Quantitative evaluation and the toxicity mechanism of synergism within three organophosphorus pesticide mixtures to Chlorella pyrenoidosa

Organophosphorus pesticide (OPP) pollutants in the environment pose toxicity risks to living organisms, and the possible toxicity mechanism needs to be further clarified. Therefore, the individual and combined toxicity of three OPPs namely acephate (ACE), trichlorfon (TRI) and glyphosate (GIY) towards a freshwater green alga Chlorella pyrenoidosa (C. pyrenoidosa) was investigated by the time-dependent microplate toxicity analysis method. Here, a ternary mixture system of the three OPPs including five rays with different concentration ratios was designed by the uniform design ray method. The standard additive reference model, concentration addition (CA), was used to analyse toxicity interaction within ternary mixtures and the toxicity interaction intensity was characterized using a deviation from CA model (dCA). Besides, the effects of the three OPPs and their mixtures on the chlorophyll (CHL) content, superoxide dismutase (SOD) activity and malondialdehyde (MDA) content of C. pyrenoidosa were also investigated to explore the possible mechanisms. The results show that toxicity of the three pesticides and their ternary mixture rays is time-dependent and the combined toxicity correlates well with the components, ACE and GLY. It is likely that there is a significant time-dependent synergism in ternary mixtures induced by ACE and GLY. The synergism intensity of the ternary mixtures is not more than 30% at the whole experimental concentration level. The CHL reduction rate and MDA content of C. pyrenoidosa increase, while the SOD activity of C. pyrenoidosa decreases with the lengthening of exposure time under the action of the three pesticides and their ternary mixtures. So, the possible mechanism of the three pesticides and their mixtures may be by affecting the photosynthesis, and then causing oxidative damage to C. pyrenoidosa cells. The results can provide reference for the combined toxicity assessment of OPPs to living organisms.