| Microalgae are one of the most promising bioresources that are receiving a significant interest with the increasing recognition of their potentials in mitigating CO2emissions as well as the removal of environmental pollutants from water, in addition to their applications in production of biofuels and diverse functional ingredients of food and Pharmaceuticals. However, harvesting microalgae biomass from diluted cultures is one of the obstacles for developing economically viable processes at large scales for the above purposes. Microalgal biomass recovery through cell flocculation, particularly the self-flocculation of microalgal cells, is a promising strategy towing to its low operation cost, simple manipulation and low energy consumption as well as less costly in equipment investment. However, studies on microalgal cell self-flocculation are still very limited. Some microalgae are naturally flocculated, and characterization of the flocculation mechanism is of importance for development of robust trains and processes for industrial application of microalgae.Chlorella vulgaris is one of the best-known unicellular green algae which is widely used in commercial production of food additive, feed and bioenergy. In this work, we studied a spontaneously flocculating microalga C. vulgaris JSC-7and optimized its culture conditions. C. vulgaris JSC-7was cultivated at temperature ranging from22℃to40℃and pH ranging from6.0to10.0, and the cell growth and chlorophyll content, as well as lipid production under different conditions were investigated. C. vulgaris JSC-7was able to grow under all the examined conditions. Higher biomass and lipid content were obtained at pH8.0than those under pH6.0, indicating that weak alkaline conditions are more suitable for this microalga. The highest biomass accumulation (0.941at OD690), chlorophyll content (19mg/L) and lipid content (39.07%of dry cell weight) were achieved at31℃and pH7.0The flocculating agent of C. vulgaris JSC-7was further characterized. The flocculating agent was purified, and identified to be cell wall-associated polysaccharides. Supplementation of the crude cell wall extract at low dosage of0.5mg/L initiated a more than80%flocculating rate of freely suspended microalgae C. vulgaris CNW11and Scenedesmus obliquus FSP3. The unique cell wall-associated polysaccharide with a molecular weight of9.86×103g/mol, and the monomers consist of glucose, mannose and galactose with a molecular ratio of5:5:2. The elemental analysis of the flocculating agent from C. vulgaris JSC-7reveals the presence of carbon, hydrogen, nitrogen and sulfate and meanwhile, the flocculating substance was thermo-stable.Due to the special cell surface properties, we attempted to use this special flocculating microalga for heavy metal removal. In brief, we performed the experiments for the removal of Zn2+or Cd2+by the self-flocculating C. vulgaris JSC-7. We obtained80%removal efficiency by C. vulgaris JSC-7from the culture within3days when0.5-5.0mg/L Zn2+or0.5-1.5mg/L Cd2+was supplemented.. This result was much better than that observed with the non-flocculating strain C. vulgaris CNW11(60%). The highest removal of Zn2+or Cd2+by the self-flocculating C. vulgaris JSC-7were92%and88%respectively. Furthermore, we explored the mechanism underlying this phenomenon by investigating the effect of Zn2+and Cd2+concentration on the growth and metabolic activities of the two microalgal strains.Collectively, in this work, the identification of the flocculating agents in C. vulgaris JSC-7provides a foundation for further understanding of microalgal cell self-flocculation. More importantly, it paves the way for further identification of genes encoding key enzymes for the biosynthesis of the flocculating agents and for engineering of the non-flocculating microalgal strains with the flocculation phenotype for cost-effective biomass recovery. Furthermore, the better tolerance capability of C. vulgaris JSC-7to heavy metal toxicity implies its potential in bioremediation applications. |
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