The Growth, Physiological And Biochemical Characteristics Of Oleaginous Greenalga Scenedesmus Acuminatus Under Different Nitrogen Levels

Scenedesmus acuminatus, which belongs to the Chlorophyta, Chlorococcales，Eucoenobianae, Scenedesmaceae, Scenedesmus, is a green freshwater alga. Previousresearch has confirmed that S. acuminatus can produce a large amount oftriacylglycerol （TAG）, large size lipid bodys（LBs）emerged and occupied almost thewhole cell, the highest total lipids content could up to more than50%of dry biomassat its aged time, which makes the alga be an potential oleaginous alga. In this study,we used S. acuminatus as the object to research the algal morphologic andultrastructural changes at different growth phases, and to analysis the algal growthcharacteristics，dynamic change of main biochemical composition and differentialexpression of proteins under four different initial supplies of sodium nitrate with thehope to reveal the relationship between the photosynthesis process and the storagelipid accumulation, and to provide the theoretical reference for further study of thephotosynthetic physiological mechanisms during the storage TAG accumulation in S.acuminatus.Main results are as followed:（1） Bubble photobioreactors were aerated with compressed air containing1%CO₂（v/v） as stirring gas and carbon source and irradiated continuously from one sidewith300μmol photons·m-2·s-1as artificial light, S. acuminatus was grown underdifferent initial NaNO3supplies, measured on growth. The biomass of S. acuminatusincreased along with the growth phase, at the end of cultivation, all the groupsachieved their highest biomass, and the highest biomass was obtained at6.0mmol/LNaNO3. It is demonstrated that comparing with the control（18mmol/L NaNO3）,mid-low nitrogen concentration at a certain extent can promote the algal biomassaccumulation. There is significant difference between different initial nitrogen groups（P<0.05）.（2）The algal morphologic and ultrastructural changes of S. acuminatus duringdifferent growth periods were observed under light microscope （LM） and transmission electron microscope （TEM）. The coenobia of S. acuminatus overlap in4crescent-shaped cells in the early stage, following with the decrease of chloroplastand starch granule,each cell began to appear small oil droplets, with the cultivationtime extending, S. acuminatus began to disperse into some single cell which becomebigger and rounder, the size of oil droplets in cells become bigger and small dropletsmerging to form larger oil body.（3） The total lipid content of algal cell increased significantly（54%DW），whichincreased by17%more than that of the control group(（P<0.05） under lower nitrogenconditon（3.6mmol/L NaNO3）, whereas the total carbohydrate and protein contentsdecreased significantly. It indicated that lower initial nitrogen supply, to some extent,could promote the accumulation of lipid in S. acuminatus。6mmol/L NaNO3was themost suitable nitrogen supply for the algal biomass accumulation and lipid yield.The contents of main photosynthetic pigments （chlorophyll a, b） and total carotenoidsin the alga were positively correlated with the nitrogen concentration.（4） The maximum efficiency of light energy conversion of PS â…¡ （Fv/Fm）, relativeelectron transfer efficiency （ETR）, and the actual energy conversion efficiency（Yield）decreased significantly accompanied with the decrease of nitrogen supply. Thephotosynthetic oxygen release rate decreased, while the respiratory rate of Rd showedan opposite tendency, which rised slowly during the whole cultivation cycle.（5） The analysis of the twodimensional electrophoresis （2-DE） maps of proteinextracts from S. acuminatus grown in medium containing low nitrogen concentrationpresented that, compared to the control group, the abundance of76proteins changedof more than2times. Glucose-6-phosphate isomerase and glutamate dehydrogenasewere found to be up-regulation and the six proteins：proteasome subunit beta type,predicted protein, putative uncharacterized protein, ornithine carbamoyltransferase,P0459B04.22and Rubisco presented as down-regulation expressed in low nitrogenlevel. The result indicated that cell protein expression changed significantly alongwith the accumulation of storage lipid, these differentially expressed proteinsparticipated in glycolysis, gluconeogenesis, calvin cycle, protein modification anddegradation, amino acid metabolism, and a series of metabolic process. Proteomic analysis proved that S. acuminatus could make corresponding changes in intracellularmetabolism regulation to adapt low nitrogen condition.