| Eustigmatos cf. polyphem is a unicellular soil microalga, belonging to theEustigmatophyceae, Eustigmatales, Eustigmataceae, Eustigmatos. Previous researchhas confirmed that E cf. polyphem can synthesize many high-value bioactivesubatances, such as chrysolaminarin, lipids, polyunsaturated fatty acids (PUFAs),beta-carotene and so on, therefore, E. cf. polyphem could be a potential microalga forcommercial application. In this study, E. cf. polyphem was used as experimentalmaterials. The growth, changes in major biochemical components and the specificacitivity of two key enzymes involved in the chrysolaminarin metabolism of E. cf.polyphem grown under two different initial supplies of sodium nitrate wereinvestigated. The aim is providing reference for further study of the relationshipbetween carbohydrate and lipids metabolism and carbon flow distribution inmicroalgal cells. The main results are as follows:(1) E. cf. polyphem cells were mainly spherical, the color of cultures changedgradually from green to yellow-green with the growth phase extension, large amountsof oil droplets accumulated inside cells during stationary stage. Under two differentinitial nitrogen supplies, there is no significant difference in the time for maximumgrowth of E. cf. polyphem between the control group (18mmol/L N) and thetreatment group (6mmol/L N). The highest biomass concentration of the treatmentgroup was obtained at the end of growth period with7.91g/L of dry biomass,0.45g/Lof dry biomass was higher than that of the control group. There was a significantdifference between the highest biomass concentrations of two groups (p<0.05).(2) The total lipids content of E. cf. polyphem had an enhancement trendfollowing the cultivation time, the fastest increase happened from the3rdday to the6thday and then reached the highest at the stationary phase. The highest total lipidscontent of the treatment group was up to62.02%of dry biomass, which increased by10.69%more than that of the control group, the total lipid contents of the two groupsfrom the6thday to the18thday period possessed significantly difference (p<0.05). Itindicated that the initial low nitrogen supply, to some extent, could promote theaccumulation of lipids in E. cf. polyphem.(3) The neutral lipids (NLs) were the major fractions of the total lipids in E. cf.polyphem, which contents existed an upward trend with the cultivation period, while the glycolipids (GLs) and phospholipids (PLs) exhibited a declining trend. Thehighest content of NLs in E. cf. polyphem under the initial low nitrogen supplyreached52.68%of dry biomass, accounting for85.51%of the total lipids. Using thinlayer chromatography (TLC), triacylglycerol (TAG) was separated from NLs,according to the gravimetric method, the highest TAG content was68%of NLs in theinitial low nitrogen supply group, the TAG content in the control group was60%ofNLs. The main fatty acids of this microalgal powder, NLs, GLs, PLs and TAG werepalmitic acid (C16:0), palmitoleic acid (C16:1) and oleic acid (C18:1), the sum ofthese three fatty acids in powder and NLs were81.50%and82.54%of the total fattyacids, respectively.(4) The total carbohydrate content in E. cf. polyphem dry biomass (%DW)increased first and then decreased, reached a peak on the3rdday. The highestcarbohydrate content of the initial low nitrogen supply group was39.59%of drybiomass, which was38.42%higher than that of the control group, the differencebetween these two groups was significant (p<0.05). Chrysolaminarin was the maincomponent of the total carbohydrate in E. cf. polyphem. The highest chrysolaminarincontent of the initial low nitrogen supply group was24.88%of dry biomass,accounting for62.84%of total carbohydrate, the chrysolamarin contents of the controlgroup were22.01%of dry biomass and57.29%of total carbohydrate, respectively.(5) The specific activities of two key enzymes involved in the chrysolaminarinmetabolism under different initial nitrogen concentration were studied, the resultsshowed that the specific activity of uridine diphosphate glucose pyrophosphorylase(UGPase) in E. cf. polyphem exerted a downward trend following the growth period,but, there was no significant difference between two groups(p>0.05). The specificactivity of UGPase was195.92μmol/g·min in the early stage of cultures, while thespecific activity of UGPase were98.01μmol/g·min and103.77μmol/g·min withtreatment and control groupds on the4thday, respectively. The specific activity ofβ-1,3-glucanase the cultures of E. cf. polyphem decreased first and then slightlyincreased during growth phase. It was approximately0.94U/g at the early stage ofcultures,0.72U/g at24h-48h of the control group and then increased to0.88U/g at96h; the minimum specific activity of treatment group was0.43U/g at48h andincreased to0.46U/g at96h. The specific activities of β-1,3-glucanase in thetreatment group was always lower than that of the control group, the differencebetween the two groups was significant (p<0.05). |
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