely utilized 
chelating agent in algal media. Note that the order in which the metals are 
chelated by EDTA is not simply related to either the metal ion activities or 
their affinities for EDTA (FE>Cr>Cu>Ni>Pb>Zn, Cd, Co). 
Other chelating agents which are commonly used include nitrilotriacetate 
(NTA), citrate and various amino acids. “Tris” (tris(hydroxymethyl)amino 
methane) commonly used as a pH buffer for biological experiments has 
received much use in recent studies of copper toxicity to phytoplankton (44). 
Used in conjunction with EDTA which chelates the other metals at a very low 
concentration, Tris permits a convenient manipulation of the cupric ion 
activity. Figure 4-3 illustrates this point by comparing how the cupric ion 
activity varies with total copper in Aquil (EDTA = 10*^*^M) and in a modified 
Aquil recipe containing Tris (EDTA = lO'^M) and Tris = 10'^M). Around 
[Cu 2 + ] = 10*^M, where many toxicity studies are run, the cupric ion activity 
in the Tris medium is less sensitive to variations in total copper concentration 
than in the EDTA medium. However, with the proper precautions, both media 
yielded the same results in a study of copper toxicity to Gonyaulax tamarensis 
( 2 ). 
44 
