Reduced uptake as a mechanism of zinc tolerance in Oscillatoria anguistissima

Oscillatoria anguistissima could tolerate 50 ppm ZnSO4. 7H(2)O, and a zinc- tolerant strain with maximum tolerance concentration (MTC) of 100 ppm ZnSO4 . 7H(2)O was obtained by stepwise transfer to higher concentrations. The ad aptation was irreversible even after three generations in metal-free medium . In the presence of metal, the tolerant strain grew with a shorter lag per iod of 4 days as against 6 days in the case of the wild strain. The toleran t strain had higher MTC than that of the wild strain for other metals also, viz., Ni2+, Co2+, Cu2+ and Cd2+. The zinc resistance in the tolerant strai n was a result of reduced uptake, since around 42% of the total metal was p resent on the surface as against only 30% in the wild strain. The calcium-s timulated uptake, as observed in the wild strain, was absent in the toleran t strain. Ultrastructural comparisons revealed no structural change in the tolerant strain on exposure to zinc, whereas in the wild strain a thick ext racellular matrix was observed.