Differences in the response of Chlorella pyrenoidosa to three antidepressants and their mixtures in different light-dark start cycles

The use of antidepressants is increasing along with the continuing spike in the prevalence of depression worldwide. As a result, more and more antidepressants are entering the water and probably does harm to the aquatic organisms and even human health. Therefore, three antidepressants, including fluoxetine (FLU), citalopram (CIT), and aspirin (APC), were selected to investigate the toxic risks of antidepressants and their mixtures to a freshwater green alga Chlorella pyrenoidosa (C. pyrenoidosa). Due light is critical for the growth of green algae, six different light-dark cycle experiments were constructed to investigate the differences in toxicity and interaction responses of C. pyrenoidosa to antidepressants and their ternary mixture designed by the uniform design ray method. The toxic effects of individual antidepressants and their mixtures on C. pyrenoidosa were systematically investigated by the time-dependent microplate toxicity analysis (t-MTA) method. Toxicity interactions (synergism or antagonism) within mixtures were analyzed by the concentration addition (CA) and the deviation from the CA model (dCA) models. The results showed that the toxicities of the three antidepressants were different, and the order was FLU > APC > CIT. Light-dark cycles obviously affect the toxicity of three antidepressants and their combined toxicity interaction. Toxicity of the three antidepressants increases with the duration of light but decreases with the duration of darkness. The ternary antidepressant mixture exhibits antagonism, and the longer the initial lighting is, the stronger the antagonism. The antagonism of the ternary mixture is also affected by exposure time and mixture components' p(i) as well as exposure concentration.