Table 4-3. Calculation of Total Metal Concentrations Needed 
to Change the Cupric Ion Activity in Aquil While Maintaining 
the Other Metal Activities Constant 
(-log (concentration) or [activity], M) 
(EDTA) t 
[CU 2+ ] 
(Copper)-p 
(Iron)-p 
(Mang)-j- 
(Zinc)y 
(Cobalt)-j- 
8.5 
3.30 
7.00 
7.63 
8.7 
8.6 
3.3 
10.9 
3.70 
4.72 
6.40 
6.49 
6.7 
11.3 
4.0 
4.6 
6.30 
6.40 
6.6 
9.8 
4.35 
6.45 
7.20 
8.30 
8.49 
4.3 
10.9 
4.70 
5.72 
7.20 
7.49 
7.7 
11.3 
5.00 
5.58 
7.15 
7.4 
7.6 
activities vary with total copper in modified Aquil medium containing “Tris”, a 
ligand known to chelate mostly copper (44). Upon variations in copper 
concentration, the other metals are seen to have a much more constant activity 
in Aquil with Tris than in Aquil with only EDTA. 
One of the principal ways by which indirect chemical effects can be initiated 
is through pH variations. For example, pH has an indirect effect on metal 
complexation due to the acid-base properties of the coordinating ligannds. 
Figure 4-9 illustrates this effect for Mn, Cu and Zn in Aquil, with EDTA and 
Aquil with Tris. In this case, Tris mediates a much greater indirect effect than 
EDTA: Zinc and especially cupric ion activities are markedly depressed by 
increasing pH in the Tris medium, while the activities of all three ions remain 
essentially constant in the EDTA medium. Increases in pH, which can be 
brought about by photosynthetic carbon uptake if the aeration of the culture 
is insufficient, can also result in precipitation as illustrated in Figure 4-6. 
Adsorption on the fresh precipitate will follow, resulting in an unquantified 
decrease in the soluble concentration of trace metals. It is apparent that pH is a 
major factor in determining directly and indirectly the activity and toxicity of 
trace metals, and should be monitored regularly in metal toxicity experiments. 
CONCLUSION 
The chemistry of metals in the external milieu of algal cells is only one of 
the determinants of their toxicity. The literature on bacteria and higher cells 
abounds with examples of how the sensitivity of a particular strain or clone to 
a particular toxicant, depends markedly on the physiological status of the cells 
(9, 4). Although it is often recognized that the same situation should apply to 
phytoplankton, this concept has received scant attention in recent algal 
literature. It stands to reason that the previous history of an algal cell, its 
55 
