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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.