A systematic review on toxicity assessment of persistent emerging pollutants (EPs) and associated microplastics (MPs) in the environment using the Hydra animal model

Emerging pollutants (EPs) are causative for teratogenic and reproductive effects. EPs are detected in all the environmental matrices at higher levels. A suitable model for aquatic toxicity assessment is Hydra, because of morphological, behavioral, reproductive (sexual and asexual), and biochemical changes. Many researchers have used Hydra for toxicity assessment of organic chemicals (BPA), heavy metals, pharmaceuticals, nanomaterials and microplastics. Various Hydra species were used for environmental toxicity studies; however H. magnipapillata was predominantly used due to the availability of its genome and proteome sequences. Teratogenic and reproductive changes in Hydra are species specific. Teratogenic effects were studied using sterozoom dissecting microscope, acridine orange (AO) and 4',6-diamidino-2-phenylindole (DPAI) staining. Reactive oxygen species (ROS) generation by EPs had been understood by the Dichlorodihydrofluorescein Diacetate (DCFDA) staining and comet assay. Multiple advanced techniques would aid to understand the effects at molecular level, such as real-time PCR, rapid amplification of cDNA end- PCR. EPs modulated the major antioxidant enzyme levels, therefore, defense mechanism was affected by the higher generation of reactive oxygen species. Genome sequencing helps to know the mode of action of pollutants, role of enzymes in detoxification, defense genes and stress responsive genes. Molecular techniques were used to obtain the information for evolutionary changes of genes and modulation of gene expression by EPs.