Cadmium effect on the growth, photosynthesis, ultrastructure and phytochelatin content of green microalga Scenedesmus armatus:: A study at low and elevated CO2 concentration

Short-term experiments were carried out to examine the toxicity of cadmium chloride (CdCl2) at a concentration of 93 mu M (EC50/24) to green microalga Scenedesmus armatus, cultured at low (0.1%) and elevated (2%) concentration of CO2. Cadmium did not affect the viability of cells cultured for 24 h in both CO2 variants but markedly inhibited the growth of algae. This inhibition was more pronounced in cultures aerated with 0.1% (about 50% of control) than with 2% CO2 (about 75% of control) and did not change during 72 h of culture. Cadmium inhibited the rate of oxygen evolution (P-oky.) (70% of control) of cells cultured at 0.1% CO2 and had no effect on P-oxy. of cells cultured at 2% CO2. The values of the chlorophyll fluorescence parameters, i.e. F-M (maximum fluorescence yield), F-V (variable fluorescence), F-V/F-M (maximum quantum yield of PSII), Phi(PII) (effective quantum yield of PSII) and qP (photochemical quenching) were reduced by cadmium treatment in algae grown at 0.1% CO2 concentration, whereas F-0 (initial fluorescence yield) remained unaffected. In high-CO2 grown cells only F-V was significantly reduced. Cd-treated cells synthesized several thiol-containing peptides identified by HPLC as a dimer (PC2), a trimer (PC3) and a tetramer (PC4) of phytochelatins (PCs). High-CO2 grown cells produced significantly more PCs than low-CO2 grown cells and their individual appearance depended on the time of exposure and CO2 level. The ultrastructural analysis of low-CO2 grown cells showed in chloroplasts an increased number of small starch grains visible around the pyrenoid. In the enhanced vacuome compartment, various types of vacuoles were clearly seen in Cd-treated cells. Vacuoles containing non-membranous, electron-opaque deposits of an undefined structure and myelin-like figures were especially observed. The results suggest that algae living in conditions of elevated CO2 are better protected against cadmium than those at ordinary CO2 level, and productive processes are less affected than the growth ones. (C) 2007 Elsevier B.V. All rights reserved.