PHYSIOLOGICAL AND BIOCHEMICAL STUDIES ON AN ACID-TOLERANT CHLORELLA VULGARIS UNDER COPPER STRESS

Abstract

This study compared the growth, photosynthesis, electron transport, ATP content, NO₃^- and NH₄⁺ uptake, Na⁺ and K⁺ loss, nitrate reductase and ATPase activities of an acid-tolerant strain and a wild-type strain of Chlorella vulgaris exposed to Cu at varied pHs (pH 6.8, 5.0, 4.0 and 3.5). A general reduction in growth, NH₄⁺ uptake, photosynthesis and nitrate reductase activity of both the strains was noticed at decreasing pH. In contrast, the acid-tolerant strain depicted an increased NO₃^- uptake, insignificant Na⁺, K⁺ loss and superactive ATPase at decreasing pH. Both pH and Cu produced a non-competitive inhibition of NO₃^- uptake. An increase in V_max of ATPase of acid-tolerant Chlorella and inhibition of the V_max of its wild-type strain at decreasing pH was noticed. Though the magnitude of Cu toxicity was significantly low in tolerant strain, a significant negative correlation (r=-0.99, p<0.01) of Cu uptake with acid pH was noticed. The acid-tolerant Chlorella was less sensitive to Cu and physiologically more efficient than the wild type. Presence of a superactive ATPase and change in membrane permeability are considered responsible for acid tolerance in Chlorella.