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

Two Chlamydomonas reinhardtii 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. 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 (FV/FM ratio) under copper stress was also significantly higher for the Cu population than for Re and control populations. The Re population showed only residual tolerance towards the elevated concentration of copper, which is revealed by an FV/FM ratio slightly higher than in the control population under Cu²⁺ stress in dim light or in darkness. We postulate that in the Chlamydomonas populations 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.