Phytochelatin formation kinetics and toxic effects in the freshwater alga Chlamydomonas reinhardtii upon short- and long-term exposure to lead(II)

Phytochelatins (PC) are metal-binding ligands synthesized by algae in response to elevated concentrations of various metals, such as Pb. Kinetics of PC synthesis and Pb accumulation in Chlamydomonas reinhardtii were investigated as a function of [Pb2+] = 10−11–10−7 M (pPb11–pPb7.1) in the exposure medium for up to 6 h. The role of PC in Pb detoxification was explored by relating PC synthesis to the effects of Pb on growth and photosynthetic yield upon exposure to pPb9 and pPb8.3 for up to 72 h. Pb accumulation increased with increasing [Pb2+], reaching a maximum concentration of 596 ± 77 amol/cell (intracellular concentration 2.98 mM) at pPb7.1. Low concentrations of PC2–PC4 were present in C. reinhardtii grown in control media without Pb addition. Upon short-term exposure, PC2 and PC3 synthesis was induced within minutes at [Pb2+] ≥ pPb8 and PC4 synthesis after a lag phase at pPb7.1. Cellular PC2–PC4 concentrations increased with time over 6 h and with increasing [Pb2+]. PC concentrations after 6 h exposure to pPb7.1 were 28.5 ± 0.2 amol/cell (142 μM) PC2, 2.8 ± 0.05 amol/cell (14 μM) PC3 and 0.30 ± 0.01 amol/cell (1.5 μM) PC4. Upon long-term exposure, induction of PC synthesis was detected at pPb9 and synthesis of PCs with a higher degree of polymerization was observed (PC5). PC concentrations were lower than intracellular Pb and were thus not present at sufficiently high concentrations to immobilize accumulated Pb. Inhibition of photosynthesis and growth up to 100% was observed upon long-term exposure, whereas in short-term experiments no inhibitory effects were detected.