Research On Photosynthetic Physiology And Biochemical Characteristics During Lipid Accumulation In Nannochloropsis Sp.

Nannochloropsis sp.is a marine microalga with high growth and are enriched in pigments??-carotene,violaxanthin and zeaxanthin?,long-chain polyunsaturated fatty acids?EPA?,chrysolaminarin and protein.It can be used to produce biodiesel,microalgae feed and environmental protection with great economic value and broad application prospects.In this research,the Nannochloropsis sp.was cultivated in ASW medium which contain different KNO₃ concentrations?14.9,7.5,5.0 and 3.0 mM?.We measured the biomass and lipid content of Nannochloropsis sp.in four different KNO₃ concentrations.In addition,the characteristics of main biochemical components and photosynthetic physiological parameters,as well as changes of differential expression of the transcriptional in Nannochloropsis sp.were investigated under nutrient recovery conditions.The results show that:?1?The biomass of Nannochloropsis sp.among four nitrogen?14.9,7.5,5.0 and 3.0 mM?grow with the increase of initial nitrogen concentration,the maximum biomass of Nannochloropsis sp.was obtained in 14.9 mM group(8.27 g·L^-1).Reducing nitrogen concentration significantly promotes total lipid accumulation in Nannochloropsis sp.,the maximum lipid contents was obtained in 3.0 mM group?44.8%DW?.Algae cells mainly contained C20:5,C18:1 C16:1 and C16:0,the content of C20:5 fatty acids decreased remarkably with the cultivation and the decrease of nitrogen concentration.?2?During the nitrogen limitation-induced lipid accumulation,the ratio of neutral lipids of Nannochloropsis sp.was significantly increased,while the ratio of glycolipids and phospholipids decreased,as well as the content of soluble protein was decreased,and the content of polysaccharide was gradually decreased after initial increase in culture,and photosynthetic efficiency parameters?F_v/F_m and rETR?related to photosynthesis showed a decreasing trend.The energy ratios of Z-scheme electron transport chain and alternative electron transport pathway were significantly reduced,and the energy ratio of Z-scheme electron transport chain was more greatly reduced.The ultrastructure shows that the formation of oil bodies was accompanied by the degradation or reduction of protein nucleus,chrysolaminarin,chloroplasts.After nitrogen nutrition recovery,the content of soluble protein and polysaccharide increased rapidly,the proportion of neutral lipids decreased,the ratio of sugar lipids and phospholipids increased,and photosynthetic efficiency parameters?F_v/F_m and rETR?related to photosynthesis rose rapidly.The energy ratios of Z-scheme electron transport chain and alternative electron transport pathway increases,and the ultrastructure shows that the oil body particles decrease,while the regular structure of the chloroplast reappears.From this it was concluded that neutral lipids,the main reservoirs under the stress conditions of Nannochloropsis sp.,gradually decompose and participate in other anabolic processes of cells after the stress is stopped,and the variable electron transport pathway of Nannochloropsis sp.plays an important role in energy supply under the stress conditions.?3?The transcriptome of Nannochloropsis sp.was analyzed by high-throughput sequencing technology during lipids accumulation and degradation lipid.The results showed that,there were 1126 differentially expressed genes between logarithm growth period and the lipid accumulation period,of which 533 genes were significantly up-regulated and 593 genes were down-regulated.The photosynthesis pathway in the lipid accumulation period was significantly changed compared with logarithm growth period.There were 1926 differentially expressed genes between the lipid accumulation period and the lipid degradation period,of which 877 genes were significantly up-regulated,and 1049 genes were significantly down-regulated.Different expressed genes were annotated and showed that the key genes involved in the process of carbon fixation in photosynthetic organisms were down-regulated during the period of lipid accumulation,such as light-harvesting system,PS?reaction center,phosphorylation and Calvin cycle.Many important genes involved in the process of TCA cycle,phosphorylation,glycolysis processes were down-regulated during the lipid accumulation period,although the expression of multiple key genes related to the amino acids metabolism pathway were up-regulated.Key genes involved in the major biochemical metabolism pathways and related pathways such as the TCA cycle,glycolysis,and Calvin cycle,were up-regulated,in addition,multiple key genes related to amino acid metabolism pathways were down-regulated.Therefore,it can be inferred that algae cells enhanced amino acid catabolism during the lipid accumulation period were due to nitrogen limitation,thus the utilization of endogenous nitrogen more efficiently.In addition,growth of Nannochloropsis sp.was reduced during the lipid accumulation period attribute to the down-regulation of the process of photosynthetic carbon fixation.Moreover,the polysaccharide and protein contents increased in lipid degradation period were due to increased of photosynthetic carbon fixation pathway,shifting of carbon flux distribution to carbohydrate synthesis,and down-regulation of amino acid metabolism.