Study On Promoting Self-flocculation Of Microalgae By Extracellular Polymers

Microalgae are widely distributed in the natural world,with a wide variety of species and rapid growth,and can synthesize abundant bioactive substances,which have high value in the fields of biodiesel,biomedicine and health food.However,the difficulty of harvesting microalgae severely restricted the industrialization of microalgae.Therefore,It is particularly important to have a simple and efficient way of harvesting microalgae.Compared to other harvesting techniques,self-flocculating harvesting has advantages in efficiency,operation and environmental protection.In this study,the microalga Heveochlorella sp.Yu was selected as the research subject,and a suitable self-flocculating culture system was established by optimizing the medium.The Extracellular Polymeric Substances（EPS）of Heveochlorella sp.Yu was analyzed at different culture stages,the correlation between self-flocculation formation and the composition of Loosely Bound Extracellular Polymers Substances（LB-EPS）was studied,to reveal the mechanism of the role of LB-EPS in self-flocculation process of microalgae,which provided a new idea for microalgae harvesting.The specific research results were summarized as follows:（1）The effects of four different carbon sources（D-（+）-maltose,sodium acetate,glucose,sucrose）and six different nitrogen sources（sodium nitrate,potassium nitrate,ammonium chloride,ammonium nitrate,ammonium sulfate,urea）on the growth and self-flocculation of Heveochlorella sp.Yu were investigated under the condition of BG-11 medium.The high self-flocculating efficiency（84.21%）and biomass(1.91 g L^-1)were obtained at the beginning of the stable period（7 days）in the self-flocculating culture system of Heveochlorella sp.Yu,which glucose and potassium nitrate were identified as carbon and nitrogen sources.（2）The effect of Extracellular Polymeric Substances（EPS）on the self-flocculation of Heveochlorella sp.Yu was studied at different culture stages（day5,day7 and day9）.It was found that Loosely Bound Extracellular Polymers Substances（LB-EPS）played a dominat role in the self-flocculation process of microalgae.The spontaneous condensation process of Heveochlorella sp.Yu cells was analyzed in terms of adhesion free energy(?₇))and total interaction energy(?₍（9）⁽（3（1））by combining thermodynamics and XDLVO theoretical model.It was found that the Lewis acid-base action energy(hydrophobic action energy?_（9）played a leading role in the spontaneous aggregation of microalgae.（3）The correlation between the ratio of protein to polysaccharide（PN/PS）in LB-EPS and its Zeta potential was studied by comparing the hydrophobicity of LB-EPS at different culture stages.It was found that the high PN/PS ratio and hydrophobicity of LB-EPS and the low absolute value of Zeta potential of LB-EPS were the main reasons for inducing microalgal self-flocculation.（4）The high hydrophobicity of LB-EPS was related to its tryptophan protein content,but also to the tightness of its protein structure by three-dimensional fluorescence spectroscopy（3D-EEM）and protein secondary structure analysis.The looser the structure of the protein,the more easily its hydrophobic groups were exposed,and the higher hydrophobicity of LB-EPS.The high hydrophobicity of LB-EPS was the key to induce the self-flocculation of Heveochlorella sp.Yu.（5）LB-EPS were extracted from Heveochlorella sp.Yu at day 7 of culture,and then added to the culture broth of five strains of microalgae without the ability of self-flocculation（Acutodesmus sp.HLGY,Tetradesmus obliquus Zhao,Desmodesmus sp.ZFY,Ankistrodesmus sp.EHY,Coelastrella sp.CHZ）.It was found that LB-EPS promoted flocculation efficiency of all microalgae to different degrees,and the higher concentration of LB-EPS,the better flocculation efficacy could be obtained.