Effects Of Anionic Surfactant LAS And Polycyclic Aromatic Hydrocarbon Naphthalene Pollution Systems On Two Kinds Of Floating Plants

Water pollution has always been one of the major issues of global concern.Anionic surfactants,mainly linear alkylbenzene sulfonates(LAS),are one of the most commonly used raw materials for industrial and household detergents.With the large discharge of industrial wastewater and domestic sewage,LAS has become a common organic pollutant in the aquatic environment.Polycyclic aromatic hydrocarbons(PAHs)are persistent toxic pollutants that widely existing in the environment.They are listed as environmental priority monitoring pollutants by the US Environmental Protection Agency because of their strong carcinogenic,teratogenic and mutagenic ability.Naphthalene(NAP)is one of the most abundant PAHs in water.Because of its high water solubility,NAP is easily enriched by plants and accumulated in aquatic ecosystems.Plants in natural waters are rarely exposed to a single contaminant.LAS often acts as a detergent to elute PAHs from sediments,and the risk of both being present in the water is increasing.As two kinds of pollutants widely distributed in aquatic environment and increasing potential hazards,it is important to explore the interaction and combined effects of LAS and NAP to evaluate their environmental behavior and ecological risk.Floating plants are often directly exposed to the aquatic environment and do not contact with sediment.They are extremely suitable as materials for ecological safety assessment of water bodies.In this study,Spirodela polyrhiza(L.)Schleid.and Hydrocharis dubia(Bl.)Backer were used as research materials to investigate the response of floating plants to individual and combined pollution systems of LAS and NAP.The results are as follows:(1)The short-term effects of individual and combined LAS and NAP pollution on S.polyrhiza were explored.A single LAS/NAP concentration gradient of 0.5,2.5,5 and 10 mg/L was set up in this experiment,the combined LAS+NAP concentration gradients of 0.5+0.5,2.5+2.5,5+5 and 10+10 mg/L were set up.The gradient was conducted for a short period of 14 days.The results showed that under short-term stress,LAS alone induced hormesis and hormesis has a weakening trend as the exposure time increases.The peroxidase(POD)enzyme and catalase(CAT)enzyme are the main antioxidant enzymes in protecting S.polyrhiza from LAS stress.NAP at a concentration of 0.5-10 mg/L had little or no toxic effect on S.polyrhiza.NAP can increase the content of malondialdehyde(MDA),and the membrane lipid is damaged.With the increase of NAP exposure time,the S.polyrhiza had a certain adaptability to NAP toxicity.The antioxidant enzyme system superoxide dismutase(SOD),POD and CAT reacted negatively,with no significant changes.After 14 days,the combined LAS+NAP pollution had a strong stress on S.polyrhiza,and the toxicity was dose-dependent.The combined toxicity assessment showed that LAS and NAP had a strong synergistic effect on S.polyrhiza when the concentration of LAS and NAP was more than 5+5 mg/L.Compared with NAP,LAS played a more important toxic role.The strong synergistic effect is probably due to the influence of LAS toxicity on the normal absorption function of roots,which enhances the uptake of NAP by S.polyrhiza and increases NAP toxicity to S.polyrhiza.POD enzyme is the main protective enzyme of S.polyrhiza when exposed to LAS + NAP.(2)Exploratory experiments on the short-term and long-term effects of individual and combined LAS and NAP pollution on H.dubia.In this experiment,four concentration gradients of 0.5,5,10,and 20 mg/L for LAS/NAP alone treatments were set and the combined LAS+NAP was 0.5+0.5,5+5,10+10,and 20+20 mg/L.Monitoring was conducted 14 days(short-term)and 28 days(mid-long-term)after treatment.We found that the toxicity of single LAS stress to H.dubia was dose-dependent.Combined with physiological and biochemical indicators,in the short-term LAS exposure,with increasing concentration,MDA content of H.dubia increased first and then decreased,soluble sugar and soluble protein showed a downward trend and then increased,SOD activity and POD activity increased,which played a protective role.After mid-long-term exposure,H.dubia can recover from low concentration(0.5 mg/L)LAS injury,and the POD enzyme activity increased to protect H.dubia cells.LAS concentration(?5 mg/L)caused some damage to H.dubia,and MDA decreased significantly in the highest treatments.Soluble sugar content and CAT activity showed a trend of increasing first and then decreasing with increasing concentration.Soluble protein content and SOD activity increased gradually.Through the changes of these indicators,we can conclude that H.dubia had certain resistance to LAS.In individual NAP study,we found that under the short-term NAP stress,the physiological responses of H.dubia were affected to some extent.CAT enzyme activity increased first and then decreased,which was a sensitive enzyme to NAP stimulation.Soluble protein and SOD enzyme were the main protective substances in H.dubia under high concentration of NAP stress.After mid-long-term exposure,H.dubia were resistant to lower NAP concentrations(?5 mg/L),which can adapt to and alleviate the toxic effects.NAP treatment(?10 mg/L)caused damage to H.dubia,the root activity decreased.SOD,APX enzyme and soluble sugar gradually increased with the increase of concentration,playing a major protective role.In the study of combined LAS+NAP on H.dubia,the short-term stress had a significant effect on the physiological and biochemical indexes of the leaf cells of H.dubia.Most indicators showed inhibition,the activities of SOD and CAT increased,which played protective role.Under mid-long-term LAS+NAP stress,most of the growth indicators of H.dubia showed dose-dependent damage response,and root activity also decreased significantly.High concentration of LAS + NAP toxicity(?10+10 mg/L)caused more significant damage to the underground part of H.dubia.When the concentration of LAS+NAP was more than 5+5 mg/L,LAS and NAP had antagonistic effects on H.dubia,and their combined effects on the aboveground and underground parts were not completely the same.It is presumed that NAP affects the uptake of LAS by the roots of H.dubia and weakens the uptake of LAS by plants,thus reducing its toxic effects and causing antagonism.Under mid-long-term combined LAS and NAP stress,the physiological and biochemical responses of H.dubia were more negative and suffered more significant damage.In this study,H.dubia were more tolerant to high concentration LAS toxicity than S.polyrhiza.The stronger resistance of H.dubia was observed to NAP and LAS+NAP combined toxicity,while S.polyrhiza was relatively sensitive.When the concentration of LAS and NAP was more than 5+5 mg/L,LAS and NAP had a strong synergistic effect on S.polyrhiza,but had antagonistic effect on H.dubia.It can be seen that the combined pollution effects of surfactants and PAHs in aquatic ecosystems are complex and have certain potential risks.On this basis,it can be concluded that S.polyrhiza is more suitable as a sensitive monitoring plant for combined LAS + NAP risk and H.dubia has the potential to become LAS and NAP bioremediation plants in aquatic ecosystems.