thermodynamically stable form, it probably is the one which forms initially in 
the medium for kinetic reasons. Regardless of the precise nature of the solid, 
good agreement has been obtained between calculated and measured copper 
concentration in the solid phase in Aquil medium with a high EDTA 
concentration, 2 hours after addition of excess copper (29). It is worth noting 
that the precipitate was very finely dispersed, and that centrifugation was 
necessary to separate it from the aqueous phase. Ignorance of the formation of 
a precipitate can obscure completely the meaning of otherwise well controlled 
experiments. In terms of metal ion activity, the situation is complicated by the 
change in the nature of the precipitate which might evolve from an active form 
to a more stable one. In copper saturated media, the cupric ion activity has 
been measured potentiometrically to decrease markedly over 24 hours, the rate 
of decrease becoming very small thereafter (22). Such conditions can create 
large uncertainties in toxicity experiments. 
ADSORPTION 
The common notion that chelating agents make iron available to algae, 
seems to be supported by experiments where addition of iron or EDTA salts 
provide similar growth and carbon uptake enhancement in a variety of algal 
cultures (3). However, aluminium salts have also been observed to enhance 
carbon uptake (24). Following Stumm and Barber (41), it is now a prevalent 
interpretation of such experiments to attribute part, or all of the beneficial 
effect of the metal additions to a scavenging of other toxic metals by 
adsorption on precipitating iron or aluminium hydrous oxides. Figure 4-7 
illustrates the beneficial effect of iron additions to a Pyramimonas culture, and 
demonstrates how iron and copper behave antagonistically under controlled 
conditions (28). The growth rate of Pyramimonas is reduced at a total copper 
concentration of 1.2 10 M, and completely stopped at 4.4 10 M when the 
iron concentration is low (1.2 10'^M). Increasing the iron concentration by a 
factor of 10 completely blocks the toxic effect of the same copper 
concentrations. The question to be resolved is how much of this 
“detoxification” of copper by iron is due to adsorption processes, effectively 
removing the copper from solution and decreasing the cupric ion activity, and 
how much is due to a genuine physiological antagonistic effect at the cellular 
level. In a recent study of the adsorption of copper on hydrous iron oxide in 
seawater (48), it has been observed that under conditions similar to the 
experiments of Figure 4-7, iron adsorbs copper up to a Fe/Cu molar ratio of 
1/3. Adsorption can then certainly account for all of the antagonistic effects in 
the Pyramimonas experiment. What becomes more difficult to explain is the 
lack of antagonistic effect at the low iron concentration (1.2 xlO’^M) since 
even then the highest copper concentration (4.4 x lO'^M) should be entirely 
adsorbed. Note, however, that this is a domain of concentrations where copper 
starts saturating the colloidal iron surface, and there must be a titration effect 
51 
