| The oleaginous microalgae is a good raw material for biodiesel production.In recent years,it has become a research hotspot to improve the oil yield of microalgae and reduce bottleneck of the production cost.Two-stage cultivation,stress conditions and plant hormones induction are effective methods to promote lipid accumulation in microalgae.Melatonin(MT),an indole derivative,widely exists in animals,plants and microalgae.It has a variety of physiological functions on plants,but its effects on microalgae are rarely studied.In this study,MT was used as a plant growth regulator.Monoraphidium sp.QLY-1 was selected as the research object.The effects of MT on lipid synthesis of microalgae under salinity stress and nitrogen deficiency stress were investigated.The effects of MT on physiological and biochemical indexes and expression of lipid biosynthesis-related genes were detected.The responses of intracellular ROS,antioxidant system and plant hormones were also analyzed.These results preliminary revealed that the regulatory mechanism of MT on the lipid biosynthesis in microalga under high salt and nitrogen starvation conditions.This study provides a new research idea and technical reference for the development and utilization of microalgae resources and the cultivation of oleaginous microalgae under extreme environment.The detailed results are as follows:(1)Under salinity stress,no significant difference in the cell growth was observed by treatment with MT,which considerably increased the lipid accumulation.The lipid content and lipid productivity were 51.74%and 248.87 mg L-1 d-1,respectively,and were enhanced by 1.21 and 1.24 folds compared with the salinity stress alone.(2)Exogenous MT upregulated and downregulated the carbohydrate and protein content.The expression levels of key lipid biosynthesis genes were also upregulated.Moreover,MT application reduced the level of cellular ROS under salinity stress and alleviated the salinity-induced oxidative damage,resulting in enhancing the microalgae salt tolerance.These results showed that exogenous MT could promote the lipid synthesis in microalgae under salinity stress by regulating carbon flow,key genes expression and cellular ROS levels.(3)Under nitrogen deficiency condition,the cell growth was suppressed.MT addition slightly increased the biomass,but the biomass under nitrogen starvation with or without MT was lower than that of control group.Combination of MT and nitrogen stress enhanced the lipid content by 22%and 36%compared with the nitrogen starvation alone and control group,respectively.The highest lipid content was up to51.38%.In addition,MT application downregulated the contents of carbohydrate and protein.These results suggested that MT could regulate the shift of carbon source from carbohydrate to lipid synthesis exposure to nitrogen deficiency stress.(4)Further investigation showed MT remarkably reduced the ROS level and upregulated the activities of antioxidant enzymes,resulting in maintaining the oxidative homeostasis.Meanwhile,MT regulated the expression of lipid biosynthetic-related genes.Compared with the nitrogen depletion stress alone,the rbc L and acc D genes levels under MT were upregulated 1.2 and 1.98 folds,whereas downregulated the pepc level.The above results indicated that MT could increase the lipid accumulation by regulating the levels of ROS and key lipid biosynthesis genes expressions.(5)Under nitrogen starvation condition,MT also regulated the plant hormones levels.The contents of GA and ABA were significantly upregulated and downregulated,and were enhanced by 1.49 and 1.3 folds compared with the nitrogen stress alone.ABA and GA act as important signal molecules in regulation abiotic stress,which might involve in mediating lipid synthesis and oxidative stress.(6)Compared with the nitrogen deficiency,MT could remarkably increase the content of neutral lipid and decrease the glycolipid.However,it had no significant effect on the phospholipids.Moreover,after MT induction,the saturated fatty acids was up to55.81%,which was slightly increased compared with the nitrogen starvation.Meanwhile,the monounsaturated fatty acids under MT was also improved.There are no significant difference in polyunsaturated fatty acids and degree of unsaturation between MT plus nitrogen deficiency and nitrogen depletion alone.Results showed that MT addition enhanced the total fatty acids by 8%.The unsaturation degree of biodiesel can meet the standard of biodiesel production. |
PDF Full Text Request