| The combination of CO2 fixation and biofuel production through microalgae represents a promising alternative to current CO2 mitigation strategies. A limitation is that photosynthesis in many microalgal species will be dramatically inhibited by the high dissolved oxygen (DO) accumulated in the culture medium. Conventionally the dissolved oxygen is eliminated using a degasser or vigorously sparging air into the medium. However, several disadvantages of these methods have been reported such as high cost, increasing of contamination risk, and low effectiveness of oxygen removal due to limited transfer efficiency of oxygen from liquid to gas phase. Therefore, it is necessary for developing a new approach of deoxygenation for biomass production of valuable microalgae. Apart from autotrophic growth, there are also other metabolic types such as heterotrophic, mixotrophic, etc. High algal cell density and biomass production are the main attractions of these growth approaches. The basic culture medium composition for heterotrophic cultures is similar to the autotrophic culture with the sole exception of adding an organic carbon. In mixotrophic growth, CO2 and organic carbon are simultaneously assimilated and both respiratory and photosynthetic metabolism operates concurrently.Sodium Isoascorbate (NaErA) is a new food antioxidant. So far, there is no report that NaErA has been used in oxygen dissolved removal of microalgal culture systems. Meanwhile, it is not sure that the NaErA could be regarded as carbon resource for heterotrophic or mixotrophic culture of microalgae to promote their growth. In this study, we explore the effect of oxygen removal by NaErA in photoautotrophic cultivation system of three species of microalgae(Chlorella vulgaris, Nannochloropsis limnetica and Dunaliella salina) in the first; then, NaErA and organic carbon resource (glucose) were added into the open cultivation system respectively, and we analyze the advantages and the reasons why NaErA is good for microalgae photoheterotrophy by measuring biological and chemical indexes during the cultivation of C. vulgaris; then, the feasibility of C. vulgaris subculture was evaluated with supernatants from heterotrophic systems; finally, we cultivate C. vulgaris in two-phase culture (photoheterotrophy with air aerated containing NaErA as the first phase for 9 days, and mixotrophy with 10% CO2 aerated as the second phase for 11 days). According to growth, carbon fixation, lipid productivity and fatty acid composition of microalgae, we will draw a conclusion that whether the high density cells of microalgae which results from NaErA, could promote CO2 fixation rate and biofuel production potential of microalgae significantly. The main results are as follows:(1)The adding of NaErA could remove O2 released by photosynthesis of microalgae quickly, but the effects of oxygen removal varied with the difference of microalgae and doses. As for the cultivation system of C. vulgaris and N. limnetica, the optical doses of NaErA were 1G (1.5744 g/200 ml) and 0.5G (1.8550 g/200 ml) respectively. Besides, the DO of microalgae suspension could be controlled at suitable low level below (1 or 2 mg·L-1) and the growth of microalgae was promoted significantly. The biomass of the two algae reached at 1.94 g·L-1 and 1.70 g·L-1 respectively, and at the end of cultivation is 5.88 times and 3.15 times of the control group, respectively. The DO was controlled at a low level with the higher dose of NaErA, however, it is not good for the growth of the microalgae. NaErA has strong inhibition effect on the growth of D. salina, so it was not appropriate deoxidant for the cultivation system of microalgae.(2) NaErA (3.1488 g/400 ml) and the same carbon concentration (2.8640 g/400 ml glucose) were added to open culture systems with illumination inoculated with C. vulgaris. The DO of the systems added with glucose dropped to below 2 mg·L-1. The photosynthesis is restrained, and microalgae was only capable of growing photoheterotrophically. On the contrary, The systems added with NaErA in the low DO level, which could promote microalgae autotrophic growth. Meanwhile, the oxidation products of NaErA can also serve as organic carbon resource for microalgae photoheterotrophic growth. As the result, the biomass of microalgae added with NaErA was 7.84 times higher than control (without organic carbon), and increased by 20.6% than treatment added with glucose. Therefore, in open culture systems, NaErA has better microalgae growth enhancement than traditional organic carbon glucose.(3) In the culture systems added organic carbon resource, the concentration of NO3--N less than 5 mg·L-1 at the end of the cultivation, leading growth inhibition of microalgae. If we mix the centrifuged and filtered supernatants with fresh medium according to a certain proportion (1:1 or 1:3), making N resource supplement, as a result, we could cultivate C. vulgaris continuously. It is important for reducing the cost of microalgae culture.(4) Compared with photoautotrophic cultivation systems aerated with 10% CO2 for 20 days, the C. vulgaris photoheterotrophic cultivation systems containing NaErA as carbon resource with air aerated for 9 days, then change the air into 10% CO2, and growth pattern of microalgae would convert to mixotrophy which main part of photoautotrophy as consequence, and average carbon fixation rate (306 mg·L-1·d-1) is 4.46 times higher than control groups (photoautotrophic culture), indicating that high density cells of microalgae comes from photoheterotrophic cultivation systems has a higher CO2 fixation rate. At the end of the cultivation, optical density, biomass, lipid productivity of photoheterotrophy are 1.12,1.62 and 1.2 times higher than photoautotrophy, and the short-chain fatty acids, polyunsaturated fatty acids and C16～C18 fatty acids are also at a high level, making it possible for biofuel production. |
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