Nickel toxicity and mechanisms of nickel resistance in Scenedesmus acutus f. alternans

grantor: University of TorontoOrganisms living in metal-polluted environments, or exposed to metals in a defined growth medium, can develop metal resistance. Ni toxicity and mechanisms of Ni resistance were studied in a resistant strain of the green alga, 'Scenedesmus acutus f. alternans', B4, isolated from a Cu- and Ni-polluted lake, and Cu- and Ni-resistant strains (Cu-Tol and Ni-Tol) derived from a laboratory sensitive strain, UTEX72, by repeated subcultures in Cu- or Ni-containing media. Effects of Ni on growth paralleled effects on photosynthesis in each strain. Growth in UTEX72 was at least 18 times, and photosynthesis at least 3 times, more sensitive to Ni than in B4, Cu-Tol and Ni-Tol. Ni dosage causing 50% inhibition of photosynthetic rate decreased sharply with the increase in duration of exposure. Active transport is the major means of Ni uptake, but passive transport also exists in these strains. Cu-Tol and Ni-Tol exclude Ni, but B4 accumulates Ni, as resistant mechanisms. Ni resistance in Ni-Tol is due to decreased intracellular Ni uptake; this may be related to changes in Mg transport systems. Neither extracellular adsorption nor active Ni release is a major contributing factor to Ni resistance in these strains. Ni resistance in B4 does not rely on a larger intracellular pool of total non-protein thiols. However, higher levels of some specific thiol compothids in B4 than in UTEX72 may be involved in Ni resistance. B4 probably has a greater ability to maintain these thiol compounds in their reduced forms through its markedly higher level of glutathione reductase than that of UTEX72. Ni increased oxidative stress to a higher degree in UTEX72 than in B4 as indicated by a fluorescent probe. B4 had larger amounts and wider distribution of small polyphosphate granules than UTEX72. X-ray microanalysis revealed that Ni was associated with these granules in B4, suggesting a possible role of these polyphosphate granules in Ni complexation, and to some extent, in Ni-resistance.Ph.D