The Research Of The Enhancement Of Growth And Lipid Production Of Nannochloropsis Oculata By Mixotrophic Cultivation With Acetate And Ammonium

With the exhaustion of the fossil fuels,seeking a new type of renewable energy becomes more and more important.Microalga because of its fast growth,high oil content,and strong adaptability to advantage,is an ideal material for the production of biodiesel.At present,the main bottleneck restricting the microalgae in industrial application is its high cost.Then,improving its growth rate is an effective means to reduce cost.In this paper we used acetate as carbon source to remove the detoxification of ammonium to Nannochloropsis oculata and enhance its growth and lipid production,and discussed its mechanism by using comparative transcriptomics.The results are as follows:1)The effects of salinity,pH,carbon and nitrogen on the growth and lipid production were studied.The optimal salinity for Nannochloropsis oculata was 25‰.The optimal pH for Nannochloropsis oculata was 7-8.Nitrite and nitrate cound be used as the nitrogen source of Nannochloropsis oculata,but ammonium is toxic to autotrophic cultivation of Nannochloropsis oculata.However,toxicity was not observed when acetate was used as a substrate.In addition to the enhancement on algal growth,mixotrophic cultivation with acetate significantly promoted lipid production,resulting in a lipid content of 38.6% when using ammonium as the nitrogen source.Compared with the lipid content of alga in mixotrophic cultivation using nitrate or nitrite as nitrogen source,the algal lipid content in mixotrophic cultivation with ammonium as nitrogen source was about 2.7–4.4 times higher.2)The effects of ammonium and acetate on algal growth and lipid production.The algal biomass in mixotrophic cultivation reached maximum production with a cell density of 543 mg/L at the concentration of 1 mM ammonium,which are significant higher than the density when grown in heterotrophic conditions.The alga lipid content also reached maximum production level under this condition,which was measured to be 279 mg/L.Compared with heterotrophic cultivation,mixotrophic cultivation had a significant effect on the lipid synthesis,resulting in the lipid contents being about 2 –5.7 times higher and the lipid production being about 2.7 –7.0 times higher at the same concentrations of ammonium?Biomass production reached to 560 mg/L at concentrations of 16–64 mM of acetate.Similar to the enhancement effect of acetate on algal growth,2.6–5.6 times higher lipid content and 3–7.7 times higher lipid production was observed when the acetate concentration ranged from 4 to 128 mM.3)In the transcriptomic analyses,the Carbon fixation of Nannochloropsis oculata was reduced when Nannochloropsis oculata was cultivated in mixotrophic with ammonium and acetate,in which the acetate was used as the major carbon source for the growth of Nannochloropsis oculata.Photosynthesis was enhanced in mixtrophic with ammonium and acetate,so much more NADPH could be generated during the cultivation and used to synthese fatty acid and produce lipid.