Physiological And Transcriptomic Response Of Skeletonema To Changing Ambient Salinity

Skeletonema is a widely distributed genus in water areas with different salinity,such as freshwater rivers and lakes,estuaries and lagoons,costal and open ocean.Members of Skeletonema are an important primary producer of water ecosystems and a component of food web,they are also the foods for fishes,shrimps and mussels.Furthermore,they play important roles in regulating biogeochemistry of biogenetic elements.Studies have reported that salinity is one of the key factors regulating survival,growth and proliferation of Skeletonema.Much effort has been devoted to effects of salinity on Skeletonema but majority of them is focused on physiological response,such as morphology and growth,we know little about response mechanisms of Skeletonema to ambient salinity changes at the molecular level,especially its tolerance mechanism to low salinity stress.To fill this gap,this thesis selected one freshwater Skeletonema species,Skeletonema potamos,and three marine Skeletonema species,Skeletonema costatum,Skeletonema dohrnii and Skeletonema tropicum as the research targets,we investigated cell growth,morphology,proline and chlorophyll contents under different salinity conditions,compared transcriptomic profiles and identified and characterized differentially expressed genes,analyzed their molecular functions and biological processes,and constructed metabolic regulatory network of Skeletonema to salinity stress.Finally,we discussed physiological and molecular mechanisms in response to ambient salinity changes.The main findings are as followings:(1)S.potamos and S.costatum could grow with a salinity range of 0-35 psu,while S.dohrnii and S.tropicum could grow with a salinity range of 10-35 psu.The optimum salinity for cell growth of S.potamos and S.tropicum were 0 psu and 35 psu respectively,for S.costatum is about 20-35 psu,while the salinity of S.dohrnii at 10-35 psu had little difference.Except for S.dohrnii,the other three Skeletonema species could accumulate proline under high salinity conditions and increased cell size under unfavorable salinity conditions.In addition,the chlorophyll content of S.potamos,S.costatum and S.dohrnii increased with the salinity increased,while it decreased in S.tropicum.(2)Transcriptome analysis showed that SOS1 gene,and key genes involved in proline and other five osmolyte biosynthesis of S.potamos,chlorophyll synthesis,photosynthesis,protein metabolism,oxidative stress kinase were significantly upregulated under high salinity stress.SOS1 gene of S.costatum was significantly down regulated but the NHX1 gene was significantly up-regulated,and key genes participating in proline and other four osmolyte biosynthesis were significantly down regulated under low salinity stress,while key genes involved in trehalose and other four osmolyte biosynthesis of S.dohrnii were also down-regulated.In addition,expression of genes related to chlorophyll synthesis,energy metabolism and oxidative stress of S.costatum and S.dohrnii were significantly down regulated,while key genes related to TAG synthesis in S.dohrnii were also significantly down regulated.However,the expression of proteasome protein genes in S.costatum under low salinity stress were significantly up-regulated,expression of genes related to photosynthesis and key gene PDAT related to TAG synthesis in S.costatum in 0 psu were significantly up-regulated,and genes related to photosynthesis in S.dohrnii were significantly up regulated.(3)Freshwater diatom S.potamos was capable to survive at 35 psu may be related to overexpression of SOS1 and accumulation of proline and other organic osmolytes,which significantly enhanced its high salinity tolerance.The survival of S.costatum in fresh water was closely related to low expression of SOS1 and high expression of NHX1 as well as the weakening of organic osmolyte synthesis.Differred from the strategy of S.costatum to cope with low salinity stress,S.dohrnii obtained more energy by enhancing photosynthesis to maintain its survival.However,it can not survive in fresh water,which may be related to the difference of Na+ regulation mechanism.The metabolism of Skeletonema was enhanced by high salinity stress,which provided sufficient energy for active transportation and material synthesis under high salinity stress,and enhance TAG synthesis and protein synthesis to enhance their own resistance.On the contrary,the metabolism of Skeletonema was depressed by low salinity stress,linking to the survival of Skeletonema under low salinity stress.The differences in response ability and mechanism of different species of Skeletonema to salinity stress may be the result of their long-term adaptation and evolution to the specific salinity environment.Therefore,it is speculated that salinity is an important environmental factor affecting the evolution of Skeletonema species,and S.costatum may have a special position in the evolution of Skeletonema.The hypothesis of "parallel evolution"that Skeletonema evolves into fresh water and sea water under hydrodynamic driving condition is proposed.Overall,in this thesis,we compared the response mechanisms of four Skeletonema species to environmental salinity changes at physiological and transcriptional levels,identified and confirmed the genes involved in salt stress response and their biological processes,and outlined the metabolic regulatory network in response to salinity changes,which provided a theoretical basis for further understanding and revealing evolution of Skeletonema and the response of marine diatoms to future salinity stress.