Characterization and optimization of endogenous lipid accumulation in Chlorella vulgaris SDEC-3M ability to rapidly accumulate lipid for reversing nightly lipid loss

BACKGROUND: During inevitable light/dark cycle, lipid productivity of outdoor microalgae photoautotrophic cultivation is lowered by nightly biomass and lipid loss. To minimize, or even reverse the nightly lipid loss, it was expected that lipid accumulation would not cease, even if at night. Without relying on photosynthesis and organic matter in media, endogenous lipid accumulation that consumes energy and carbon sources derived from cells themselves, namely endogenous accumulation, is the only way for lipid production. The main aims of the present study was to characteristic endogenously accumulated lipid, confirm feasibility to reverse nightly lipid loss, and determine optimal conditions and its quality suitability for biodiesel feedstock production under stress conditions. RESULTS: Chlorella vulgaris SDEC-3M ability to rapidly accumulated lipid under stress conditions was cultivated for 12 h in darkness, and the effects of various conditions on lipid accumulation and biomass loss were analyzed. Under non-stress conditions, lipid contents dropped. Under certain stress conditions, conversely, the lipid contents were substantially improved so that net nightly endogenous lipid accumulation was observed. Under the optimal conditions (aeration mode with 0.10 vvm and 15% CO(2), 5–10 mg L(−1) of NO(3)(−)-N, 30–35 °C, approximate 2500 mg L(−1) of biomass), the lipid content was doubled and increased lipid was approximately 180 mg L(−1). Among stress conditions, N-deficiency had the most significant effect on endogenous lipid accumulation, and the optimum results were characterized under relatively low-N concentrations. Higher consistency between loss in carbohydrate and gain in lipid confirmed accumulated lipid endogenously conversed from carbohydrate. Based on the analyses of fatty acids profiles and prediction of kinematic viscosity, specific gravity, cloud point, cetane number and iodine value, it was confirmed that the quality of lipid obtained under optimal conditions complied with biodiesel quality standards. CONCLUSION: Via triggering endogenous lipid accumulation by stress conditions, even in darkness, SDEC-3M can synthesize enough lipid suitable for biodiesel feedstock. It implies that the lipid accumulation phase in two-phase strategy can be scheduled at night, and following biomass production stage in light, which should be a solution to improve the lipid yield and quality of large-scale outdoor photoautotrophic microalgae cultivation for biodiesel production.