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 x 10'^) since 

 even then the highest copper concentration (4.4 x 10 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 



