Research On Filamentous Fungi-assisted Microalgae Immobilization Technology And Recycling Water From Spent Medium

To confront with energy crisis,microalgae has great potential in exploring the field of renewable energy,but the harvesting process has become the main bottleneck due to the cumbersome steps as well as the large amount of resource input.Although culturing filamentous fungal mycelium pellets for cell immobilization is an emerging,efficient,and sustainable method,the effects on bioenergy production and water conservation were still unknown.Therefore,through selecting representative filamentous fungal species Aspergillus oryzae and oil-rich freshwater microalgae Chlorella vulgaris for the construction of symbiotic system,four key parameters(temperature,p H,rotational speed,algal-bacteria ratio)were identified and optimized,then the specific mechanism of the formation of mycelium-microalgae copolymers from multiple perspectives(spatial structure,surface potential,functional groups,extracellular secretions)were explored.Finally,the specific impacts of the recycling of harvested water on the production of microalgae biodiesel were analyzed to provide theoretical basis for further applying the practical results.The main results obtained were as followed:(1)Under the optimized conditions of 30 °C,130 rpm,and the dry weight ratio of fungi:algae on 1:1,the highest efficiency of 99.23% could be obtained within 5 h,and the p H value of the algal solution did not need to be adjusted(the initial value was9.68).After harvesting,the separated medium for the second round of cultivation could effectively support the regrowth of Chlorella vulgaris.The biomass concentration of in medium with 50% circulation rate was higher than that in the group cultivated in fresh medium,while the intracellular lipid content in the medium with 100% circulation was increased from 24.37% to 33.97%,indicating that the remaining substances in the medium harvested by the hyphal pellets could play key role.(2)In terms of mechanism,the structure of Aspergillus oryzae mycelium was relatively loose and porous,and the loose accumulation of mycelium allowed it to form a network structure to actively capture and adsorb microalgae.Charge neutralization was not the dominant mechanism for the formation of copolymers between Aspergillus oryzae and Chlorella vulgaris,and the specific differences of microbial species were the main reason for the varied results.Notably,functional groups on cell surface had changed during immobilization,indicating that interactions between specific groups could play vital role in harvesting.Not only that,the results of three-dimensional fluorescence spectroscopic analysis had shown that the metabolites secreted by mycelium and the microalgae during this process were also the important factors for the formation of aggregates.(3)In recycled water with different N/P ratios,the absorption of nitrogen by microalgae would be significantly limited due to lack of phosphorus,which would lead to changes in intracellular photosynthesis-related chlorophyll content and response from the intracellular antioxidant enzyme system.The function of phosphorus was to act as an energy storage element for cell metabolism,which was the main reason for why nitrogen could be absorbed by Chlorella vulgaris at an accelerated rate under sufficient nutrient condition.The organic matter content in the harvested medium had increased,but it was difficult to support a complete period of cultivation,so it would limit the growth in latter stages.Nutrient-deficient conditions served as stress factors could restrict the carbohydrate and protein synthesis of microalgae,but increased lipid content by 12.69%.The highest biomass concentration(1.37 g/L)and lipid yield(0.42 g/L)were found in the recycling group with N/P ratio of 6:1,and the composition of fatty acid methyl ester was also more uniform.Compared with the traditional single-batch cultivation in artificial wastewater,the recycling mode can save 52.4% of the cultivation cost.This study can provide a theoretical basis for the large-scale operation of filamentous fungal mycelium-enriched energy microalgae.Furthermore,there is a certain practical significance and development space for the transformation of microalgal biomass energy with industrial application value into an alternative green energy technology.