ON THE UPTAKE AND RELEASE OF ZINC (ZN-65) IN THE GROWING ALGA SELENASTRUM-CAPRICORNUTUM PRINTZ

A Zn- and pH-buffered medium was used to study the accumulation of zn in batch-cultured algae (Selenastrum capricornutum Printz.). All exper iments were carried out using Zn-65 radiotracers, which were measured by gamma-ray spectrometry. Zinc was applied in the presence of nitrilo triacetic acid (NTA), leading to free Zn2+ concentrations ranging from 4.2 x 10(-11) to 1.6 x 10(-7) mol litre(-1). During the 75-h experime nts, the rates of Zn influx and efflux were determined, the latter wit h full consideration for algal growth rates. Algal growth stopped comp letely at a free Zn2+ concentration of 1.6 x 10(-8) mol litre(-1). Ini tial Zn binding was determined and, based on free Zn2+ concentrations, presented by the apparent dissociation constant K-diss and capacity C as 4.6 x 10(-9) mol litre(-1) and 19.8 x 10(-6) mol gDW(-1), respecti vely. The rate constant of Zn afflux could be calculated as a constant 0.021 +/- 0.003 h(-1), irrespective of the Zn concentrations applied. The rates of Zn influx were expressed by the V-max and K-M values as 3.3 x 10(-10) mol gDW(-1) s(-1) and 6.3 x 10(-9) mol litre(-1), respec tively. These values, which are based on applied free Zn2+ concentrati ons, are compatible with the operation of a high-affinity low-concentr ation carrier mechanism. The results suggest that, under the condition s applied, free Zn2+ may be the most relevant Zn species for cn uptake ; furthermore, relative growth rate may be regarded as a relatively se nsitive signal for Zn stress circumstances.