Identification Of An Alga-symbiotic Bacterium Gordonia And Characterization Of Its Exopolysaccharides

In recent years,bacterial exopolysaccharides（EPS）have received widespread attention due to their non-toxic,safe,biocompatible,and degradable environmental-friendly properties,as well as their unique physical structure and chemical properties.In this study,an EPS-producing Gordonia terrae was isolated from the culture medium of Scenedesmus,and found that there were few reports on the strain.Therefore,in this paper,the EPS yield was improved by optimizing the medium,and conducts a preliminary exploration on the composition structure and physicochemical properties of EPS.The main researches are as follows:（1）Isolation and identification of strains.The strain TD18 producing exopolysaccharide（TD18-EPS）was obtained from the culture medium of Scenedesmus by dilution coating method.According to morphological analysis,physiological and biochemical tests and 16S r RNA sequencing,the strain TD18 was identified as Gordonia terrae and named as Gordonia terrae TD18.（2）Optimization of medium composition.First,the single factor method was used to screen out the most suitable carbon and nitrogen sources,and then the Plackett-Burman（PB）design was used to screen 9 single factors,and 3 important factors were selected for the Central composite design（CCD）,and it was concluded that the best optimization scheme is:glucose 21.01 g/L,Na NO₃1.63 g/L,Mg SO₄0.16g/L,KH₂PO₄0.5g/L,K₂HPO₄0.5g/L,citric acid C 5m L/L,Ca CO₃0.03g/L,trace amount Metal T 1m L/L.The maximum yield of TD18-EPS was 3.45 g/L,which was a 130%increase in yield compared to the initial medium.（3）Extraction,purification and structural characterization of TD18-EPS.The purified TD18-EPS was obtained by ethanol precipitation extraction and separation by Q-Sepharose Fast flow anion exchange column and Sephadex G-200 column.Structural analysis showed that the molecular weight of TD18-EPS was about 23Kda,and it contained carboxyl and acetyl functional groups.It was composed of glucose,mannose,galactose and glucuronic acid.The types of glycosidic bonds are:（1→）,（1→3）to glucose,（1→3）to glucuronic acid,（1→4）to galactose,（1→6）,（1→3,6）,（1→2,6）linked mannose.（4）Physicochemical properties of TD18-EPS.The degradation temperature of TD18-EPS is 308℃,and it has good thermal stability.Its solution viscosity decreases with the increase of shear rate,showing pseudoplastic non-Newtonian fluid characteristics.TD18-EPS can form stable emulsion to mineral oil and edible oil under different temperature,p H and salt concentration conditions.The antioxidant research on TD18-EPS showed that TD18-EPS had a good antioxidant activity in a dose-dependent manner.he maximum scavenging abilities of DPPH radicals,ABTS⁺radicals,hydroxyl radicals and superoxide anion-radicals were 72.8±1.73%,81.1±1.51%,87.65±1.96%and 34.13±1.88%,respectively.Hence,Gordonia terrae TD18 is the first alga-symbiotic Gordonia strain identified so far,while TD18-EPS is unique in composition and structure different form the known Gordonia EPS with excellent physiochemical properties and thus of potential industrial application.