REGULATION OF HEAVY-METAL TOXICITY IN ACID-TOLERANT CHLORELLA - PHYSIOLOGICAL AND BIOCHEMICAL APPROACHES

To explore the mechanism(s) of survival of the organism in acid- and m etal-stressed environment and to test certain hypotheses, the growth r ates, uptake of NH4+, Na+, K+, Ca2+, NO3-, PO43-, efflux of Na+ and K, activities of nitrate reductase, acid phosphatase and ATPase and int ernal pH of an acid-tolerant and an acid-sensitive strain of Chlorella vulgaris exposed to Cu and Ni at different pHs (pH 6.8, 5.0, 4.0 and 3.5) were compared. A general reduction in all variables was noticed w ith decrease in pH; however, the acid-tolerant strain was metabolicall y more active than the acid-sensitive strain. Reduced cation (NH4+, Na +, K+, Ca2+, Cu2+ and Ni2+) uptake coupled with an increased concentra tion of anions (NO3- and PO43-) supported the hypothesis of the develo pment of positive membrane/zeta potentials in the acid-tolerant strain . Further an accelerated accumulation of NO3- and PO43- in the cells a nd a significant increase in ATPase activity with decreasing pH at neu trality in the acid-tolerant strain either by exporting H+ ions into t he external medium or by producing OH- ions from NO3-. The lower toxic ity of Cu and Ni to the acid-tolerant strain compared to the acid-sens itive strain also supported our hypothesis of low metal uptake in the acid-tolerant strain due to cation repulsion by the membrane having lo w permeability and positive zeta and membrane potentials. The developm ent of a superactive ATPase and a change in membrane potential and per meability not only offer protection against acidity by keeping interna l pH neutral but also confer co-tolerance to metals.