Improvement of Chemical Composition of Tisochrysis lutea Grown Mixotrophically under Nitrogen Depletion towards Biodiesel Production

Abstract

In the present study, the marine microalga Tisochrysis lutea was cultivated mixotrophically in F2 growth medium with sodium acetate as exogenous carbon source. The medium was composed of different concentrations of nitrogen to determine the impact of nitrogen depletion on cellular growth and chemical composition. Nitrogen depletion led to severely decreased growth and protein content. However, mild nitrogen depletion (0.22 mM NaNO₃) led to maximum lipid yield. The fatty acid methyl ester profile also showed increased unsaturation as the nitrogen content decreased. Growth in nitrogen-free medium increased the proportions of mono- and poly-unsaturated fatty acids, while the proportion of saturated fatty acids decreased. Growth under all tested nitrogen levels showed undetectable fatty acids with ≥4 double bonds, indicating these fatty acids had oxidative stability. In addition, all tested nitrogen concentrations led to specific gravity, kinematic viscosity, iodine value, and cetane number that meet the standards for Europe and the U.S.A. However, growth in the presence of nitrogen deficiency enhanced the higher heating value of the resulting biodiesel, a clear advantage from the perspective of energy efficiency. Thus, mixotrophic cultivation of T. lutea with nitrogen limitation provides a promising approach to achieve high lipid productivity and production of high-quality biodiesel.

Keywords: biodiesel; carbohydrates; fatty acids; microalgae; pigments; protein.

Figure 1

The cellular dry weight of Tisochrysis lutea grown mixotrophically at different concentrations of NaNO₃. * p < 0.05, ** p < 0.01, *** p < 0.001 with respect to the control (0.88 mM).

Figure 2

Crude protein (A), soluble protein (B), and true protein (C) content (as % dry weight (DW)) of Tisochrysis lutea grown mixotrophically in different concentrations of NaNO₃. ** p < 0.01, *** p < 0.001, ns: not significant with respect to the control (0.88 mM).

Figure 3

Total carbohydrate content (as % DW) of Tisochrysis lutea grown mixotrophically in different concentrations of NaNO_3. * p < 0.05, *** p < 0.001, ns: not significant with respect to the control (0.88 mM).

Figure 4

Contents of total chlorophyll (A) and carotenoids (B) in Tisochrysis lutea grown mixotrophically in different concentrations of NaNO₃. ** p < 0.01, *** p < 0.001, ns: not significant with respect to the control (0.88 mM).

Figure 5

Lipid content (A) and volumetric lipid productivity (VLP) (B) of Tisochrysis lutea grown mixotrophically in different concentrations of NaNO_3. * p < 0.05, ** p < 0.01, *** p < 0.001, ns: not significant with respect to the control (0.88 mM).