Effect of Heat Stress on the Biosynthesis of Exopolysaccharides from Rhodotorula glutinis YM25079 and Its Underlying Mechanisms

In this study, the effect of heat stress on the synthesis and the structural and physicochemical properties of exopolysaccharides (EPSs) from <i>Rhodotorula glutinis</i> YM25079 as well as its underlying mechanisms were explored. The results showed that the monosaccharide compositions of the purified YM25079 EPSs produced under normal culture conditions and heat stress (named EPS Y-1 and EPS Y-2, respectively) were consistent. Analyses of ion-exchanged chromatography, Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy suggested that these two EPSs should be dextran, consisting mainly of α-(1→6)-linked glucopyranose units with α-(1→3) branches. Scanning Electron Microscope observed obvious differences in their surface morphologies, with EPS Y-1 showing a smooth, glossy lamellar structure and EPS Y-2 showing an irregular porous structure. According to Atomic Force Microscopy analysis, they formed aggregations with different cohesive structures. EPS Y-2 also had higher molecular weight and thermal stability than EPS Y-1, while EPS Y-1 had better α-amylase inhibitory activity. In addition, transcriptomic analysis unveiled changes in the metabolic pathways related to the uptake and utilization of carbon, nitrogen and phosphor sources, the biosynthesis of steroid and the oxidoreductase activity, as well as the regulatory genes implicated in the EPS biosynthesis under heat stress.