On the uptake and release of zinc ( 65Zn) in the growing alga Selenastrum capricornutum Printz

Abstract

A Zn- and pH-buffered medium was used to study the accumulation of Zn in batch-cultured algae ( Selenastrum capricornutum Printz.). All experiments were carried out using 65Zn radiotracers, which were measured by γ-ray spectrometry. Zinc was applied in the presence of nitrilotriacetic acid (NTA), leading to free Zn 2+ concentrations ranging from 4·2 × 10 −11 to 1·6 × 10 −7 mol litre −1. During the 75-h experiments, the rates of Zn influx and efflux were determined, the latter with full consideration for algal growth rates. Algal growth stopped completely at a free Zn 2+ concentration of 1·6 × 10 −8 mol litre −1. Initial Zn binding was determined and, based on free Zn 2+ concentrations, presented by the apparent dissociation constant K diss and capacity C as 4·6 × 10 −9 mol litre −1 and 19·8 × 10 6 mol gDW −1, respectively. The rate constant of Zn efflux 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 × 10 −10 mol gDW −1 s −1 and 6·3 × 10 −9 mol litre −1, respectively. These values, which are based on applied free Zn 2+ concentrations, are compatible with the operation of a high-affinity low-concentration carrier mechanism. The results suggest that, under the conditions applied, free Zn 2+ may be the most relevant Zn species for Zn uptake; furthermore, relative growth rate may be regarded as a relatively sensitive signal for Zn stress circumstances.