Lipid biosynthesis mediated by the cyclic adenosine monophosphate (cAMP) signaling pathway in Chlorella pyrenoidosa under salt-induced osmotic stress

An increase in lipid accumulation in algal cells often occurs under salt-induced osmotic stress, but the link between the two is unclear. In this study, the effects of osmotic stress as a kind of biomechanical stress on lipid biosynthesis and the cAMP signaling pathway were investigated using Chlorella pyrenoidosa, and a plausible and novel mechanism by which cAMP regulates lipid accumulation in C. pyrenoidosa under osmotic stress conditions was proposed. The maximal lipid content in C. pyrenoidosa exposed to osmotic stress (453 mOsm/kg) (38.31 ± 0.99%) was increased by 63% compared to that in the control group (32 mOsm/kg) (23.45 ± 0.61%). Under osmotic stress, the levels of lipogenic genes, cAMP (3.24 ± 0.09 nM), and reactive oxygen species (ROS) (285.46 ± 26.51 DCF fluorescence intensity) and the total antioxidant capacity (T-AOC) (12.14 ± 1.03 U mgprot−1) increased. Activated cAMP downregulated Snf1 levels, resulting in enhanced expression of accD. As a result, the metabolic flow of carbon in C. pyrenoidosa was shifted to lipid biosynthesis, resulting in an improvement in biodiesel quality. This study provides novel insight into the role of signaling pathways in modulating lipid accumulation in algal cells under osmotic stress.