Acetic acid stress and utilization synergistically enhance squalene biosynthesis in Candida glycerinogenes

Squalene is widely used in industries such as vaccines and cosmetics. The traditional carbon source for microbial synthesis of squalene is glucose, but its long metabolic pathway leads to inefficient synthesis, limiting industrial production. This study uses acetate as a carbon source, taking advantage of its stress effects and the ability to shorten metabolic pathways to enhance the synthesis of squalene. To promote the accumulation of squalene, the MVA pathway and NADPH supply pathways were enhanced. To alleviate acetic acid stress, acetic acid conversion was accelerated by overexpressing ACS1, and the strain’s tolerance to acetic acid was improved by overexpressing HOG1. Subsequently, the strain was subjected to fed-batch fermentation under acetic acid stress with a carbon source of 6 g/L acetic acid, resulting in a squalene titer of 1834.74 mg/L in shake flasks. The culmination of these strategies, applied during a 96 h fed-batch fermentation with acetic acid-mediated pH control in a 5 L bioreactor, resulted in a squalene titer of 8.76 g/L, a content of 411.24 mg/g DCW, and a productivity of 4.28 mg·h⁻¹·g⁻¹ DCW, marking the highest productivity reported to date. This study provides a novel perspective for the green, economically efficient biosynthesis of squalene.