Effects of the timing of a culture temperature reduction on the comprehensive metabolite profiles of Chlorella vulgaris

This study investigated the effects of different temperature conditions on the comprehensive metabolite profiles of Chlorella vulgaris using gas chromatography–mass spectrometry coupled with multivariate statistical analysis. C. vulgaris cells cultivated at 20 °C were transferred to 10 °C incubators at different time points of cultivation [days 0 (TR0D), 7 (TR7D), and 14 (TR14D)], then they were cultivated at 10 °C until harvesting at day 21 to compare the growth and comprehensive metabolite profiles with those cultivated under a constant cultivation temperature of 20 °C (T20). There was no significant difference in algal cell growth between cultivation under the T20 and temperature reduction (TR) conditions. Algal fatty-acid profiles under TR were different from those of the T20 condition. Specifically, the contents of octadecanoic acid (C18:0), octadecenoic acid (C18:1), hexadecadienoic acid (C16:2), and octadecadienoic acid (C18:2) increased the most under TR0D. The relative levels of metabolites such as β- alanine, glutamine, glycine, isoleucine, proline, valine, and myo-inositol, which act as osmolytes, and bioactive compounds such as neophytadiene and ascorbic acid were increased under TR conditions on day 21. Among the metabolites, the contents of neophytadiene and ascorbic acid were further investigated, and the content of ascorbic acid was highest on day 14 under the TR7D condition, while the content of neophytadiene was highest on day 21 under the TR0D and TR14D condition. Therefore, we suggest that a TR from 20 to 10 °C could enhance the production in C. vulgaris cultures of bioactive fatty acids such as C18:1, C16:2, and C18:2 (TR0D), organic osmolytes such as β- alanine, glutamine, glycine, isoleucine, proline, valine, and myo-inositol (TR conditions), ascorbic acid (TR7D), and neophytadiene (TR0D and TR14D).