The Occurrence,fate And Hazards Of Antibiotics In The Aquatic Environment

The discovery of antibiotic started a new era for medicine and developed a revolution of infectious diseases control,nowadays,there are hundreds of antibiotics that have been registered in open market and extensively used for human and animal anti-infective therapy,as well as growth promotion in agricultural activities.However,overuse of antibiotics has generated a serious ecological problem worldwide,because of the rapid increase of antibiotics production and consumption in last several decades,and the insufficient of relevant regulation.After intake by human and animals,a significant fraction of as much as 30-90% antibiotics is excreted in the forms of parent or metabolites via urine and feces,which would result in continuous release of excreted antibiotics into the environment,and they are therefore also regarded as “pseudopersistent” contaminants.With the properties of hydrophilic compounds,antibiotics are generally accumulated in the aquatic environment,and watersheds receiving wastewater from human activities are invariably faced with serious problems of antibiotics contamination.However,limited studies have explained the following questions: in what degree that anthropogenic activity effect on the antibiotics contamination in the aquatic environment;how antibiotics transport between different aquatic systems under the impact of different patterns of hydraulic connection;and what toxicity effects would be generated on the aquatic organisms with the exposure of antibiotics at environmental detected residual levels.Two study areas,one located at Jianghan Plain of central China and the other at Ozark Highland of central USA,were selected to identify antibiotic contamination sources and seasonal and spatial variation of antibiotics and their migration in the aquatic environment,to evaluate the hazards and risks evaluation of environmental detected antibiotic residues to the aquatic organisms,and to implement toxicity test by selecting the most sensitive organism exposed to antibiotics applied with environmental measured ratios.Target antibiotics in this study included macrolides,tetracyclines,fluoroquinolones,and sulfonamides.Major results and conclusions of this study are as follows.1.By selecting watersheds applied with different intention of anthropogenic pressure(reference,moderate,and intensive)in Ozark Highland,central USA,to visually identify the source of antibiotics in surface water,and address the extent of human activities influenced on antibiotics contamination in the aquatic environment.The highest concentrations of antibiotics were found in surface water from intensive area with WWTP effluents of massive discharge.Water samples from forest area with the least human disturbance(reference)and pasture land with poultry farming(moderate)were detected with much lower residual levels of selected compounds,which clearly indicates that the urban WWTP effluents were the major source of antibiotics for the surface water.The intensive human activities(urban area in this study)caused the most serious antibiotics contamination with annual average total concentration up to 202.87 ng/L,which was over 30 and 70 times higher than that detected in reference and moderate watersheds,respectively.Sulfamethoxazole and trimethoprim,widely used together to enhance the therapeutic effect,were predominantly detected in all water samples and with an average ratio of 6:1 in concentrations in water samples from WWTP effluents discharging watershed(intensive).Concentrations of total selected antibiotics in water samples were detected with higher value in spring than in winter,which could be attributed to more antibiotics usage for epidemic disease control and the monitored relative slower flow rates of surface water in spring.2.Further study aimed to explore the fate of antibiotics in the aquatic environment after being transported into surface waters was conducted in a rural area in Jianghan Plain,central China,where there is intensive livestock farming and no centralized treatment for domestic sewage and agricultural wastewater.Livestock wastewater,river/rivulet water,and groundwater from different depths of the aquifers were sampled to investigate antibiotics contamination,especially their seasonal and spatial patterns in the aquatic environment.With information about the seasonal variation of groundwater hydrochemistry,the transport of antibiotics between surface water and groundwater was studied.The variation of residual antibiotics in the aquatic environment in this study was contributed to not only the spatial correlation between sampling sites and pollution sources,but also water flow and dilution effect in rainy seasons.Total concentration levels of detected antibiotics in the aquatic environment were in the order of piggery wastewater > river/rivulet water > groundwater,and the highest antibiotic residues in wastewater occurred in winters with sulfonamides and macrolides as predominant contaminants.However,it was in spring when the highest concentration levels of selected antibiotics in river/rivulet water were detected.For river/rivulet waters,macrolides were dominated during three sampling seasons except in winter 2013;groundwater sampled from three different depths of the aquifers during four seasons were all detected with fluoroquinolones and tetracyclines at significant higher residual levels among four selected antibiotic classes.There was no significant and consistent tendency of attenuation under the effect of horizontal flow from upstream to downstream in rivers.However,due to filtration effect of the aquifer matrix,the residual antibiotics concentrations in groundwater decreased with the aquifer depth(10 m > 25 m > 50 m below land surface).The elevated values of DOC,EC,Cl-,and SO42-,and the decrease of Fe and Mn concentration in groundwater detected in spring and summer indicate that the recharge of surface water resulted in changes in hydrogeochemical conditions during rainy seasons.Considering the highest recharge and lowest concentrations of target antibiotics in groundwater during summer sampling campaign,and the similar seasonal variations of antibiotics concentrations in surface water and groundwater,it could be deducted that antibiotics in groundwater in the study area were initially originated from the antibiotics-contaminated surface water through the process of surface water recharge of groundwater,and the lower concentrations of antibiotics in groundwater during rainy seasons were the result of both antibiotics introduction and water dilution derived from contaminated surface water.3.The study then translated field observations of antibiotics derived from two study areas to water quality hazards assessment using resistance hazard approaches derived from minimal selective concentrations for antibiotic resistance,and potential aquatic environmental risks of both individual and mixture of antibiotics to three trophic levels(algae,daphnids,and fish).Results derived from the more conservative method of predicted no effect concentration(PNEC)for resistance selection indicated that antibiotic residues in waters applied with intensive human activities(intensive area,Ozark Highland,USA)and drained with agricultural and domestic wastewater(wastewater and river/rivulet in Jianghan Plain,China)would potentially result in the antibiotic resistance selection with the resistance hazard ratios(RHR)exceeding one.The lower residual level of trimethoprim in the environment with higher probability of antibiotic resistance selection than sulfamethoxazole(based on the RHR values)indicates that the low residual level of individual antibiotic was not always associated with the low hazard or risk to the ecosystem.Besides,the mixture of sulfamethoxazole and trimethoprim showed higher RHR values than that of individual antibiotics,suggesting that the mixture residues in the environment would more likely trigger the production of resistance genes than single antibiotic under the same circumstances.The risk assessment was further conducted to determine the probability and severity of the variety of hazards under the environmental residual levels of antibiotics.The sensitivity to the antibiotic residues among three trophic levels was as follows: algae > daphnids > fish.Same to the hazard assessment,waters with the most serious potential ecological risks posed by antibiotic residues were surface waters with frequent anthropogenic activities in both study areas.Antibiotics detected in groundwater from different depths of the aquifers were within the “relatively safe” scale to aquatic organisms with the RHR value lower than 1 in hazard assessment,while in the risk assessment,risk quotients(RQs)of individual antibiotics for fish were all below 0.01,as to the lower forms of life algae and daphnids,the potential risks were elevated to medium or high level.What's more,the risks calculated from evaluation approaches of mixture risk were all assessed to be significantly higher than individuals.The hazard and risk assessment for mixtures poses more serious threat to ecosystem than that for individual antibiotics in this study,suggesting the urgent need in carrying out the hazard assessment of aquatic organisms to the antibiotic mixture residues in the environment.4.Since algae was demonstrated as the most sensitive organism response to the hazards of antibiotics,a species of micro-algae,P.subcapitata,was selected to conduct the toxicity test.The green alga was exposed to sulfamethoxazole and trimethoprim individually,and then to the mixture of them with environmental detected ratio of 6:1.Toxicity data of half maximal effective concentration(EC50)was derived from the growth inhibition test lasted for total of 96 h with cell number endpoint as a surrogate for growth,the photosynthetic related endpoint,pigment contents,was then detected to further assess the physiological impacts on P.subcapitata.The toxicity data derived from 72 h cultivation was lower than that from 96 h culturing for all of toxicity test batches.Considering that the toxicity data would be used for the further hazard and risk assessment and combined with the duration range of logarithmic phase for P.subcapitata,72-h toxicity values(72-h EC50)would be with more practical significance for the accuracy and conservative evaluation perspective.The toxicity endpoint based on the growth inhibition was less sensitive than algal photosynthesis,including the pigment production of chlorophyll A,chlorophyll B and total carotenoid.Values of EC50 detected in the growth inhibition test indicated stronger toxicity of sulfamethoxazole than trimethoprim,with 72-h EC50 of 0.37 mg/L and 56.46 mg/L,respectively.Furthermore,the mixture of two antibiotics dosed at the environmental detected ratio of 6:1 determined with rather higher adverse effects on target species than individual exposure based on both growth and photosynthesis endpoints,and resulted in 72-h EC50 of 0.257 mg/L for sulfamethoxazole coupled with 0.043 mg/L for trimethoprim in the growth inhibition test.The detected maximum environmental concentration of sulfamethoxazole and trimethoprim in two investigation areas of this study were at level of ng/L,which were not high enough to exert obvious inhibitory effect on selected algal species of P.subcapitata based on both growth and physiology.