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Accumulation of zinc protects against cadmium stress in photosynthetic Euglena gracilis

Sánchez-Thomas, R., Moreno-Sánchez, R., García-García, J.D.
Environmental and experimental botany 2016 v.131 pp. 19-31
Euglena gracilis, cadmium, calcium, cell growth, chlorophyll, chronic exposure, culture media, cultured cells, glutathione peroxidase, growth retardation, heavy metals, inhibitory concentration 50, oxygen, oxygen consumption, photosynthesis, polyphosphates, toxicity, zinc
To determine the interplay between zinc and cadmium on the heavy metal accumulation capacity of Euglena gracilis, the effects of increasing Zn2+ concentrations (13–65.4ppm or 200–1,000μM) were analyzed on growth; O2 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 (EgZn20; grown with 20μM Zn2+) showed a half-maximal inhibition of growth (IC50) of 1,700μM by external Zn2+. O2 consumption, and APX and GPX activities were unaltered by Zn2+ treatments. Cells cultured with 500 or 1,000μM 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 400μM Zn2+ (EgZn400 cells) was less susceptible to Cd2+ than that of EgZn20 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 20μM Zn2+. The thiol-metabolites increased at similar levels in both EgZn20 and EgZn400 cells when further exposed to 200μM 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 0.5mM 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.