Study On Improving Productivity Of Microalgae For Biofuel By Polyculture In Municipal Wastewater

Using municipal wastewater as nutrient resource to cultivate microalgae for biofuel production is a prospective research direction for wastewater resource utilization.The establishment of a high efficient open pond microalgae culture system is the key to realizing the commercial application of this technology.The research on improving biofuel microalgae productivity in the open culture system is mainly focused on adding bacterial agents and growth promoters to the culture system.Through algae-bacteria co-metabolism improves the utilization of nutrients in environment to improve productivity.With the development of synthetic ecology,studies on the combination culture of multiple microalgae have been reported in recent years,which is another way to improve the productivity of the system,and it is urgent to carry out relevant basic research.In this paper,an exploratory and systematic study was carried out focusing on the construction method of microalgae combined culture system and the evaluation method of energy utilization development potential.The main research contents and results are as follows:（1）The productivity and nitrogen and phosphorus utilization of 10 microalgae monoculture were compared in the experiment.Through comprehensive analysis of the productivity,nitrogen and phosphorus nutrient niche of each microalgae culture system,microalgae with high potential for polyculture were selected.The results showed that:The biocrude yield of 10 microalgae in simulated municipal wastewater ranged from 1.85 to 15.4mg L^-1 d^-1.There were significant differences in the utilization rates of nitrogen and phosphorus among the 10 microalgae species.The highest nitrogen utilization rate was 46%,and the highest phosphorus utilization rate was 82%.The microalgae with high potential for microalgae combination culture were P（Pediastrum sp.）,D（Desmodesmus sp.）and S（Scenedesmus obliquus）,M（Micractinium sp.）.（2）According to the nitrogen and phosphorus nutrient utilization niche,four kinds of microalgae dualculture systems were constructed,and their productivity and nutrient utilization were analyzed.The PM（Pediastrum sp.and Micractinium sp.）combined culture system had the highest biocrude yield,17.5 mg L^-1 d^-1,which was 14%and 69%higher than Pediastrum sp.and Micractinium sp.monoculture system,respectively.Compared with the microalgae monoculture system that participated in the combination of microalgae,the nitrogen and phosphorus utilization rates of the microalgae dualculture system were higher.The DS（Desmodesmus sp.and Scenedesmus obliquus）combination had the highest N utilization（39%）,which was 15%higher than D,and 7%lower than S respectively.The PM had the highest P utilization（85%）,which was 27%and 22%higher than P and M,respectively（3）The bacterial communities of ten microalgae monoculture systems and four microalgae dualculture systems were analyzed,and the results showed that:Compared with the microalgae system with a lower bacterial species diversity,the bacterial species diversity in the microalgae dualculture system both Observed＿species and Faith＿PD index was improved.The differences in bacterial species diversity in microalgae monoculture systems were larger than in microalgae dualculture systems.The bacterial community structure of the microalgae dualculture system was closer to one of the two microalgae monoculture systems.（4）Based on Spar CC,the bacterial species correlation analysis was carried out on the 16S r RNA sequencing results of the culture system.The fast greedy algorithm was used to calculate and divide the community modules,and the topology analysis within and between modules was carried out.Combined with bacterial abundance,productivity and nutrient utilization,the keystone bacteria and its associated microalgae were identified.The results showed that:Paraburkholderia（genus）was the keystone bacteria that could improve the productivity of microalgae culture system.The Nannochloropsis Oceanica with the highest correlation with the keystone bacteria.（5）The three-microalgae culture system PMN constructed by Nannochloropsis Oceanica was introduced into the PM.The culture system PMp was constructed by adding keystone bacteria Paraburkholderia into the microalgae combination PM,and the productivity and the abundance of keystone bacteria of the two culture systems were compared.At the end of microalgae culture in the first and second culture cycles,the biocrude yield of PMN was 18.5mg L^-1 d^-1 and 22.8 mg L^-1 d^-1,respectively,and the biocrude yield of PMp was 22.2 mg L^-1 d^-1 and 17.6 mg L^-1 d^-1,respectively.The total productivity of PMN was higher.The relative abundance of keystone bacteria was 14.3%and 15.5%in PMN,and 2.5%and 6%in PMp,respectively.In the PMp culture system,the relative abundance of keystone bacteria was high only at the beginning of the first cycle and could not maintain high abundance during the two cycles of culture.（6）The keystone bacteria Paraburkholderia have the functions of denitrification and phosphorus dissolution,and can also secrete the growth promoter indole-3-acetic acid,and may establish contact with microalgae through the type VI secretion system.Nannochloropsis oceanica can produce a large amount of unsaturated fatty acids and can synthesize more isoprenoids.The Paraburkholderia prefer to use unsaturated fatty acids.And Isoprenoids are important raw materials for the synthesis of biomass by Paraburkholderia.These close connections in growth and metabolism between Paraburkholderia and Nannochloropsis oceanica may be the internal mechanism reasons for enhancing the productivity of the microalgae culture system PMN.（5）According to the aims of biofuel microalgae cultivation system in municipal wastewater.From the growth rate,biomass output,nitrogen and phosphorus utilization and stability four aspects,we established a method,based on entropy weight,to evaluate the performance of the biofuel microalgae culture system,and the application of the method to evaluate the development potential of microalgae combination culture system,the results showed that:Among the four microalgae dualculture systems,PM had the highest development potential.In the first culture cycles,the development potential of the PMN and PMp culture systems was 2.8%and 8.9%higher than PM,respectively.In the second culture cycle,the development potential of the PMN and PMp culture systems was 15.1%higher and 0.2%lower than PM,respectively.The development potential of PMN in the two culture cycles was 5%higher than that of PMp.In this paper,the method of building microalgae polyculture system was established based on nitrogen and phosphorus nutrition niche and keystone bacteria associated with microalgae,which improves the productivity.And the development potential of microalgae for biofuel production was evaluated.The technical bottleneck of the low level of system productivity has been usefully explored.