| Microalgae,considered as a potential feedstock for biofuel production,have several features—such as extremely rapid growth,short generation time,and high content in oil.Although microalgae are regarded as the next-generation source of biodiesel,large-scale commercialization is still a challenge,due to additional cost and energy consumption.Therefore,how to achieve the rapid and efficient accumulation of lipid without weakening the growth is the key to enhance and economic the algal biomass generation.Seawater,combining the two stress factors of low nitrogen and high salinity,is a good choice to promote the lipid productivity.However,pure seawater cannot support the growth of freshwater microalgae.The appropriate frequency of ultrasound could stimulate the growth of microalgae,so the seawater coupled ultrasound is expected to be a new method to cultivate freshwater Chlorella.In this study,two-stage approach combined with seawater and ultrasonic treatment was applied to cultivate Chlorella sorokiniana SDEC-18 at stationary phase in order to boost growth and lipid content.Chlorella sorokiniana SDEC-18 was cultivated through two stages,which comprised fast cell growth in stage ? in BG11,followed by lipid stimulation in stage ? in seawater and ultrasound treatment to improve lipid productivity.The effect of seawater coupled ultrasound on microalgal growth and lipid accumulation was studied to explore the lipid synthesis mechanism of freshwater algae SDEC-18.The mechanism on the synthesis and secretion of AES in algae under stresses were analyzed.And the influence of ultrasound on the cell permeability of algae was also clarified.The main studies and conclusions reached in the present study are as follows:(1)The introduction of two-stage culture can effectively solve the adverse effects of seawater culture on the growth of microalgae.In the stage ?,SDEC-18 was cultivated in BG11 medium,and the highest biomass in BG11 medium is up to 0.52 g/L.Ultrasound,introduced in stage ?,improved microalgae growth greatly by removing 19%of the algal extracellular substances(AESs),and further increased biomass concentration and cell density to 0.63 g/L and 5.68×1010 cells/L,which were 1.4 and 1.5 times of that in seawater control,respectively.(2)A novel combination of seawater and ultrasound was able to realize a satisfactory lipid content.After 6 days of stage II,lipid content increased from 25.45%to 64.38%,which was significantly higher than that in the seawater medium(50.53%),almost three times of that in the BG11 control.Lipid productivity value of 28.78 mg/L/d was obtained,which was about 3.2 times of that in the BG11 control.(3)It seems that the application and duration seawater are the main factors influencing the cell size.The sizes of C.sorokiniana SDEC-18 under stress were all greater than those in the BG11 control group.In BG11,the average diameter of cells in the range of 5.0-7.0 pm accounted for the majority(77%).In the seawater culture medium,the majority of microalgae cells were in the range of 6.5-9.5 ?n.After the introduction of ultrasound,the cell size concentrated in the range of 7.0-9.5 ?m.After 6 days' stress with S&U,the diameter was up to 7.6?m in mean value,which is 1.3 times larger than cells in BG11 medium(mean diameter of 5.83 ?m).The largest cell diameter was 11.02 ?m,and the size of most algal cells increased with the increase of cultivation time.(4)At the stage of lipid accumulation,the content of Chl-a and total Chl decreased,while the content of carotenoids increased.After 10 days of cultivation in BG11 medium,the contents of Chl-a(5.3 mg/L)and Chl(7.25 mg/L)reached the highest.After SDEC-18 was transferred to seawater in stage II,the content of Chl-a decreased to 2.98 mg/L,and Chl-a/Chl-b decreased,indicating that seawater caused the decrease of cell activity.At the same time,the amount of carotenoids in cells increased with the addition of seawater.The maximum concentration of carotenoids reached 0.98 mg/L and Caro/Chl increased in the 6th days of S&U,showing the response of Chlorella to the stress environment. |
PDF Full Text Request