Growth, biochemical composition and photosynthetic performance of Scenedesmus acuminatus under different initial sulfur supplies

The effects of sulfur availability on growth, biochemical composition and photosynthetic capacity of the freshwater green microalga Scenedesmus acuminatus were investigated. Modified BG-11 media with Na2SO4 concentrations of 0.076 mM (0.25S), 0.153 mM (0.5S), 0.306 mM (control group), 0.459 mM (1.5S) and 0.612 mM (2.0S) were used to culture S. acuminatus. Sulfur repletion promoted the growth and reproduction of this species, while low sulfur supply markedly decreased the reproduction, nitrogen uptake, soluble protein and chlorophyll content, and photosynthetic activity. Elemental analysis showed that nitrogen, sulfur and carbon content per cell was significantly higher in the sulfur-limitation groups than in the control group on day 18. The metabolic response to sulfur limitation was divided into two stages: first, carbon equivalents were diverted from soluble protein to carbohydrates; then, carbohydrates were converted into neutral lipids. The total lipid content in the 0.25S and 0.5S groups reached 55.2% dry weight (DW) and 52.5% DW, respectively; neutral lipids accounted for 92.1% and 90.3% of the total lipids. Low-sulfur (0.5S) and sulfur-replete conditions both enhanced the lipid productivity. The initial sulfur level caused significant changes in the fatty acid composition of S. acuminatus. The sulfur-limited groups accumulated C16:0 and C18:1, while the sulfur-replete groups were mainly enriched in C16:0. Levels of free leucine, isoleucine, valine, lysine, glycine, alanine, aspartic acid and proline were closely related to the initial sulfur concentration. Changes in free phenylalanine, tryptophan, threonine and serine were due more to the duration of culturing. The results of this study will be useful as a reference for regulation of the initial sulfur supply to enhance lipid productivity of S. acuminatus for use in applications such as biodiesel production.