Copper excess-induced large reversible and small irreversible adaptations in a population of Chlamydomonas reinhardtii CW15 (Chlorophyta)

Abstract

<p>Two <em>Chlamydomonas reinhardtii</em> CW15 populations modified by an excess of copper in growth medium were obtained: a “Cu” population that was continuously grown under the selection pressure of 5 µM Cu²⁺ (for at least 48 weeks) and the “Re” population, where a relatively short (9 week) exposure to elevated copper, necessary for acquiring tolerance, was followed by a prolonged period (at least 39 weeks) of cultivation at a normal (0.25 µM) copper concentration.</p><p>Cells of the Cu population were able to multiply at a Cu²⁺ concentration 16 times higher than that of the control population at a normal light intensity and at a Cu²⁺ concentration 64 times higher when cultivated in dim light. The potential quantum yield of photosystem II (F_V/F_M ratio) under copper stress was also significantly higher for the Cu population than for Re and control populations.</p><p>The Re population showed only residual tolerance towards the elevated concentration of copper, which is revealed by an F_V/F_M ratio slightly higher than in the control population under Cu²⁺ stress in dim light or in darkness.</p><p>We postulate that in the <em>Chlamydomonas populations</em> studied in this paper, at least two mechanisms of copper tolerance operate. The first mechanism is maintained during cultivation at a standard copper concentration and seems to be connected with photosynthetic apparatus. This mechanism, however, has only low adaptive value under excess of copper. The other mechanism, with a much higher adaptive value, is probably connected with Cu²⁺ homeostasis at the cellular level, but is lost during cultivation at a normal copper concentration.</p>