THE INFLUENCE OF pH ON ALGAL CELL MEMBRANE PERMEABILITY AND ITS IMPLICATIONS FOR THE UPTAKE OF LIPOPHILIC METAL COMPLEXES

Uptake of lipophilic metal complexes by freshwater algae has recently been shown to be pH dependent. Here we look at different physiological aspects that could influence the diffusion of the lipophilic Cd complex, Cd(diethyldithiocarbamate)20 (Cd(DDC)20), into algal cells at different exposure pH values. Changes in cell membrane permeability were assessed as a function of pH for three species of green algae [Chlamydomonas reinhardtii P. A. Dang., Pseudokirchneriella subcapitata (Korshikov) Hindak, and Chlorella fusca var. vacuolata Shihira et R. W. Kraus] using two neutral, nonionic probes, fluorescein diacetate (FDA) and D-sorbitol. In parallel experiments, we exposed algae to inorganic Cd or to Cd(DDC)20 and monitored Cd intracellular metal distribution, together with phytochelatin synthesis. For the three algal species acclimated at pH 5.5 (w/wo DDC 1 mu M) and exposed at this pH, their permeability to FDA and D-sorbitol was consistently lower than for algae growing at pH 7.0 and exposed at this pH (P < 0.001). The ratio of the FDA hydrolysis rate measured at pH 7.0 with respect to the rate measured at pH 5.5 (both in the presence of DDC) correlated with the ratio of the Cd(DDC)20 initial internalization rate constant obtained at pH 7.0 versus that obtained at pH 5.5 (three algae species, n = 9, r = 0.85, P = 0.004). Our results strongly suggest that acidification affects metal availability to algae not only by proton inhibition of facilitated metal uptake but also by affecting membrane permeability.