Effect Of Environmentally Relevant Antibiotics Long-term Exposure On The Growth And Antioxidant Responses Of Marine Algae

Antibiotics are considered as a kind of emerging environmental pollutants.Antibiotics are continuously discharged into the aquatic environment and will inevitably enter into marine by runoff,then affect marine unicellular algae in estuary and marine areas.Recently,there have been many studies about the toxicity of antibiotics to freshwater algae,however,only limited researches concentrated on the effect of environmentally relevant concentration antibiotics on marine unicellular algae.This study was performed to explore the long-term effect of environmentally relevant concentration of antibiotics on the growth and antioxidant responses of marine unicellular algae.Sulfamethoxazole(SMX)and norfloxacin(NFX)were chose as representatives of antibiotics,and marine Chlorella sp.and Prorocentrum Lima were chose as tested species.10 ng/L and 100 ng/L was set as exposure concentrations,and there were four single antibiotic exposure groups,four mixed antibiotics exposure groups and a control group in this study.We analysed and discussed the effect of antibiotics on the growth,the activities of antioxidant enzymes,and antioxidant activity content of two algae.The main results are as follows:The effects of antibiotics on the growth of the two algae were different,and Chlorella sp.was more sensitive to the two antibiotics.The growth of Chlorella sp.were significantly inhibited by environmentally relevant concentrations of SMX and NFX,and the highest inhibition rate was 35.01%.The growth of Prorocentrum Lima was significantly promoted by SMX,but not significantly effected by NFX.However,cell densities of high concentration NFX treatment groups(100 ng/L)was increased,indicated that the high concentration(100 ng/L)of NFX may also promote the growth of Prorocentrum Lima.Low concentration(10 ng/L)treatments had no significant effect on the growth of the two algae.Additionally,the toxicity of NFX was higher than SMX to Chlorella sp.,and the effect of combined exposure of the two antibiotics on Chlorella sp.was considered as a synergistic effect,the toxicity of SMX was higher than NFX to Prorocentrum Lima,and the combined effect between SMX and NFX was not obvious.The antioxidant responses of Chlorella sp.and Prorocentrum Lima were different.The activity of superoxide dismutase(SOD)in Chlorella sp.was significantlyincreased,while the activities of peroxidase(CAT)and peroxidase(POD)was significantly inhibited,and the glutathione(GSH)content in Chlorella sp.was increased.The activities of SOD(not significant),CAT and POD were increased in Prorocentrum Lima.SMX and NFX induced higher oxidative pressure on Chlorella sp.than Prorocentrum Lima,which was due to the thicker cell wall of Prorocentrum Lima and higher resistance ability of induced stress.The effect of SMX and NFX on the growth were mainly due to the balance between the activity of SOD and the activities of CAT and POD.For Chlorella sp.,the activities of CAT and POD activity were significantly decreased,while the activity of SOD was increased,this imbalance resulted in that hydrogen peroxide could not be eliminated in time,thus the growth of Chlorella sp.were inhibited.For Prorocentrum Lima,the changes of activities of CAT,POD and SOD were consistent,therefore the growth of Prorocentrum Lima was not inhibited,and even be promoted.The effect of combined exposure of SMX and NFX on the antioxidant system of the two algae was considered as a synergistic effect.High concentrations of mixed antibiotics induced greater oxidative stress.Our results indicated that green algae and dinoflagellate have different sensitivity to environmentally relevant antibiotics in marine,and the long term existence of antibiotics in marine environment may affect the influence the species interaction between marine algae,and thus influence the outbreaks of dinoflagellate red tide.