Variations in growth, photosynthesis, oxidative stress and microcystin production in Microcystis aeruginosa caused by acute exposure to Benzalkonium Chloride and Benzalkonium Bromide

Due to the COVID-19 pandemic, the use of quaternary ammonium salts (QACs) and their release into the aquatic environment increased dramatically. The ecological risks of QACs still need to be explored. Two extensively used QACs, Benzalkonium Chloride (BAC) and Benzalkonium Bromine (BAB), were selected to explore their acute toxic effects on a common bloom-forming cyanobacterium M. aeruginosa. Both QACs had the same 96 h-EC50 concentration on M. aeruginosa, which was 0.97 mg L-1. Nevertheless, BAC was able to inhibit the growth of M. aeruginosa at real environmental concentrations. Under the EC50 concentration, the inhibition of photosyn-thesis was more intense induced by BAC than BAB. The ROS levels under BAC and BAB stress were 5.88 and 4.31 times of the control, demonstrating that the oxidative stress caused by BAC was stronger. Notably, the increase of antioxidant substances such as SOD, CAT and GSH and the significant upregulation of genes such as sod2 proved that oxidation defense mechanisms were activated in response to the stressors. Both QACs significantly promoted the synthesis and release of microcystin-LR but the mechanism was different. The effects of QACs on microbial and algal communities and on microcystins synthesis should be paid special attention in future environmental management work.