Physiological And Molecular Regulation Mechanism Of Nannochloropsis Oceanica Under Ocean Acidification

With the atmospheric CO₂ increasing,a large amount of CO₂ is absorbed by oceans,causing ocean acidification,which changes the carbonate system and affects the survival of marine organism,especially marine microalgae.Nannochloropsis oceanica is a unicellular eukaryotic marine microalgae,belonging to Stramenopiles,Eustigmatophyceae,Nannochloropsis,with a diameter of 2?5?m,plays an important role in the global C and N cycle.At present,most laboratory studies only focus on the changes of physiology and biochemistry of microalgae in response to short-term acidification,lack of experimental demonstrations on long-term adaptability,and elucidate the molecular mechanisms of related metabolic pathways through omics methods.Therefore,this study carried out a predictive analysis of the physiology,biochemistry and molecular mechanisms in long-term acidification(LT)and short-term acidification(ST).Animal feeding experiments were conducted to evaluate the possible ecological effects,and to analyze the impact of the changes in nutrient composition of the acidifying N.oceanica on the food chain.The results showed that the chlorophyll content,Fv/Fm and carbon fixation rate were increased under LT and ST,and increased the growth and photosynthesis of N.oceanica significantly.In addition,the contents of C,N,and C/N were both increased significantly under elevated CO₂ conditions,maintaining the homeostasis of algal cells.In cells under ST,N.oceanica significantly increased the content of carbohydrate,increased the protein content,and enhanced the proportion of intracellular USFAs,but the activity of T-SOD was decreased.In cells under LT,the total phenols content increased significantly,but the protein content and T-SOD activity were significantly decreased.Compared with normal culture condition,the fatty acid profiles had specific changes,and the proportion of PUFAs was increased significantly under LT.Integrated transcriptome and metabolome analyses demonstrated that gene expression of partial metabolic pathways was not significant under ST.Only the Calvin cycle was found that most genes were significantly downregulated.However,physiological and biochemical results showed the carbohydrate content was increased significantly under ST,which might be caused by the significant co-upregulation of tkt A and PC genes.There was a global downregulation of genes involved in Calvin cycle,fatty acid biosynthesis,TAG biosynthesis and nitrogen metabolism but PGAM(gene?9748)and PK(gene?4717)of glycolysis and?-oxidation were significantly upregulated in the LT compared with CK.The metabolic data indicated that some amino acids were increased and the content of carbohydrate was decreased,and the proportion of PUFAs was increased,and the nutritional quality(amino acids and PUFAs)of the algal cells was improved under LT.In order adapt to the long-term acidification environment,N.oceanica changed its metabolic regulation mechanism,and consumed sugars and fatty acids through glycolysis and?-oxidation to provide a large amount of energy for algal cells,thus maintaining stable and rapid growth under long-term high carbon and low p H environment,forming a specific adaptive evolution.The rotifers(Brachionus plicatilis)were fed with acidification induced algal fluid and normal algal fluid,respectively.The results revealed that rotifers grow slowly under acidification conditions.It may be that lower seawater p H causes stress to the growth and development environment of rotifers and inhibits feeding.Combined analysis of the fatty acid profiles of N.oceanica and B.plicatilis demonstrated that the changes in the nutritional quality of microalgae by acidification will be transferred to the rotifers through the transmission of trophic levels.Especially the content of EPA was both increased significantly in the LT and RL populations,which may affect the biogeochemical cycle through nutrient transfer in the food chain.