Abstract: Tetracycline is one of the widely used antibiotics for animals as prescription drugs and as growth promoters in livestock.As a matter of fact,much of the tetracycline is excreted with urine or feces and released into the aquatic environment through many pathways,thus posing a potential threat to aquatic organisms.Microcystis aeruginosa is an important algae in the aquatic ecosystem.In order to determine toxic effects of tetracycline on non-target aquatic organisms,M.aeruginosa was used as a subject in this study,and effects of tetracycline on photosynthesis and antioxidant enzymes of M.aeruginosa were investigated.Results show that exposed to tetracycline(0.80-35.00 mg·L-1),the algae declined significantly in chlorophyll fluorescence and potential maximal relatively electron transport rate(Re,t,max).The inhibiting effects intensified with the increase in concentration of tetracycline,with the maximal inhibitation rates being 39.95% and 44.08%,respectively,after 4 days,and rising up to 59.48% and 91.90% after 7 days.The activity of superoxide dismutase(SOD) in the algae dulled by 30.36%-35.92% and 25.03%-35.51% after 4 and 7 days of exposure,respectively,and no significant difference was found among treatments different in tetracycline concentration.The activity of peroxydase(POD) was intensified after 7 days of exposure.The findings indicate that tetracycline hinders photosynthesis of M.aeruginosa,disturbs balance of the antioxidant enzymes system,and then inhibits growth of the algae.

Key words: tetracycline, Microcystis aeruginosa, photosynthesis, antioxidant enzyme