Role Of Extracellular Polymeric Substances In Alleviating The Toxicity Of Nano-ZnO On Chlorella Vulgaris And Its Mechanism

With the wide application of nanomaterials,more nanoparticles inevitably are introduced into the environment through the manufacture,use and disposal of commercial products,potentially threatening aquatic organisms and affecting the balance of entir aquatic ecosystem.The behavior and biological effects on the environment of nanoparticles have also received wide attention from researchers in related fields.Algae in a specific environment can secrete high molecular polymer substance,named extracellular polymeric substances?EPS?,which would form a protective layer outside the cell and interact with nanoparticles to protect cells from damaging.Currently,the behavior and biological effects on the environment of nanoparticles with EPS are still unclear.In this study,nano-ZnO was employed as the representative of nanoparticles,and a green alga C.vulgaris widely distributed in water environment was selected as the test organism.Based on investigating the role of extracellular polymeric substances in alleviating the toxicity of nano-ZnO on C.vulgaris,the effect of EPS on the dissolution and sedimentation behavior of nano-ZnO was studied.Furthermore,the mitigation mechanism of EPS on the toxic effects of nano-ZnO C.vulgaris was investigated.The obstained results are as follows:?1?Growth of C.vulgaris could be significantly inhibited by nano-ZnO,however,EPS weakened the inhibitory effects.When the concentration of nano-ZnO was 0.04 mM,the growth inhibition rate of EPS-Free?EPS-F?cells at 24 h was 7%higher than that of EPS-Covered?EPS-C?cells;the content of chlorophyll a,b and total chlorophyll of EPS-F cells at 96 h was 80%,76%and 78%lower than that of the control group?0 mM nano-ZnO?,17%,12%and 14%lower than that of EPS-C cells,respectively.EPS could combine Zn²⁺dissolved from nano-ZnO,and nano-ZnO promoted the secretion of EPS from C.vulgaris.Under the treatment of 0.04 mM nano-ZnO,the content of Zn²⁺in S-EPS and B-EPS of C.vulgaris increased by 37%and 43%from 24 to 96 h,respectively.Under the treatment of 0.01 mM nano-ZnO,the polysaccharide and protein contents of EPS increased by 33%and 38%at 24 h,respectively,and the fluorescence intensity of tryptophan and tyrosine increased by8%and 11%,respectively.?2?EPS and its components,tryptophan?protein?and glucuronic acid?saccharide?,could promote the dissolution of nano-ZnO.When the concentration of nano-ZnO was low?0.01 mM?,tryptophan and glucuronic acid could promote the dissolution of nano-ZnO compared to ultrapure water,and the content of Zn²⁺in glucuronic acid was higher than tryptophan.When the concentration of nano-ZnO was increased to 0.15 mM,the dissolution rate of nano-ZnO in glucuronic acid was32%lower than that in ultrapure water.The ratio of free Zn²⁺to total dissolved Zn in glucuronic acid was lower than that in tryptophan.EPS could increase the sedimentation performance of nano-ZnO;compared with tryptophan,nano-ZnO in glucuronic acid was more likely to settle.When the concentration of nano-ZnO was0.15 mM,the relative absorbance of nano-ZnO in EPS,tryptophan and glucuronic acid was 0.38,0.63 and 0.25 at 5 h,respectively,which was 7%lower,54%higher and 39%lower than that in ultrapure water,respectively.?3?In the presence of EPS,the zeta potential of nano-ZnO increased,and gradually approached the zeta potential of EPS as the concentration of nano-ZnO increased.When the concentration of nano-ZnO was 0.15 mM,the zeta potential in EPS,tryptophan and glucuronic acid was-3.77,-4.48 and-1.80 mV,respectively,which was 34%,21%and 68%higher than that in ultrapure water.The zeta potential of nano-ZnO?0.01 mM?also increased with the concentration of tryptophan and uronic acid increase.The-OH or-NH group,C=O group in proteins,and C-O group of polysaccharides could combine with nano ZnO,N-H/C-N of amide II groups and C-O in carboxyl groups were the main binding sites.Based on the aboved results,the mechanism of alleviation effects of nano-ZnO are as follows:nano-ZnO accerlated the production of EPS,then EPS and its main components,protein and saccharide binded with the dissolved Zn²⁺.