| Nannochloropsis oculata, which is belonging to photoautotrophic microalgae, is rich in eicosapentaenoic acid, and was selected as the experimental object in this paper. To deal with the problem of low photosynthesis efficiency, low biomass and productivity of photoautotrophic microalgae grown outdoor, this paper took advantage of the photobioreactor design and the optimization of cultivation strategy to increase the biomass and productivity of N. oculata.Through the light attenuation experiment, the light attenuation formula of pure cultures of N. oculata was established:I=I0 e-(206.3x-1.2)L (0.1≤<2.0 g·L-1). The experimental results confirmed that the optimal CO2 concentration for N. oculata cultivation was 2%, and the concentration of 4% CO2 inhibited the growth of algal cells.TL-BBR (Thin layer-Bubbling bioreactor) was established and operated successfully, and by comparing the performance differences of N. oculata which was cultured in TL-BBR and BBR, TL-BBR was found to be more suitable for the high cell density cultivation of photoautotrophic microalgae, with the highest biomass, productivity and specific growth rate being 1412 mg·L-1,59.4 mg·L-1·d-1 and 0.1 d-1 respectively. The different light conditions of algal cells grown in the two reactors resulted in the difference of the biochemical composition of the cells. The intracellular chlorophyll a, protein and lipid content of cells grown in the TL-BBR was lower than that in the BBR, and only the carbohydrate content of cells grown in the TL-BBR was higher than that in the BBR. The ability of uptake and assimilation of nitrogen and phosphorus by algal cells grown in the TL-BBR was better than that in the BBR. N. oculata contained higher EPA (31.8%) could be produced in the TL-BBR within a shorter production cycle.Ammonium acetate was chosen to be the best nitrogen source for biomass production of N. oculata through optimization experiment of nitrogen sources, with which the highest biomass and productivity were obtained, being 873 mg·L-1 and 25.3 mg·L--1·d-1 respectively. N. oculata was easy to absorb ammonium nitrogen, which can be quickly converted into intracellular protein. Addition of ammonium acetate promoted the accumulation of intracellular lipid and protein simultaneously. The lipid content of algal cells cultured with ammonium acetate (13.5%) was the highest among all, and the carbohydrate (16.5%) and chlorophyll a (3.32%) contents of algal cells cultured with sodium nitrate were the highest among all. Nitrogen sources also had effects on the intracellular fatty acid profiles of N. oculata, and the highest relative content of EPA of sodium nitrate group was 26.6%.The dilution effects of renewal part of culture broth helped to increase the productivity of N. oculata. The results showed that the biomass productivity increased with the increasing renewal rates, and the highest productivity was 35.5 mg·L-1·d-1 with 20% renewal rate. However, the production stability could be reduced by excess renewal, resulting in diluted culture broth and increased recovery costs. The determination of renewal rate needed to weigh the productivity and production stability of N. oculata simultaneously. Under our experimental condition,10% was chosen to be the best renewal rate to produce N. oculata. Increasing renewal rate helped to increase the intracellular lipid content, the highest value (12.8%) was achieved with 10% and 20% renewal rate. The average intracellular chlorophyll a and protein of N. oculata increased with increasing renewal rate. Renewal of part of culture broth also affected the fatty acid profiles, which caused the increase of the relative content of saturated fatty acids, and was not conducive to the accumulation of PUFAs.With ammonium acetate as the nitrogen source, TL-BBR was used to carry out high cell density cultivation of N. oculata, and the maximum biomass and productivity were 2.3 g·L-1 and 104 mg·L-1·-d-1 respectively. |
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