Light attenuates lipid accumulation while enhancing cell proliferation and starch synthesis in the glucose-fed oleaginous microalga Chlorella zofingiensis

Abstract

The objective of this study was to investigate the effect of light on lipid and starch accumulation in the oleaginous green algae Chlorella zofingiensis supplemented with glucose. C. zofingiensis, when fed with 30 g/L glucose, synthesized lipids up to 0.531 g/g dry weight; while in the presence of light, the lipid content dropped down to 0.352 g/g dry weight. Lipid yield on glucose was 0.184 g/g glucose, 14% higher than that cultured with light. The light-mediated lipid reduction was accompanied by the down-regulation of fatty acid biosynthetic genes at the transcriptional level. Furthermore, light promoted cell proliferation, starch accumulation, and the starch yield based on glucose. Taken together, light may attenuate lipid accumulation, possibly through the inhibition of lipid biosynthetic pathway, leading to more carbon flux from glucose to starch. This study reveals the dual effects of light on the sugar-fed C. zofingiensis and provides valuable insights into the possible optimization of algal biomass and lipid production by manipulation of culture conditions.

Figure 1. Time course of dry weight of C. zofingiensis cells with light and without light (A).

Changes in culture color on the second, fourth, and eighth day with light and without light (B).

Figure 2. Time course of lipid content (A), starch content (B), lipid yield on glucose (C), and starch yield on glucose (D) in C. zofingiensis cells grown on glucose with or without light.

Figure 3. Time course of cell density (A), per cell weight (C), per cell lipid content (D) in C. zofingiensis grown with light and without light.

Cell cycle determined by flow cytometry (B) with labels of M1, M2 and M3 representing G1/G0 phase, S phase and G2/M phases, respectively, and M4 to M8 representing polyploid phases. BODIPY stained intracellular lipids (E) of C. zofingiensis cells grown on glucose with or without light. Bars in E represent 5 μm.

Figure 4. Relative transcript levels of SAD (A) and BC (B) in C. zofingiensis grown in four culture conditions: Dark culture, light culture, conversion from dark to light after three days and conversion from light to dark after three days; SAD, stearoyl ACP desaturase; BC, biotin carboxylase.

The levels of gene expression were normalized relative to the cultures with light on day 1, which was set to 1.

Figure 5. Fatty acid profiles of C. zofingiensis lipids in four culture conditions: L (light culture), D (dark culture), LD (conversion from light to dark after three days), and DL (conversion from dark to light after three days); The numbers following the letters represent culture days.

Data are expressed as percentages of total fatty acids.