Metal Toxicity to Algae: A Highly pH Dependent Phenomenon

Abstract

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