Inhibition of photosystems and respiratory complexes enhances lipid biosynthesis via ROS in Chlorella pyrenoidosa...

The suppression of photosystem II, photosystem I, respiratory complex I, and respiratory complex III triggers lipid biosynthesis via increased ROS in Chlorella pyrenoidosa SHOU-1002 under nitrogen limitation. This study investigated the regulation of photosynthesis and respiration in lipid biosynthesis. Chlorella pyrenoidosa SHOU-1002 was analyzed using biochemical and molecular biological approaches under nitrogen limitation, chemical treatments, and RNA interference (RNAi). The results showed that nitrogen limitation redirected carbon flux from chlorophyll, carbohydrates, and proteins to lipids, yielding biodiesel-suitable lipids with a calculated degree of unsaturation ≤ 92.80%. Mechanistically, nitrogen limitation inhibited the activities of photosystem II (PSII), photosystem I (PSI), respiratory complex I (RCI), and respiratory complex III (RCIII) by downregulating the expression of their constituent genes. This suppression led to increased levels of reactive oxygen species (ROS) and subsequent lipid accumulation. RNAi of these complexes similarly enhanced ROS and lipid accumulation. These results support a conclusion that photosynthetic and respiratory inhibition drives ROS-mediated lipid accumulation in C. pyrenoidosa SHOU-1002. This finding enhances our understanding of microalgal lipid biosynthesis under nitrogen limitation and could contribute to the development of the microalgae-biofuel industry through metabolic engineering.