Sensitivity of Chlamydomonas reinhardtii to cadmium stress is associated with phototaxis
Cadmium (Cd) is a common hazardous pollutant to aquatic environment and easy to be accumulated in living organisms. The roles of phototactic behavior in Cd tolerance in motile organisms are poorly explored. In this study, two Chlamydomonas reinhardtii strains, a wild type with positive phototaxis (CC125) and a negatively phototactic mutant (agg1) were used to assess the effects of phototaxis on Cd-induced toxicity to algae. Exposure to Cd inhibited the cell growth and photosynthetic activities, reduced photosynthetic pigment contents, and enhanced intracellular oxidative stress of algae. Well buffered by EDTA in algae medium, the concentration of Cd causing 50% of growth inhibition (EC50) of CC125 and agg1 of 72 h duration exposure were 55.96 and 77.20 µM L-1, respectively. Photosystem II activities in CC125 were more sensitive to Cd than agg1 at 60 µM L-1 of Cd. In addition, agg1 accumulated less intracellular Cd than CC125. The changes of extracellular polymeric substances and intracellular response to Cd stress might be related to the different tolerance to Cd of the two algae. Taken together, phototaxis was demonstrated to be associated with Cd-induced toxicity to C. reinhardtii.