[Toxicological Effects of Enrofloxacin and Its Removal by Freshwater Micro-Green Algae Dictyosphaerium sp.]

ABSTRACT

Enrofloxacin (ENR), a fluoroquinolones antibiotic, is widely used in the medical and aquaculture fields. Its residues in surface waters in China are high. However, few studies have evaluated both its toxicity to phytoplankton and the degradation or removal by microalgae. In this study, the growth, photosynthetic activity, and exopolysaccharides (EPS) of freshwater micro-green algae Dictyosphaerium sp. and the dynamics of ENR concentrations (0, 5, 25, 50, and 100 mg·L-1) were studied through an exposure experiment for 12 days. Results showed that the biomass and photosynthetic pigment content of Dictyosphaerium sp. increased with increasing exposure time in each treatment; however, it showed a significant inhibitory effect on the growth and pigment accumulation of Dictyosphaerium sp. compared with the control group (P<0.01). The LC50 of ENR to Dictyosphaerium sp. was (241.29±7.33) mg·L-1 after 96-h exposure, indicating that Dictyosphaerium sp. could adapt to the stress conditions of high concentration ENR. Meanwhile, when the concentration of enrofloxacin was<5 mg·L-1, it was found to promote the maximum photosynthetic rate (Fv/Fm) of Dictyosphaerium sp. On the contrary, when the concentration of enrofloxacin was>5 mg·L-1, photosynthetic inhibition was observed (P<0.01). The actual photosynthetic rate (Yield) and the maximum electron transfer rate (ETRmax) showed a trend of initially decreasing and then increasing in 12 days. It can gradually adapt to the stress conditions and recover certain photosynthetic activity after 6 days' exposure. In addition, ENR can also stimulate the EPS (RPS and CPS) release. At the end of the experiment, the removal rates of ENR in the four control groups (no algae addition groups) (5, 25, 50, and 100 mg·L-1ENR) were 7.27%, 5.56%, 5.30%, and 4.88%, respectively, while the removal rates of the treatment groups were 3.21, 3.01, 2.69, and 2.83 times of the no algae groups, indicating that Dictyosphaerium sp. had a significant promoting effect on the removal of ENR (P<0.01). Overall, our results can provide new insights for the understanding of the ecological toxicity of fluoroquinolone antibiotics to primary producers in the aquatic system and also provide new ideas for the ecological removal of antibiotic residues in water bodies and the biological resource utilization of freshwater microalgae.