Effect Of Extracellular Polymeric Substances On Cadmium Accumulation And Detoxification By Two Diatoms

Diatoms are the primary producers of marine ecosystems,accounting for about 40%of marine primary productivity.Diatoms have the advantages of fast growth and high biodiversity,and have been widely used in wastewater treatment.Diatoms secrete a large amount of extracellular polymer(EPS)during their growth process.These polymers are rich in functional groups that can complex heavy metals to remediate heavy metal pollution.So far there are few reports on the cadmium(Cd)accumulation ability of diatom EPS,and effects of EPS on photosynthesis and antioxidant enzyme system of diatom cells under Cd stress remain unclear.In this paper,P.tricornutum and Nitzschia sp.were used to investigate the growth,changes of intracellular photosynthesis and antioxidant system,and interactions of extracellular EPS content,surface functional groups and fluorescent substances with Cd in two diatoms with and without EPS.The effects of EPS on Cd accumulation and detoxification in diatom were investigated.The results were as follows:1.Effects of EPS on growth,photosynthetic parameters and antioxidant system of two diatoms under Cd treatmentUnder Cd treatment,compared with the normal algae cells,the growth of P.tricornutum and Nitzschia sp.without EPS increased,Fv/Fm had no significantly change,ΦPSⅡ decreased,chlorophyll content of P.tricornutum increased,and chlorophyll content of Nitzschia sp.decreased.With the increase of Cd concentration,the MDA content of P.tricornutum without EPS decreased,the SOD content increased,and the CAT content first decreased and then increased.The contents of MDA,SOD and CAT increased after removing EPS,which indicated that the detoxification process of the two diatoms was different under Cd stress.By analyzing the changes of intracellular GSH content of P.tricornutum and Nitzschia sp.with and without EPS,we found the GSH content of two diatoms increased after EPS removing.These results indicated that EPS of two diatoms could affect the synthesis of GSH.2.Interaction of Cd with two diatoms and their EPSThe results of ATR-FTIR analysis showed that the surface functional groups C-O-C and C-OH of P.tricornutum and Nitzschia sp.changed significantly after the application of Cd,indicating that the functional groups of polysaccharides on the surface of diatom reacted with Cd.With the increase of Cd concentration,the content of polysaccharide and protein in EPS of two diatoms increased.Compared with the control group without Cd treatment,the content of polysaccharide(16.51 mg/g)and protein(2.58 mg/g)in EPS of P.tricornutum increased by 2.6 times and 7.8 times under 3 mg/L Cd treatment;The polysaccharide(13.34 mg/g)and protein(3.15 mg/g)in EPS increased 2.5-fold and2.1-fold under the treatment of 7 mg/L Cd.3D-EEM analysis showed that tryptophan and aromatic protein in EPS of P.tricornutum and Nitzschia sp.quenched with the increase of Cd concentration.3.Effect of EPS on Cd accumulation in two diatomsThe Cd removal rate of P.tricornutum was 31.4%～52.18%,and it decreased with the increase of the Cd concentration.The maximum concentration of Cd by P.tricornutum was 8.44 mg/g,and the absorption was the dominant process in Cd accumulation.The Cd removal rate of P.tricornutum without EPS was significantly reduced,the accumulation reduced by 69.2%,and the adsorption ratio showed an upward trend.Under the treatment of 3 mg/L,the adsorption dominated Cd accumulation by P.tricornutum without EPS.These data showed that EPS played an important role in the removal of Cd by P.tricornutum,and EPS was involved in the accumulation mechanism of P.tricornutum.The removal rate of Cd by Nitzschia sp.was 12.12%～37.15%,and the maximum concentration of Cd by Nitzschia sp.was 3.93 mg/g.The Cd removal rate of Nitzschia sp.without EPS was about 15.73%～28.53%.The maximum accumulated Cd was 3.03 mg/g,with absorption and adsorption accounted for 50% each.The results indicated that Nitzschia sp.accumulated Cd and EPS played a certain role in the adsorption and resistance of this metal.In conclusion,the effects of EPS on the growth,photosynthetic and antioxidant system of different diatoms and Cd accumulation under Cd treatment were studied.Mechanisms of Cd detoxification by were investigated in and out of two diatom cells,and the roles of EPS in the Cd tolerance and enrichment by diatoms were clarified.Diatoms increased the tolerance to Cd by the EPS secretion and responses of intracellular antioxidant enzyme system.