The growth characteristics and metabolic profiling of Δ 5,7 -sterols and fatty acids in Schizochytrium sp. S31 under different culture conditions

Abstract Schizochytrium is a promising thraustochytrid cell factory for co-producing high-value bioactives; however, its growth and bioactive accumulation depend heavily on culture conditions. This study systematically investigated the growth characteristics and the changes in Δ 5,7 -sterols, fatty acids and astaxanthin of Schizochytrium sp. S31 under different concentrations of glucose (0–120 g/L), sea salt (0–130 g/L) and pH (3.5–7.5). Sea salt exerted a markedly stronger influence on sterol profiles and fatty acid contents than glucose or pH. Low salinity (10 g/L), with ergosterol constituting the largest proportion (41%), maximized total fatty acid (41.3% DCW) and DHA (7.3% DCW) contents, whereas higher salinities shifted sterol composition toward C28-Δ 5,7 and increased trace astaxanthin levels. The maximum cell dry weight of 17.4 g/L was obtained at 60 g/L glucose, 20 g/L sea salt and pH 5.5, which was also the optimum condition for the simultaneous accumulation of Δ 5,7 -sterols (50.73 mg/L), docosahexaenoic acid (DHA, 1097 mg/L) and astaxanthin (0.11 mg/L). Transcriptomic analysis revealed a crosstalk among sterol, DHA and astaxanthin biosynthetic pathways in Schizochytrium sp. S31, and low salinity enhanced fatty acid β-oxidation to elevate the acetyl-CoA supply toward the mevalonate (MVA) pathway, while downregulating crtI and crtR to suppress downstream astaxanthin accumulation. Additionally, candidate genes involved in key phytosterols synthesis were identified. These findings provide critical insights into the biosynthetic regulation of sterols, astaxanthin, and DHA in thraustochytrids, highlighting their potential for the co-production of these high-value bioactives.