Effects of pH Changes on Zinc Uptake by Chlamydomonas variabilis Grown in Batch Culture

Abstract

The effects of changes in H+ activity on the adsorption and intracellular accumulation of Zn by Chlamydomonas variabilis Dangeard have been studied over the pH range 7–5. Other experimental variables included the dissolved free Zn concentration, [Zn2+]b, the antecedent growth conditions (pH of the growth medium = 7, 6, or 5), and the physiological state of the harvested cells. After short-term incubations with 65Zn, algal cells were collected and the concentrations of EDTA-extractable Zn ([Zn]a ~ surface-bound Zn) and nonextractable Zn ([Zn]c ~ transported Zn) were determined. Values for [Zn]a and the flux (F) of Zn across the cell membrane decreased with increasing culture age, but cells harvested at comparable growth stages behaved similarly in the subsequent short-term incubations with Zn, irrespective of their original growth pH. In the incubation solutions, however, pH changes did affect both [Zn]a and F. At constant [Zn2+]b, a decrease in pH from 7 to 5 led to lower values of [Zn]a (~70%), suggesting either a pH-induced change in algal surface potential or a competition between H+ and Zn2+ for specific binding sites at the cell surface; a concomitant decrease in Zn flux (50–65%) was noted. The decrease in pH from 7 to 5 also had the effect of minimizing the dependence of Zn flux on the Zn2+ concentration. Two Zn transport mechanisms may coexist in C. variabilis cells, one involving a diffusion pathway sensitive to pH changes in the range 7–5 and the other a high-affinity porter system operative at both pH 7 and pH 5. These results suggest that the net effect of lake acidification may well be a decrease in the overall bioavailability of Zn to algae.