Cadmium accumulation and toxicity affect the extracytoplasmic polyphosphate level in Chlamydomonas reinhardtii

Cadmium is a non-essential metal and highly toxic for biological functions. Depending on the dose of Cd2+, the biochemical response will differ. In this study, we investigated the level of extracytoplasmic polyphosphate (polyp) when Chiamydomonas reinhardtii was exposed to the effect of CdCl2. When compared to control cells, Cd2+-treated cells to 400-600 mu M showed a decrease in the growth rate from 0.78 to 0.38 d(-1) for the strain CC 125, and a decrease from 0.81 to 0.32-0.35 d(-1) for CC-503. This indicated a strong toxicity effect on the population growth of cells during 72 h. In addition, the results demonstrated the decrease in the synthesis and/or the degradation of polyP that was correlated with the accumulation of Cd2+ in both algal strains. Furthermore, the level of polyP decreased in relation to the decrease of F-V/F-M value. The toxicity of Cd2+ induced a high level of cell necrosis for CC-503, and the level of polyp could not be recovered at 72 h. In response to the toxic effects of Cd2+, the observed formation of palmelloid colonies by CC-125 cells was correlated with the recovery of the polyp level. Nevertheless, both algal strains were able to accumulate significant amount of Cd2+ in their biomass. Therefore, our study demonstrated a distinct impact of Cd2+ on the metabolism of polyP (synthesis and/or degradation), which was dependent on the concentration of CdCl2 and the Chlamydomonas strain. Based on this study, the level of polyP can be used as a biomarker of Cd2+ toxicity at 24-48 h, even with the cell wall-deficient strain CC-503.