Revealing the main toxic factor and mechanisms of shale gas flowback water on the growth of algae Chlorella pyrenoidosa.

Flowback Water (FBW) have been proved to show toxicity towards some aquatic organisms, but the toxic effect across different flowback periods and underlying mechanism remained unclear due to their complex and changeable constitutes. In this study, the toxicity of FBW, the respective activated carbon treated FBW (AC-FBW) with most organic compounds removed, and equivalent salt control (SC) towards Chlorella pyrenoidosa were tested in three different flowback periods. Their toxicity increased with the duration of flowback time due to the change of composition. In comparison of above three groups, the AC-FBW showed the strongest toxicity through growth inhibition, cell membrane, and oxidative stress tests, followed by SC and original FBW, indicating that the inorganics in the FBW played primary toxicity factors on algae growth, which was further verified by the toxicity test of simulated FBW. Proteomic analysis demonstrated that FBW inhibited algal photosynthesis via inhibiting electron transport. The suppression of electron transport also led to the increase of superoxide radicals thereby inducing oxidative stress. Furthermore, glycolysis and Tricarboxylic Acid (TCA) cycle were up-regulated in response to FBW stress. AC-FBW caused stronger inhibition of algal growth through greater altering electron transport and reaction center activation, and then inducing more substantial oxidative stress and membrane damage. The up-regulation of glycolysis and TCA cycle was also greater in AC-FBW. This study elucidated the toxic effect in different flowback periods and identified the primary toxic factors of FBW, providing a reference for the further optimization of wastewater treatment technologies.