Accumulation of zinc protects against cadmium stress in photosynthetic Euglena gracilis

To determine the interplay between zinc and cadmium on the heavy metal accumulation capacity of Euglena gracilis, the effects of increasing Zn2+ concentrations (13-65.4 ppm or 200-1,000 mu M) were analyzed on growth; O-2 consumption; photosynthesis; ascorbate (APX) and glutathione peroxidase (GPX) activities; chlorophyll a and b (Chl a+b) content; essential metals, thiol-metabolites and polyphosphates (polyPs) levels; as well as on zinc and cadmium accumulation capacities. Control cells (EgZn(20); grown with 20 mu M Zn2+) showed a half-maximal inhibition of growth (IC50) of 1,700 mu M by external Zn2+. O-2 consumption, and APX and GPX activities were unaltered by Zn2+ treatments. Cells cultured with 500 or 1,000 mu M Zn2+ showed photosynthesis impairment but normal Chl a+b contents. Zn2+ preconditioning increased the intracellular contents of zinc (25-54 times) and calcium (2-27 times); thiol-metabolites and polyPs were only marginally altered. The growth of cells preconditioned to 400 mu M Zn2+ (EgZn(400) cells) was less susceptible to Cd2+ than that of EgZn(20) cells, although no differences in photosynthesis and respiration were observed. In cells chronically grown with Zn2+, the cadmium accumulation capacity was unchanged or slightly increased in the same culture media with high Zn2+, and increased by 42-90% in media with 20 mu M Zn2+. The thiol-metabolites increased at similar levels in both EgZn(20) and EgZn(400) cells when further exposed to 200 mu M Cd2+ and polyPs were at high levels independently of Zn2+ or Cd2+ treatments. It was concluded that chronic exposure to high Zn2+ (1) was innocuous for E. gracilis at concentrations lower than 0.5 mM and (2) promoted protection against Cd2+ toxicity and increased cadmium accumulation; and (3) these zinc effects involved GSH and polyPs metabolism and were associated with high intracellular zinc contents. (C) 2016 Elsevier B.V. All rights reserved.