The Process And Mechanisms For Enhancing Lipid Production Of Monoraphidium Sp.QLY-1 In Molasses Wastewater By Myo-inositol

Compared with the first and second generation of biodiesel feedstocks such as the oil crops and waste oil,microalgae,the third generation of bioenergy feedstock,have received worldwide attention because of their advantages of short growth period,rich oil content,high photosynthetic efficiency,and low land requirement.Meanwhile,microalgae-based energy has great economic and environmental benefits in energy conversion and greenhouse gas emission reduction.However,the industrial application and market development of microalgal bioenergy are still limited by production cost and demand for fresh water resources.In view of these problems,the use of wastewater as cheap alternative medium for microalgae cultivation to reduce the input of fresh water resources and nutrients has been a research hotspot in recent years.But the complex and challenging wastewater conditions that contain heavy metal ions and hazardous substances significantly affect the cell activity and intracellular oil synthesis of microalgae,which would eventually lead to the decrease of oil productivity.Thus,to improve the cell growth and lipid accumulation capacity of microalgae in wastewater is the key to increase the final lipid yield.In the process of microalgae culture,exogenous application of plant growth regulators is an effective method to promote the growth and lipid production.Myo-inositol(MI),a natural six-fold alcohols of cyclohexane,functions as the growth factors for certain microbes and the structural elements for many secondary messengers of eucaryote.But the effects and regulatory mechanisms of MI on microalgal lipid synthesis remain unclear.In this paper,from the perspective of low-cost and high-yield lipid production,the effects of MI on the lipid productivity of Monoraphidium sp.QLY-1 and wastewater bioremediation of molasses wastewater(MW)were investigated,and the MI-regulatory mechanisms of microalgal lipid metabolism was revealed.Secondly,the strategy of integrating salinity stress and MI treatment was applied in order to further increase the lipid productivity.Furthermore,the mechanism of ethylene(ET)on regulating lipid production of microalgae in MW under combined salt stress and MI treatment was studied.And the microalgal response mechanisms to MI were further investigated.A culture system that integrates microalgal wastewater cultivation technology with plant growth regulators is expected to obtained to simultaneously reduce the production cost and improve the lipid productivity.Specific research results are as follows.(1)The molasses wastewater(MW)was used as the basic medium,and different concentrations of MI were added for induction.When the concentration of MI in the medium was 0.50 g L^-1,the lipid content and productivity of microalgae reached 58.07%and 134.70 mg L^-1 d^-1,which were 1.20-and 1.35-fold higher than those of the control group(48.22%and 99.94 mg L^-1 d^-1),respectively.Meanwhile,during a 4-day cultivation period,under 0.50 g L^-1 MI treatment,the removal rate of total phosphorus(TP)from MW by microalgae reached 57.36%,which was 51.87%higher than that of the control group.And the removal rates of total nitrogen(TN)and chemical oxygen demand(COD)under MI treatment were also 20.02%and 9.10%higher than the control group.In addition,MI enhanced the absorption of Mg,Mn,Mo,Cd and other metals by microalgae in MW.The results show that appropriate concentration of MI can promote the cell growth and lipid production of microalgae in the MW,and improve the removal rate of TN,TP,COD and metal ions by microalgae from the MW.(2)Exogenous application of MI promoted lipid accumulation,but decreased the protein and carbohydrate content,and up-regulated the expression levels of lipid biosynthesis-related genes(acc D,GPAT,DGAT1).In addition,MI increased the levels of phytohormones ET and abscisic acid(ABA),and improved the glutathione(GSH)content,but decreased the level of reactive oxygen species(ROS)in algal cells.These results show that exogenous MI may be involved in regulating carbon metabolism flow,oxidative stress response and various signal molecule transduction to promote the lipid synthesis of Monoraphidium sp.QLY-1 in wastewater.(3)Salt stress is an effective method to promote lipid accumulation in microalgae.In this study,MW was continued to be used as the basal medium.Adding 0.50 g L^-1 MI under 10 g L^-1 Na Cl treatment,the lipid productivity of microalgae reached 146.42 mg L^-1 d^-1,which was 1.19-and 1.39-fold higher than that of the single salt stress treatment(122.75 mg L^-1 d^-1)and pure wastewater condition(105.65 mg L^-1 d^-1),respectively.Meanwhile,MI significantly increased ET levels and induced autophagy in microalgae.Further studies showed that under MI combined with salt stress treatment,exogenous addition of ET donor 2-chloroethyl phosphoric acid(CEPA)could further promote the lipid productivity and autophagic activity of microalgae in wastewater.Meanwhile,the expression of genes related to lipid synthesis was up-regulated and the antioxidant system was activated to alleviate the oxidative damage caused by salt stress.In addition,the autophagy process and lipid content in alga cells were decreased when the ethylene receptor inhibitor 1-methylcyclopropene(1-MCP)was added exogenously.The results showed that under salt stress,MI may mediate the ET to regulate the antioxidant system and autophagy signaling pathway to improve the resistance of microalgae under salt stress and induce the lipid biosynthesis in microalgae.