Effects of Nitrogen Limitation on Biochemical Composition and Photosynthetic Physiology during Lipid Accumulation in Chlorella vulgaris Beijierineck

Microalgae of the genus Chlorella are capable of accumulating lipids when exposed to nutrient limitation (especially nitrogen) and are therefore considered promising organisms for biodiesel production. This study explored the effects of nitrogen limitation on biomass, biochemical components and photosynthetic physiological parameters of C.vulgaris Beijierineck, an oleaginous microalga. Growth experiments were carried out in modified BG-11 medium with four different initial concentrations of sodium nitrate (18.0, 9.0, 4.5 and 3.6 mmol/L). These four nitrogen concentrations had no remarkable influence on growth, but exerted considerable influence on lipid accumulation in C. vulgaris. Maximum lipid content and productivity were obtained in the 4.5 mmol/L group (48.32% of dry weight and 0.0931 g·L^-1·d^-1, respectively), which increased significantly (P<0.05) compared with those of the 18.0 mmol/L group. The content of carbohydrate and soluble protein in the four groups decreased with increased lipid accumulation; however, in the low nitrogen groups (4.5 mmol/L and 3.6 mmol/L), the soluble protein degraded rapidly for carbohydrate synthesis at the initial stage, but the accumulated carbohydrate at the initial stage eventually converted to lipid at the later stage of cultivation. The maximum efficiency of light energy conversion of PSⅡ (F_v/F_m), actual energy conversion efficiency (Yield), and relative electron transfer efficiency (ETR) all decreased significantly, and the change in F683/F718 suggested optical energy distribution and state transition between PSⅠ and PSⅡ. In addition, the initial and total Rubisco activity of C. vulgaris in the 18.0 mmol/L and 9.0 mmol/L groups both peaked on the third day and then declined, while the Rubisco activity in the low nitrogen groups declined constantly. The ratio of initial and total Rubisco activity tended to decline at first and then increase. In conclusion, nitrogen limitation promoted lipid accumulation in C. vulgaris, and carbon distribution and photosynthetic physiology changed markedly.