The Toxicity Mechanisms Of Silver Nanoparticles To Aauatic Plants And Their Effects To Aauatic Ecosystem

Silver nanoparticles(AgNPs)are used in numerous commercial products,such as medical equipments and antibacterial or deodorant products.They are inevitably discharged into the environments in the process of their production,transportation,consumption and disposition.Aquatic systems are believed to be one of the pools of waste AgNPs.To investigate do the discharged AgNPs aggravate the change of structure and loss of function of aquatic systems because of pollution and eutrophication?We choose a higher aquatic plants Spirodela polyrhiza and a green alga Chlamydomonas reinhardtii as models to determine the toxicity of AgNPs and its mechanism(s)to aquatic plants.We also set the aqutic microcosms those contain water,sediment,aquatic plant(Hydrilla verticillata),aquatic invertebrate(Radix spp)and aquatic vertebrate(Gambusia affinis)to determine the chronic effects of AgNPs to aquatic system.The results showed that there was a dose-dependent increase in levels of reactive oxygen species(ROS),superoxide dismutase and peroxidase activity,and the antioxidant glutathione content under 0-10 mg/L AgNP treatments in S.polyrhiza tissues.The transmission electron micrograph(TEM)showed that 10 mg/L AgNP treatment drastically changed the structure of S.polyrhiza chloroplast which contained larger starch grains and fewer intergranal thylakoids than those in control.The changed chloroplast ultrastructure may be caused by ROS production induced by AgNPs or chloroplast is one of the targets of AgNPs in S.polyrhiza.Furtherly,using chlorophyll fluorescence,we found that 5 and 10 mg/L AgNPs significantly changed the parameters of chlorophyll fluorescence that suggesting AgNPs could demage the reaction centers of photosystem II.The rapid light curves(RLC)showed that AgNPs not only inhibited the capcity of photosynthesis but also decreased the capcity of photoprotection of S.polyrhiza under high light tensity.Ribulose-1,5-bisphosphate carboxylase-oxygenase(Rubisco),the key enzyme for CO2 fixation,was very sensitive to AgNPs and it was easiest inhibited by AgNPs.The fluorescence microscope images comfirmed that more of ROS generated within the chloroplasts.In plant tissues,Ag,whatever it originates from Agpor AgNPs,has similar capacity to induce ROS accumulation in the aquatic plant,S.polyrhiza.Similarly,AgNPs also can significantly promote ROS accumulation in C.reinhardtii indicating that AgNPs can induce oxidative stress to aquatic plants.The accumulation of ROS in C.reinhardtii was significant higher under light condition than under dark condition.The activity of carbohydrate related enzymes was significantly inhibited by AgNPs,especially the Rubisco and Glyceraldehyde-3-phosphate dehydrogenase(GAPDH).The internalized AgNPs(or their dissolved Ag+)could bind on the thiol groups(-SH)in the protein and cause the reduction of ezymes activity.GAPDH was more sensitive to AgNPs and Ag+ than malic dehydrogenase(MDH)because it contains-SH in reaction center but MDH reaction center dose not contain.Mass spectra and circular dichroism(CD)data showed that Ag element bound on-SH and changed the secondary structure of protein,as a consequence,enzyme activity decreased.The microcosm’s results showed that the surface layer of sediment was the main sink of Ag element for both AgNPs and AgNO3.The biomass of phytoplankton,aquatic plant and animals was not significantly different between control and samples treated with AgNPs or AgNO3 for 90 d.However,both aquatic plant(Hydrilla verticillata)and animals(Gambusia afnis and Raix spp)significantly accumulated Ag(60,70 and 12 μg/g dry weight respectively)that indicated the healthy risk from the food web transportation.The nitrification rate and its related microbe(Nitrospira)abundance significantly decreased suggesting AgNPs could disturb nitrogen cycle in aquacit system.Our present studies were performed from laboratory pure culture and mimic aquacit system to investigate the mechanism(s)of AgNPs toxcicity to aquatic plants and to estimate the effects of AgNPs to aquatic systm.All results suggested that:1)AgNPs could internalize by aquatic plant,then AgNP dissociate into high toxic Ag+and interacted with-SH in protein.Rubisco inhibition leads to slowing down of CO2 assimilation.Consequently,the solar energy consumption decreases and then the excess excitation energy promote ROS generation and disturb plant metabolisms;2)In aquatic system surface layer of sediment was the main sink of AgNPs.Aquatic plants and animals could significantly accumulate Ag that makes a heath risk from fodd web.The decrease of nitrification rate and its related microbe could disturb nitrogen cycle in aquatic system.