| Microalgae have been considered as superior candidates for the continuous stable production of biodiesel in recent years due to their intrinsic advantages such as rapid growth,high photosynthetic efficiency,low land demand and strong adaptability.They are being the third-generation biofuels to replace soybean,palm and many other crops.However,the problems of large freshwater consumption and high energy demand lead to the high cost of microalgal biomass production,which has severely limited the large-scale production of microalgae.To reduce the cultivation cost,many efforts have been made on culturing microalgae in wastewater,but the algal biomass and lipid productivity are not very satisfactory because of the severe conditions in wastewater.So,it is especially critical to find efficient strategies to increase the lipid productivity of microalgae in adverse environments.Moreover,due to the low concentration.of microalgae biomass and high harvest costs in traditional suspended culture systems,microalgae biofilm attached systems are becoming new trends,so the choice of culture system also becomes very meaningful.This project starts from reducing the production cost and increasing the lipid accumulation of microalgae in unfavorable environments.First,four of the ten microalgae with good tolerance to KWADE:Scenedesmus SDEC-8,Scenedesmus SDEC-13,Monoraphidium SDEC-17 and Chlorella SDEC-18 were screened.The biomass concentration was 0.52,0.39,0.29 and 0.54mg/L,and the lipid content reached 33.85%,24.16%,23.88%and 34.72%,respectively.Low-phosphorous condition in KWADE encouraged the carbon fixed by photosynthesis to mainly flow to the TAG biosynthetic pathway,increasing the lipid biosynthesis in algal cells.Moreover,53.1 mg/g of nitrogen average yield coefficient(N-AYC)was achieved by SDEC-18,which showed higher nitrogen removal capacity than the other algal strains.It was found that the biodiesel synthesized by SDEC-8 and SDEC-18 in KWADE met the standards of China,the U.S.and the EU.Based on the biodiesel properties,lipid content and productivity,the four microalgae were further sorted using PROMETHEE-GAIA analysis method,and SDEC-8 and SDEC-18 were found to be the most suitable algae strains for biodiesel production.Second,the exogenous agricultural phytohormones were added to stimulate the performance of the two lipid-rich microalgae.It was found that the optimum dosage of phytohormone was 20 mg/L,and that phytohormone clearly promoted the growth and lipid accumulation of the microalgae.The biomass concentration of SDEC-8 and SDEC-18 was 59.3%and 76.6%higher than that in BG11 medium and the lipid content was 3.0 and 2.8 times,respectively.Phytohormone promoted the conversion of carbohydrates to lipids,resulting in the obvious increase in lipid content.In normal BG11 medium,the effect of phytohormone on the compositions of photosynthetic pigments was not obvious,showing that phytohormone may mainly manifest the dark reaction stage.In addition,exogenous phytohormone increased the activity of the dehydrogenase(DHase),optimizing the intracellular environment for lipid biosynthesis in the cells.Simultaneously,the cell membrane permeability was increased and the proton pump on the cell membrane was activated,so that superfluous H+in the cells was excreted to adjust the pH in the medium,optimizing the extracellular environment for algal growth.The N-AYC/P-AYC ratio of SDEC-8 and SDEC-18 increased to 1.8 and 2.4 times compared to that without phytohormones,indicating that the ability of nitrogen assimilation was clearly improved.Therefore,phytohormone presented bright prospects in the microalgae-based technology for wastewater treatment,especially for the wastewater with high nitrogen concentration.Third,the phytohormone was applied to the suspended system and a phytohormone-nitrogen-deficient microalgae suspended culture system was established.It was found that the phytohormone largely improved the performance of microalgae under adverse conditions.When the phytohormone dosage was 20 mg/L,the biomass concentration of SDEC-8 and SDEC-18 under nitrogen-deficient condition was 1.4 and 1.5 times,and the lipid productivity was 2.4 times higher than those without phytohormone,respectively.The conversion of carbohydrates to lipids was one of the key factors to increase the lipid biosynthesis of the two microalgal cells,while the mutual conversion of polar and neutral lipids is not obvious.Under the nitrogen depletion condition,the presence of phytohormone reduced the reactive oxygen radicals(ROS)level and malondialdehyde(MDA)content in the cells,resulting in the decrease in superoxide dismutase(SOD)activity and increase in DHase activity.Thus,the oxidative damage caused by nitrogen-depleted stress was alleviated so that the cell viability and biomass concentration were maintained.In addition,through analysis of changes in photosynthetic pigments,it was found that phytohormone effectively prevented the decomposition of chlorophyll,implying that chlorophyll was not the only nitrogen reservoir in the cells.Finally,the phytohormone was applied to the attached system and a self-designed inclined algal biofilm photobioreactor(IABPBR)based on phytohormone and seawater-diluted KWADE was established.The discarded fluffy polyester cloth,with the advantages of being cheap,easily available,nondegradable and reusable,was screened as algal biofilm carrier among ten common materials.With 20 mg/L of phytohormone in IABPBR,the biomass productivity of SDEC-8 and SDEC-18 biofilm reached 3.66 and 5.66 g/m2/d,while the lipid productivity was 2.81 and 3.98 g/m2/d,respectively,and all the results were clearly higher than those without phytohormone.In the IABPBR system,the synthesis of photosynthetic pigments was regulated to improve the photosynthetic efficiency in seawater-diluted KWADE and the DHase was increased by the presence of phytohormone,leading to the increase in cell viability and decrease in SOD activity and MDA concentration,so that the oxidative injury caused by the wastewater was alleviated.Moreover,651.70 mg/m2/d and 579.99 mg/m2/d of TN and NH4+-N removal capacity was achieved by SDEC-18 biofilm with phytohormone,respectively,while the COD removal efficiency and capacity reached up to 97.0%and 3.31 g/m2/d.In conclusion,whether in suspended or attached system,phytohormone has the excellent ability to improve the performance of microalgae,which obviously increases the feasibility of large-scale production of microalgae biomass.Especially,the IABPBR based on phytohormone and seawater-diluted KWADE efficiently solves the bottleneck of large consumption of freshwater and high demand of energy in suspended systems,which is significantly important for nutrient recovery in KWADE and cost reduction in microalgae-based biodiesel production. |
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