The Ecological Effects And Mechanism Study Of ZnO Nanoparticles To Microorganism In Estuarine Aquatic Environment

With the rapid development of nanotechnology,more and more nanomaterials have been and will be applied in household and industrial products,and inevitably discharged into the natural environmental systems.As one of the most widely used metal oxides,zinc oxide nanoparticles(NPs)are used in a wide range of applications,such as fillers in rubber industry,component of creams,powders in the pharmaceutical and cosmetic industries,and absorbers of UV radiation in the sunscreen and textile industries 1,and they are also used as an additive in tire rubber,released to roadways with wear,which could bring some risk to ecological environment.As an emerging pollutants,nanoparticles,including ZnO NPs can be released into the environment intentionally or accidentally,and further enter the estuarine and coastal environment,and may bring certain ecological risks.The Yangtze River Estuary is located at the mouth of the Yangtze River as it flows into the East China Sea past the city of Shanghai,one of the largest city proper in the world,with a population in excess of 24 million people.This vitally important estuary has great ecological and economic significance,yet clearly it has been dramatically influenced by human activities.ZnO nanoparticles have been continuously entered into various environmental media as a pollutant,and a considerable part of it has entered the environmental water body,affecting the microorganisms in it.Microorganism is an important part of ecosystem,and the toxicity effect of ZnO nanoparticles to the microorganisms is likely to affect the stability and balance of the whole ecosystem.Therefore,it is very important to study the ecological risk of ZnO nanoparticles.This research mainly focuses on the short-term and long-term effects of ZnO NPs on the native bacteria in estuarine water with different salinity.The study found a time-dependent effect of ZnO NPs to native bacteria in estuarine waters,i.e.bacterial inhibition for short term effect(3h),and bacterial recovery for long term effect(more than 24h).In general,the abundances of bacteria were all recovered with time,and this recovery trend is most obvious at 24h in water dosed with high ZnO NPs concentrations.The bacteria that lived in higher salinity water showed stronger tolerance to ZnO NPs than in water with lower salinity.Furthermore,we found that although exhibiting acute toxicity of ZnO NPs,bacteria could reproduce and recover in long-term exposure in the nutrient rich estuarine aquatic system.Furthermore,we have measured the Zn ion concentration during the ZnO-NPs experiment.By comparing the Zn ion concentrations released by ZnO during the experiment to those in Zn ion control experiment,we were able to the dissolution of ZnO NPs was the primary cause for ZnO ecotoxicity in this study.At the same time,the identification of cultivatable bacteria was done,and the mainly bacteria are Bacillus spp.and Acinetobacter spp.Additionally,cultivable dominant bacteria and Zn-resistance bacteria were selected from each water sample to further elucidate the time-dependent effect(up to 24h)of ZnO NPs.The result proved that the ZnO nanoparticles have different impact on different bacteria,and the bundance and diversity of the native microbial community could be significantly disturbed or disrupted in estuarine environments.In order to find more details about the mechanism of the bacteria resistence.As a typical terrestrial bacterium,E.coli was selected and tested for the eotoxicological effect of ZnO NPs in different culture,medium.We also investigated the expression of two zinc resistance genes in E.coli at increasing salinity.The results showed that the viability of E.coli under different salinity rapidly decreased to nearly 0%at high concentrations of ZnO NPs.However,it appeared a significant rebound for the abundance of E.coli in the relative high salinity with the ZnO NPs concentration of 1 mg/L and 10 mg/L.In addition,the acute cytotoxicity of ZnO nanoparticles in environmental water was obviously higher than in pure water,which suggested that there are some pollutant in the environment,and the pollutant could have compound toxicity with ZnO nanoparticles,which would enhance the toxicity.The result of gene expression showed that the zntA genes in all groups treated with ZnO-NPs were in general highly expressed as compared to the control groups without ZnO,which prove the ZnO nanoparticles could stimulate zinc resistance genes(zntA and zntR)expressed in E.coli.For E.coli in SDK water at salinity of 0.2‰,both of these two genes were down-regulated within 24h with/without ZnO.However,in the groups with ZnO-NPs under salinity of 3‰ and 6‰,zntA expression was generally decreased within 3-6 h,and then increased till 24h.These results were consistent with the results of the cell viability assays,suggesting that Zn resistance system in E.coli may be activated to defense the stress of ZnO NPs,and salinity(up to 6 ‰)may promote this process in the aquatic system.