Acclimation of Primary Photosynthetic Reactions in the Cells of Chlamydomonas reinhardtii to Cadmium: Analysis of Cell Population Heterogeneity

The mechanisms of acclimation of primary photosynthetic reactions in the green microalgae Chlamydomonas reinhardtii to the toxic action of cadmium were investigated by analyzing the dynamics of the distribution of key photosynthetic parameters in the cell population. A synchronous culture of microalgae was incubated for 96 h in the presence of 25 μM Cd, and OJIP-transient curves of individual cells were recorded at different stages of incubation with the toxicant using an original microfluorometer. The analysis of OJIP-transient curves made it possible to determine distributions of key JIP-test parameters: F_V/F_M, ET_O/ABS, RC/ABS, which represent photochemical activity of PS II, electron transport in PS II, and light absorption per active reaction center in PS II, respectively. Acclimation of primary photosynthetic reactions in microalgae to cadmium was accompanied by the appearance of two dominating cell fractions, which demonstrated the stable values of photosynthetic parameters when energy imbalance occurred due to the presence of the toxicant. The cells related to the first fraction retained moderate PS II activity by the effect of reduced PS II antenna size, while the cells of the second fraction showed low photochemical activity of PS II keeping antenna size similar to that of the control cells.