Balancing tolerance, toxicity, and detoxification in Chlamydomonas reinhardtii under silver stress

In this study, we investigated how a freshwater alga Chlamydomonas reinhardtii developed tolerance to Ag following pre-exposure to sublethal concentrations of 4.13 nM and 10.33 nM for 7-30 days. These concentrations were selected to induce adaptive responses without causing acute toxicity. Algae pre-acclimated to these conditions showed increased tolerance, with the maximum quantum yield of photosystem II (Fv/Fm) and the effective concentration inhibiting 50 % of growth (EC50) increasing by 33.8 % and 15.4 %, respectively. Intracellular Ag accumulation decreased to 70.2 %, while the Ag elimination rate increased by 1.3- to 4.8-fold compared to naive cells, indicating enhanced detoxification capacity. The intracellular distribution of Ag shifted from the pyrenoid to vacuoles and chloroplasts, suggesting improved compartmentalization for detoxification. Upon re-exposure, naive cells shrank to 93.2 % of their original size, whereas pre-acclimated cells maintained a stable size. Chloroplast stress responses were reduced to 84.8 % of the level observed in naive cells, indicating decreased sensitivity. Mitochondria exhibited increased fusion during toxicity tests, reflecting more interconnected and resilient networks. However, despite these adaptive responses, ROS levels increased to 1.4 times those of control, and GSH synthesis did not recover during prolonged exposure, suggesting a trade-off between enhanced tolerance and sustained redox homeostasis. These findings reveal how freshwater algae adapt to Ag stress and offer quantitative insights for ecological risk assessments.