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 Chlamydomonas reinhardtii was exposed to the effect of CdCl₂. When compared to control cells, Cd²⁺-treated cells to 400-600 µM 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.