Effects Of Water Acidification On Microstructure And Nutritional Composition Of Several Microalgae

The increase of atmospheric CO₂ concentration changes the carbonate system of water and affects aquatic organisms.Microalgae are important producers in water ecosystems and are highly valuable to be commercially exploited.Nannochloropsis oceanica,Dunaliella salina,Chlorella variabilis,Chaetoceros mulleri and Chlamydomonas reinhardtii are typical bait alga.Researches about effects of CO₂concentration on microalgae most focus on short-term acidification（ST）,however long-term acidification（LT）studies are relatively fewer.Not only the microstructure of Nannochloropsis oceanica,Dunaliella salina and Chlorella variabilis but also the fatty acid and amino acid changes of Chaetoceros mulleri,Dunaliella salina and Chlamydomonas reinhardtii were investigated under short-and long-term acidification conditions based on present（400μatm）and future（1000μatm）CO₂ concentrations.Furthermore,in order to explore the internal causes of physiological changes in microalgae,the molecular response mechanism of Chaetoceros mulleri under ST and LT was investigated.Our results showed that the number of chloroplasts in N.oceanica increased under ST and LT conditions.Chloroplast in D.salina grown well under ST while partial thylakoids were absent under LT,which was consistent with its photosynthesis result.On the other hand,the number of starch granules in D.salina increased under ST and LT conditions.For C.variabilis,the number of starch granules encapsulated by thylakoids under ST and LT decreased but the number of oil body increased,which indicated that an increase in CO₂ concentration may decrease carbohydrate content but promote oil accumulation in C.variabilis.Fatty acids and amino acids are important nutrients.Our results showed that the effects of elevated pCO₂ on nutritional composition of C.mulleri,D.salina and C.reinhardtii were species-specific.The high CO₂ concentration increased the SFA and C16:0 contents but decreased PUFA,C18:0,C18:3n3,EPA,ΣEAA,ΣAA,andΣEAA/ΣAA in C.mulleri.The contents of PUFA,C18:0,C18:3n3,ΣEAA,ΣAA and∑EAA/∑AA in D.salina increased under ST condition,but SFA and C16:0 decreased.LT promoted the accumulation of C16:0,ΣEAA,ΣAA but inhibited the accumulation of SFA,PUFA,C18:0,C18:3n3 and decreased∑EAA/∑AA in D.salina.For C.reinhardtii,LT decreased the content of SFA,PUFA,C16:0,C18:0,C18:3n3,∑EAA,∑AA and the ratio of∑EAA/∑AA,but these changes were different under ST condition.The results indicated that the increased CO₂ concentration reduced the nutrient levels of C.mulleri,while ST increased the nutritional value of D.salina,but LT reduced the nutritional quality of C.reinhardtii.Changes in the transcriptome revealed that a wide range of cellular and metabolic processes were differentially affected.A global upregulation of genes involved in the calvin cycle,glycolysis,lipid synthesis and nitrogen metabolism occurred under long-term acidification.Genes involved in some amino acids synthesis were down regulated.Thus,C.muelleri may have evolved new features to adapt to the elevated p CO₂ level.