Selection Of Alkalihalophilic Microalgae Strains And Optimizing Their Lipid Production

Microalgae are a promising feedstock for biodiesel production with high efficiency because of their fast growth rate, high photosynthetic efficiency, and high lipid content. However, the cost must be significantly reduced to commerilize this process. The cost of CO2 supply is one of significant cost in current algae culture processes, and must be reduced. Bicarbonate has advantages of easy transport and storage, as well as high utilization rate. When combined with the process that uses carbonate to capture CO2, a Bicarbonate-based Integrated Carbon Capture and Algae Production System (BAICCAPS) can be realized, in which bicarbonate can supply carbon to algae culture with low cost and high efficiency. However, the consumption of bicarbonate leads to pH drift, and the algae cultivated in this system should be tolerant to haloalkali. Currently, there are few studies on lipid production with alkalihalophilic microalgae. Thus, we conducted some research in this study.Firstly,7 strains of oleaginous microalgae, including Nannochloropsis maritime, Nannochloropsis oculata, Picochlorum oklahomensi (UTEX 2795, Chinese name is not known), Chlorella sp. (marine), Chlorella sp. (freshwater), Neochloris oleoabundans (UTEX 1185) and Dunaliella tertiolecta, were tested in different concentration of NaHHCO3. The result showed that N.maritime and N.oculata could grow in 0.01 mol/L NaHHCO3; P.oklahomensi and Chlorella sp. (marine) could grow in 0.04 mol/L NaHHCO3; Chlorella sp. (freshwater) could grow in 0.10 mol/L NaHCO3; N. oleoabundans could grow in 0.16 mol/L NaHHCO3, and D.tertiolecta could grow in 0.5 mol/L NaHCO3. When bibarbonate was used as inorganic carbon source, the highest lipid content were 15.7% for freshwater chlorella sp. at 0.04 mol/L NaHCO3,18.1% for N. oleoabundans at 0.08 mol/L NaHCO3,20.5% for D.tertiolecta at 0.4 mol/L NaHHCO3, respectively. These three microalgae stains growing in high concentration of bicarbonate and producing lipid are suitable for application in BAICCAPS for biodiesel production.Nitrogen limitation was then investigated to induce lipid production for selected N. oleoabundans and D. tertiolecta. It was found that these two strains had highest lipid content at 0.12 g/L NaNO3, and the correspongding lipid content was 34.1% for N. oleoabundans and 23.3% for D. tertiolecta, respectively. At this condition, the lipid productivity were 60 mg/L/d for N. oleoabundans, and 22 mg/L/d for D. tertiolecta. Also the same lipid productivity was reached at 0.48 g/L NaNO3 for D.tertiolecta because of higher biomass. Furthermore, these two algae fatty acid compositions were mainly saturated fatty acids and monounsaturated fatty acids at 0.12 g/L NaNO3, which is suitable for high quality of biodiesel production from microalgae.Due to higher lipid productivity for N. oleoabundans, the microalgae was finally cultivated in a 4.5 L photobioreactor and produced lipid under nitrogen depletion. The dry weight was 0.72 g/L, the lipid content was 33.7%, and average lipid productivity was 80 mg/L/d at 3rd day. This result indiated that N. oleoabundans could be an ideal algae for BAICCAPS, which can be applied for efficient biodiesel production with futher development.