Exogenous 24-epibrassinolide (EBL) facilitates cell growth of Chlorella pyrenoidosa under high temperatures by enhancing the photosynthetic energy utilization and alleviating oxidative damage

The microalga Chlorella pyrenoidosa is cultivated extensively for its constituents, which are of significant economic worth. Large-scale growth of C. pyrenoidosa in outdoor environments is subject to various stressors such as elevated temperature. The purpose of this study was to assess the protective effects of exogenous 24-epibrassinolide (EBL) on C. pyrenoidosa under high-temperature conditions. Compared to a temperature of 30°C, increasing the temperature to 43°C reduced the enzymatic capacity for carbon assimilation and resulted in the buildup of reactive oxygen species (ROS), thus reducing photosynthesis and proliferation. It was observed that exogenous EBL protected C. pyrenoidosa cells against high temperatures, with an optimal EBL concentration of 100 nM, resulting in enhanced capacity for photosynthetic carbon assimilation with a notable reduction in the imbalance between the absorption of light and energy used under high-temperature conditions. The addition of 100 nM EBL resulted in a 25.4% increase in cell density when exposed to elevated temperatures for 7 days. In addition, exogenous EBL reduced ROS production and increased the activities of critical antioxidant enzymes. This, in turn, mitigated heat-induced oxidative damage, resulting in advantageous outcomes in terms of cellular development and maintenance.