Study On The Interactions Of Algae-bacteria Symbiotic During Cultivating Chlorella Vulgaris In The Centrate

Microalgae have great potential for development in food,health care,new energy and other fields due to their characteristics of fast growth rate and high oil content.Wastewater contains a large number of nutrients such as carbon,nitrogen and phosphorus for the growth of microalgae.Using wastewater to cultivate microalgae can not only reduce the cost of microalgae culture,but also relieve the pressure of wastewater treatment.However,the wastewater contains a large number of microorganisms such as bacteria and fungi,which may affect the growth of microalgae,oil accumulation and nutrient removal.Therefore,it is necessary to study the complex relationship between algae and their own bacteria in wastewater.In this paper,Chlorella vulgaris was cultured with centrate.By comparing the effects of different pretreatment methods of centrate on Chlorella vulgaris growth,the appropriate pretreatment methods was determined;and three systems of co-cultivation of algae and bacteria,Chlorella vulgaris isolated cultivation and wastewater-borne bacteria isolated cultivation were set to investigate the changes of algae and bacteria biomass,metabolites,CO₂and O₂,as well as the species and abundance of bacterial community,and the symbiotic mechanism of algae bacteria was analyzed;finally,the changes of algal-bacteria in the three systems were studied under the condition of adding crude glycerol and CO₂.The main research results are as follows:(1)The biomass of Chlorella vulgaris cultured in centrate after different pretreatment had great difference.The unsterilized static centrate supernatant is more suitable for the growth of Chlorella vulgaris,and the maximum biomass of Chlorella vulgaris was 1.59 g/L,the COD removal rate was 84.9%.The number and species of bacteria in supernatant decreased after standing.The dominant bacteria in the co-culture system were Proteobacteria and Bacteroidetes,with the abundance of 62.7%and 20.7%,respectively.(2)Compared with the inoculated Chlorella vulgaris group after sterilization,the growth of Chlorella vulgaris in the coexistence system was better,and the maximum biomass was1.54 g/L.The maximum bacterial biomass was 136×10⁴CFU/m L,which was higher than that of the single culture group(106×10⁴CFU/m L),indicating that Chlorella vulgaris and wastewater-borne bacteria in centrate could promote the growth of each otherr.The COD removal rates in the single culture system of Chlorella vulgaris and the single culture system of wastewater-borne bacteria were 66.7%and 69.8%,respectively,which were lower than the COD removal rates of 84.8%in the co-culture system of algae and bacteria.The TP removal rates in the single culture system of Chlorella vulgaris and the single culture system of wastewater-borne bacteria were 74.1%and 53.7%,respectively,which were lower than the TP removal rates of 81.4%in the co-culture system of algal and bacteria.The removal rate of TN in the co-culture system of algal and bacteria was 75.5%,which was higher than the removal rate of 70%and 25%in the single culture system of Chlorella vulgaris and single culture system of wastewater-borne bacteria,indicating that the co-culture of algal and bacteria could improve the utilization rate of carbon,nitrogen and phosphorus in centrate.There is an exchange of CO₂and O₂in the coexistence system of algal and bacteria.The results of extracellular polymeric substance showed that both Chlorella vulgaris and wastewater-borne bacteria secreted proteins and polysaccharides.Scanning electron microscope showed that the algae and bacteria grew in clusters under the adhesion of extracellular polymeric substance.The dominant bacteria that could co-exist with Chlorella vulgaris were Pseudomonas,Alicycliphilus,Gemmobacter,Flavobacterium.(3)The growth of Chlorella vulgaris was further promoted by adding crude glycerol.When 2 g/L crude glycerol was added,the maximum biomass of Chlorella vulgaris in the coexistence system was 1.99 g/L,which was higher than that of 1.83 g/L when Chlorella vulgaris was cultured alone.When 2 g/L crude glycerol was added,the co-culture of algae and bacteria was more conducive to the growth of Chlorella vulgaris,which indicated that the wastewater-borne bacteria could promote the growth of Chlorella vulgaris after adding crude glycerol.The dominant bacteria was Proteobacteria in the centrate after adding crude glycerol.And the abundance of Proteobacteria was 87.5%and 92.6%respectively after adding 2 g/L and 5 g/L crude glycerol.(4)The maximum biomass of Chlorella vulgaris was 1.74 g/L when Chlorella vulgaris was cultured alone,which was higher than that of 1.69 g/L obtained in the coexistence system,when 5%CO₂was added.When 5%CO₂was injected into the centrate,the cultivation of Chlorella vulgaris alone was more conducive to the growth of Chlorella vulgaris,which indicated that the growth of Chlorella vulgaris could be inhibited by the bacteria in the centrate with the passage of CO₂.When 5%and 20%CO₂were added into the algae-bacteria coexistence system,the wastewater-borne bacteria had a negative effect on the growth of Chlorella vulgaris.Cyanobacteria was enriched when CO₂was added.The abundance of Cyanobacteria was 28.1%and 46.8%after 5%and 20%CO₂were added,respectively.Cyanobacteria was easy to secrete toxins,which had an adverse effect on the growth of Chlorella vulgaris.