Calcium availability but not its content modulates metal toxicity in Scenedesmus quadricauda

- Impact of pre-culture on medium with 0.17 or 4.40 mM Ca on metal toxicity was studied.
- Ca content rather decreased but Na content increased in metal-exposed high Ca samples.
- Amount of metals was in order Cd < Mn < Pb and high Ca samples contained less metal.
- Content of AsA, GSH and PC2 was affected mainly by Cd, GSH by Mn and PC2 by Pb.
- Elevated activities of SOD, APX, and CAT were typically found in high Ca samples.

Impact of calcium nutrition (pre-culture on solid medium with standard or elevated Ca dose, i. e. 0.17 and 4.40 mM marked as low and high Ca) on acute metal toxicity (Cd, Mn and Pb, 24 h of exposure to 10 µM) in freshwater green alga Scenedesmus quadricauda was studied. Surprisingly, Ca content differed only slightly between low and high Ca samples and applied metals rather suppressed its amount. Na content was higher in metal-exposed high Ca samples, indicating that Ca/Na ratio may affect accumulation of metals. Content of heavy metals increased in order Cd < Mn < Pb and high Ca samples contained less metal than low Ca samples at least in absorbed fraction. Accumulation of ascorbic acid and thiols (GSH - glutathione and PC2 - phytochelatin 2) was affected mainly by Cd, GSH also by Mn and PC2 by Pb with often significant differences between low Ca and high Ca samples. Calcium nutrition also affected responses of algae to metals at the level of antioxidative enzyme activities (SOD, APX, and CAT) and elevated values were typically found in high Ca samples while ROS (hydrogen peroxide and superoxide radical) were mainly depleted in Mn treatment. These data confirm that Ca nutrition affects accumulation of metals in algae and metabolic parameters as observed in vascular plants but, unlike them, rather Ca/Na ratio than absolute Ca content seems to regulate the uptake of metals.

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