Through uptake of carbon dioxide for photosynthesis, algae can modify the 

 inorgiuiic 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 effect 

 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 CO-) uptake due to the predominance of the 

 resulting pH increase over tlie 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 of "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 = lO'^-^M) and in a modified 

 Aquil recipe containing Tris (EDTA = 10"^-^M and Tris = lO'^M). Around 

 [Cu^"^] = lO'^^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 tamaremis 

 (2). 



44 



