Metal Toxicity to Algae: A Highly pH Dependent Phenomenon

The inhibitory effect of Cd and of Cu on P uptake by the green alga Scenedesmus quadricauda is a highly pH dependent process. We used a simple inorganic medium to minimize the effects of variables other than pH so that changes in toxicity should reflect changes related only to H+concentration. The toxicity of Cd to P uptake increases strongly (almost 200-fold) with increasing pH over the range 5.5–8.5. Hydrated Cd2+dominates the dissolved Cd pool over this pH range. The logarithm of the Cd concentration causing 50% inhibition of P uptake is linearly related to pH. Total Cu toxicity increases 76-fold from pH 5.0 to 6.5, where hydrated Cu2+is the dominant dissolved Cu species, and then remains relatively constant at higher pH, where uncharged inorganic Cu complexes are dominant. The logarithm of the Cu2+concentration causing 50% inhibition of P uptake was also linearly related to pH, but total Cu was not. We explain this variation in metal toxicity with pH as a competition between H+and free metal cations for cellular binding sites. Possibly this is a general phenomenon for metals that form a significant amount of free cations relative to the total metal pool.