Research On Growth And Physiological And Biochemical Characteristics Of Scenedesmus Acuminatus In Different Sulfur Concentrations

Scenedesmus acuminatus is a unicellular green microaglae with high growth and lipid accumulation,which can be used to produce biodiesel.In this study,S.acuminatus was used as material cultivated in the medium of different initial sulfate concentrations?0.25 S,0.5S,0.75S?sulfur-deficiency?,1.5S and 2.0S?sulfur-rich??by ?3.0 cm × 60 cm columnar photo-bioreactor that compared to the improved BG-11?1.0S?.We studied the effects of growth and the carbon,nitrogen and sulfur-assimilation characteristics of S.acuminatus and analyzed the dynamic changes of biochemical components and sulfur-containing lipid SQDG in different sulfur levels.Based on the characteristics of chlorophyll fluorescence parameters,we also investigated the effects of sulfur nutrition on photosynthetic activity and explored the variation of alternative electron flow of S.acuminatus after cells were treated with inhibitors,which will provide the theoretical basis for a further study on the metabolism engineering of lipids in S.acuminatus.The results are as follows:?1?The growth rate of S.acuminatus was positively correlated with the initial sulfur concentration.The biomass and cell density of the 2.0S group were 7.47 g/L and 19.6 × 107 cells/mL,respectively,which were significantly higher than those in 1.0S control group and low sulfur groups?p<0.05?.The level of sulfur affected the assimilation and uptake of NO₃^-on S.acuminatus.The concentration of NO₃^-in the sulfur-rich group was 0 in the 3rd day,and the 0.25 S and 0.5S low-sulfur groups were remained relatively higher NO₃^-levels till the end of cultivation,were 9.8 mmol/L and 6.46 mmol/L,respectively.The results of carbon,nitrogen and sulfur analysis showed that the low-sulfur condition was beneficial to the accumulation of carbon in the unit cell of S.acuminatus As the culture time lasted longer,the intracellular C/N and carbon contents increased gradually to the 18 th day;the carbon accumulation and the ratio of C/N achieved the highest in the 0.25 S group,which was 50.36 pg/cell and the carbon content was account for 62.80% of the dry weight.?2?During the whole culture period,the content of soluble protein in six sulfur concentration groups decreased markedly,and the carbohydrate gradually decreased after a transient increased in the initial period,while the content of lipid increased with the prolongation of culture time.The initial sulfur concentration was positively correlated with the content of soluble protein and negatively correlated with the lipid content.The low sulfur conditions in the earlystage was favorable for the accumulation of carbohydrate in algae cells.The carbohydrate content in the 0.25 S group was the highest,accounting for 44.37% of the dry weight,14.43% and 13.78% larger than the 1.0S and 2.0S groups?p<0.05?,respectively.On the 18 th day,the total lipid content in 0.25 S and 0.5S groups was 55.15% and 52.45% respectively,which was significantly higher than that in 1.0S control group?42.95%?and 2.0S group?42.85%??p<0.01?,indicated that it is beneficial to the accumulation of lipid under the low sulfur condition.The content of SQDG in S.acuminatus was gradually decreased with the extension of time,and the content of neutral lipid increased continuously,of which 0.25 S group of the highest lipid,accounting for 92.12% of the total lipid.?3?The contents of Chlorophyll a,Chlorophyll b and Carotenoids in S.acuminatus were positively correlated with the initial sulfur concentration,and the 2.0S group was obviously higher than that of the three low sulfur groups?p<0.05?,the results showed that sulfur-rich conditions could promote the rapid synthesis of pigment in S.acuminatus The ratio of Car/Chl in 0.25 S and 0.5S groups were continued to increase during the incubation period,indicated that the lower sulfur concentration promoted the accumulation of Carotenoids in S.acuminatus Meanwhile,The photosynthetic oxygen evolution rate,the maximum light energy conversion efficiency of PS??Fv/Fm?,the actual light energy conversion efficiency Yield???and the relative electron transfer efficiency rETR???were positively correlated with the initial sulfur concentration,the whole culture period showed a trend of increasing first and then decreasing significantly.The results of fluorescence emission spectroscopy at 77 K showed that there was light energy allocation between the two photosynthetic systems at the prophase of cultivation.?4?The optimal concentration of DCMU was 100 ?M,and the optimum treatment time was 1.5-2.0 h.There were cyclic electron flow around photosystem??PS??at different initial concentration of sulfur in S.acuminatus On the 1st day ofcultivation,the ratio of cyclic electron flow to total electron flow?A = rETR?DCMU/rETR?no DCMU?in the low sulfur groups is higher than sulfur-rich groups.On the 6th day,the ratio of A in sulfur-rich groups was higher than that of the low-sulfur groups,and the highest in the 1.25 S and 1.5S groups were 91.49% and 91.95% respectively.In conclusion,the proportion of the cyclic electron flow in the S.acuminatus was related to the level of sulfur in the culture medium.