Effects Of Sediment And Salinity On The Bioavailability Of Typical Antibiotics To Zebrafish

Currently antibiotic residues in the environment have attracted worldwide attention due to their large-scale consumption,ubiquitous occurrence,and potential risks for wildlife and human.These compounds are only partially metabolized in organisms,and as high as 80-90%are excreted as parent compounds via urine and feces before being discharged into the aquatic environment.It has been reported that antibiotics possess high migration ability.Sulfamethoxazole and roxithromycin have been widely detected in water,soil,sediment,and aquatic animals.Contaminant concentrations may increase at high trophic levels once they enter into a food chain or food web,which will threat the ecological environment and human health.Generally,aquatic sediment has been identified as an important medium to retain organic contaminants including relatively hydrophobic antibiotics.Aquatic organisms incorporate chemicals through gills,integument,and the digestive system.To some extent,the bioaccumulation of chemicals can be affected by the presence of suspended particles in aqueous phase such as sediments and colloids.In addition,the combined exposure of antibiotics also affects their bioavailability.Research has shown that antibiotics display significant toxic effects on fish,although of which there are major gaps in our knowledge.One of the key assessment indicators of the antibiotics toxicity on aquatic organisms is their bioavailability.Zebrafish are widely used as the model organism as they have many advantages e.g.small size and easy breeding.This study therefore aimed to determine the fate and effects of sulfamethoxazole,roxithromycin and their combination in a microcosm using zebrafish as the model organism.Specifically the study examined the distribution of the antibiotic between water,sediment and zebrafish phases in microcosm;the bioavailability of antibiotic to zebrafish;and the effects of sediment particle size,salinity and combined exposure of antibiotics on bioavailability.The main findings are as follows:(1)Sediment particle size and salinity are the focus of this study,in identifying their effects on the bioavailability and bioaccumulation of antibiotic sulfamethoxazole to zebrafish as a model organism in microcosm.Sulfamethoxazole concentrations in water were gradually reduced,while in sediment and zebrafish gradually increased,suggesting active adsorption and bioaccumulation processes.The presence of sediment particles and their interactions with water reduced the bioavailability of sulfamethoxazole in zebrafish.The sediment of smaller particle size with more organic carbon content adsorbed sulfamethoxazole more extensively and decreased its bioavailability most significantly.The effect became more severe with increasing salinity in water due to the salting out of the antibiotic.At equilibrium,the distribution of sulfamethoxazole in different phases was quantified,with most sulfamethoxazole being associated with water(92.3%),followed by sedimentary phase(2.5%)and finally zebrafish(0.05%).(2)The concentrations of roxithromycin in different single or combined systems(water,water-sediment,water-zebrafish and water-sediment-zebrafish)were analyzed by UHPLC-MS/MS,to quantify their respective contributions to roxithromycin distribution in the simulated environment.Following exposure,roxithromycin concentrations in water were gradually reduced,while in sediment and zebrafish gradually increased.It was confirmed that the presence of sediment in the water reduced the bioavailability of roxithromycin.Under the condition of high salinity,roxithromycin was more likely to adsorb to sediment as a result of salting out effect,leading to a reduction in roxithromycin bioavailability and body burden of roxithromycin residues in zebrafish.At equilibrium,in the water-sediment-zebrafish system,sediment could adsorb 42.0%of roxithromycin as the main accumulation place.Meanwhile,water and zebrafish accounted for 48.7%and 0.16%of roxithromycin.(3)This study attempts to investigate the effects of sulfamethoxazole and roxithromycin on the bioavailability of zebrafish.The presence of roxithromycin reduced sulfamethoxazole in sediment due to the competitive adsorption.Sulfamethoxazole in zebrafish gradually increased due to the combined exposure.In the presence of sulfamethoxazole in the system,there was no significant effect of combined exposure on roxithromycin in sediment and zebrafish.Under the combined exposure,sulfamethoxazole and roxithromycin in water reduced more in the co-existence of sediments and zebrafish than in the existence of either sediments or zebrafish in system.The concentration of dissolved organic carbon in water was gradually increased in water,water-sediment,water-zebrafish and water-sediment-zebrafish system.The more the number of phases in the system,the high the dissolved organic carbon concentration in the water.This study also observed that there was a negative correlation between antibiotic concentration and dissolved organic carbon concentration in water.These results presented provided a reference for the study of environmental behavior and ecotoxicology of antibiotics.