Hormetic dose-response of halogenated organic pollutants on Microcystis aeruginosa: Joint toxic action and mechanism

Quinolones (QNs), dechloranes (DECs), and chlorinated paraffins (CPs) are three kinds of new halogenated organic pollutants (HOPs), which originate from the use of flame retardants, lubricants and pesticides. Since QNs, DECs, and CPs are frequently detected in waters and sediments, it is necessary to investigate the toxic effects of these HOPs with dwelling phytoplankton, especially for cyanobacteria, to explore their potential hormetic effects and contributions to algal blooms. In the present study, we investigate single and joint toxicity of QNs, DECs and CPs on Microcystis aeruginosa (M. aeruginosa), a cyanobacterium that is frequently implicated with algal blooms. The results indicate single QNs and DECs induce marked hormetic effects on the proliferation of M. aeruginosa but CPs do not. The stimulatory effect of hormesis is linked with accelerated replication of DNA, which is considered to stem from the moderate rise in intracellular reactive oxygen species (ROS). Joint toxicity tests reveal that both QNs & CPs mixtures and DECs & CPs mixtures show hormetic effects on M. aeruginosa , but QNs & DECs mixtures show no hormetic effect. QNs & DECs mixtures exhibit synergistic toxic actions, which may be caused by a sharp rise in intracellular ROS simultaneously produced by the agents. Joint toxic actions of both QNs & CPs, and DECs & CPs shift from addition to antagonism as concentration increases, and this shift may mainly depend on the influence of CPs on cell membrane hydrophobicity of M. aeruginosa. This study provides data and toxic mechanisms for the hormetic phenomenon of single and joint HOPs on M. aeruginosa. The hormetic effects of HOPs may benefit the proliferation of M. aeruginosa in the aquatic environment, aggravating the formation of algal blooms. This study also reflects the important role of hormesis in environmental risk assessment of pollutants.

Automated summary

Quinolones, dechloranes, and chlorinated paraffins are new types of halogenated organic pollutants that come from flame retardants, lubricants, and pesticides. This study investigates the toxic effects of these pollutants on cyanobacteria, particularly Microcystis aeruginosa, and finds that quinolones and dechloranes have hormetic effects on their proliferation, while chlorinated paraffins do not. The joint toxicity tests show that mixtures of quinolones & chlorinated paraffins and dechloranes & chlorinated paraffins also exhibit hormetic effects on Microcystis aeruginosa. However, mixtures of quinolones & dechloranes do not have a hormetic effect. It is also found that mixtures of quinolones & dechloranes have synergistic toxic actions, while the joint toxic actions of both quinolones & chlorinated paraffins, and dechloranes & chlorinated paraffins shift from addition to antagonism with increasing concentration. The study provides important data and mechanisms for understanding the hormetic phenomenon of these pollutants on Microcystis aeruginosa and highlights the role of hormesis in the risk assessment of environmental pollutants.