Role Of Extracellular Polymeric Substances In Nitrogen And Phosphorus Uptake By Chlorella Vulgaris In The Presence Of Nano-ZnO

The environmental safety and biological effects of nanomaterials have received widespread attention with their extensive usage.Algal biofilm technology has the advantages of high removal efficiencies for nitrogen and phosphorus,low energy consumption,and resource utilization.However,the process are often affected by nanoparticles in the sewage,then inhibited the algal growth and the secretion of extracellular polymers substances(EPS).As a macromolecule substance,EPS can fix matrix and prevent the harm of exogenous toxicants into cells.However,the role and mechanism of EPS in the uptake of nitrogen and phosphorus in the presence of nanoparticles is still unclear.In this paper,a green alga Chlorella vulgaris was selected as the target,and nano-ZnO was selected as the representative of nanoparticle.Based on the investigation of the role of EPS in the algal growth and the uptake of nitrogen and phosphorus at the presence of nano-ZnO,the effects of nano-ZnO on the promotion of EPS secretion and its relationship with the uptake of nitrogen and phosphorus was studied.Moreover,we employed ultraviolet spectroscopy(UV-vis),three-dimensional fluorescence spectroscopy(3D-EEM),fourier infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)to analyze the interaction between nano-ZnO and EPS.The following conclusions were obtained:(1)Nano-ZnO showed significant inhibitory effects on the growth of Chlorella vulgaris.However,EPS weakened the inhibitory effect of nano-ZnO on the biomass,chlorophyll synthesis and photosynthetic activity of C.vulgaris.When the nano-ZnO concentration was 0.15 mM,the average growth rates of EPS-covered(EPS-C)and EPS-free(EPS-F)cells were 48.44%and 60.55%of the control group(without nano-ZnO),respectively,which of EPS-F cells was 23.48%lower than EPS-C cells;the contents of chlorophyll a,chlorophyll b and total chlorophyll of EPS-F cells were 45.54%,68.09%and 51.29%of the control group,respectively,which decreased by 16.30%,11.20%and 14.90%compared with EPS-C cells;the maximum fluorescence energy conversion efficiency(Fv/Fm),relative photosynthetic electron transport rate(rETR),and PSII potential activity(Fv/F0)of the three fluorescence photosynthetic parameters of EPS-F cells were 13.88%,5.83%and 6.67%of the control group,respectively,which decreased by 18.18%,11.35%and 16.67%compared with EPS-C cells.Moreover,in the presence of 0.15 mM nano-ZnO,the uptake efficiencies of NH4+-N by EPS-C and EPS-F cells were 73.64%and 77.04%of the control group,respectively,which of EPS-F cells increased by 20.43%compared with EPS-C cells after 7-d cultivation,indicating that EPS can maintain the stability of the intracellular NH4+-N content,and reduce the inhibitory effect of nano-ZnO on the uptake of NH4+-N by C.vulgaris.While the uptake efficiencies of PO43--P by EPS-C and EPS-F cells were 80.79%and 79.20%of the control group,respectively,which of EPS-F cells decreased by 4.18%compared with EPS-C cells,indicating that EPS had littler effects on the PO43'-P uptake by C.vulgaris than that of NH4+-N.(2)Nano-ZnO promoted the secretion of EPS from C.vulgaris and the secretion of polysaccharides and proteins in EPS,and the the promoting effect of proteins in soluble EPS(S-EPS)and polysaccharides in bound EPS(B-EPS)was more obvious.When the nano-ZnO concentration was 0.04 mM,the S-EPS and B-EPS contents in EPS-F cells were 12.57 mg/L and 5.69 mg/L,respectively,which were 36.78%and 16.84%higher than the control group;the polysaccharide contents of S-EPS and B-EPS in EPS-F cells were 3.05 mg/L and 2.57 mg/L,respectively,which were 18.33%and 23.58%higher than the control group;and the protein contents of S-EPS and B-EPS in EPS-F cells were 8.61 mg/L and 3.15 mg/L,respectively,which were 45.86%and 10.17%higher than the control group in the 3-d cultivation time.The 3D-EEM spectrum verified that S-EPS was mainly composed of tryptophan-like,tyrosine-like and humic acid-like substance,while B-EPS mainly contained tryptophan-like and tyrosine-like substance.The humic acid-like and protein-like(tryptophan-like and tyrosine-like)substance in the S-EPS or B-EPS were enhanced in the presence of nano-ZnO.The EPS contents had a satisfactory correlation with the uptake efficiencies of NH4+-N(R2=0.9466)in the presence of 0.04 mM nano-ZnO,while the correlation between EPS content and the uptake efficiencies of PO43--P was smaller than that of NH4+-N(R2=0.8128).(3)Nano-ZnO can interacte with EPS.UV-vis spectrum verified that EPS changed the intensity of ultraviolet absorption characteristic peaks and made the blue shift of nano-ZnO.3 D-EEM spectrum verified that nano-ZnO quenched the phenomenon of protein-like substances in EPS,because the protein-like substance combined with dissolved Zn2+,and tryptophan-like and Zn2+ had stronger binding ability.FTIR spectrum identified that the-NH and C=O groups of proteins,the-OH and C-O-C groups of polysaccharides,the C-H groups of carboxylic acids and-CH3 and-CH2 groups of lipids in EPS interacted with Zn2+.XPS spectrum showed that-COOH,-OH and-NH2 groups were involved in the interaction of EPS and Zn2+.