THE RESISTANCE OF DUNALIELLA-ACIDOPHILA AGAINST HEAVY-METALS - THE IMPORTANCE OF THE ZETA-POTENTIAL

The resistance of photosynthesis and growth of the acid-resistant green alga Dunaliella acidophila (optimal growth at pH 1.0) and the salt-resistant Dunaliella parva (grown at pH 7.6) against Al(III), La(III), Cu(II), Cd(II), Hg(II) (applied as chlorides) and W(IV) (applied as Na2WO4), and the effects of these compounds on the zeta potential of the cells were measured in glycerol media of comparable ionic strength. Dunaliella cells with a positive zeta potential (Dunaliella acidophila, pH 1-2) are extremely resistant to toxic di- and trivalent cations, but are sensitive to toxic anions. On the other hand, cells with a negative zeta potential (D. acidophila and D. parva, pH 7.0) are resistant to toxic anions, but sensitive to toxic cations. The non-dissociated HgCl2 causes similar inhibitions of photosynthesis in both Dunaliella species, independent of the pH of the medium. Di- and trivalent cations cause an increase of positive zeta potentials, but a decrease of negative zeta potentials. Toxic polyvalent anions decrease positive zeta potentials, but increase negative zeta potentials. Almost no effect of HgCl2 on the zeta potential was observed. The results demonstrate that the zeta potential plays an important role in cation and anion toxicity. The interpretation of effects of toxic ions on growth is more difficult than the interpretation of the effects of these ions on photosynthesis, since the chemical composition and electrical properties of the culture media are very different, leading to different activity coefficients of the toxic ions as well as competition effects between non-toxic and toxic ions. This applies especially to divalent cations, whereas trivalent cations exhibited similar differential inhibition patterns for growth to those observed for photosynthesis. The actual sensitivity of growth of D. parva cells to toxic divalent cations was masked by the high salinity of the culture medium: The growth of D. acidophila e.g., was more sensitive to Cu(II) than the growth of D. parva. Mechanisms of toxicity of polyvalent cations and anions are discussed as well as the ecophysiological benefit to an acidophilic organism of high resistance against heavy metals.