ROS and Ca2+ signaling involved in important lipid changes of Chlorella pyrenoidosa under nitrogen stress conditions.

Main conclusion: Nitrogen stress altered important lipid parameters and related genes in Chlorella pyrenoidosa via ROS and Ca²⁺ signaling. The mutual interference between ROS and Ca²⁺ signaling was also uncovered. The changed mechanisms of lipid parameters (especially lipid classes and unsaturation of fatty acids) in microalgae are not completely well known under nitrogen stress. Therefore, Chlorella pyrenoidosa was exposed to 0, 0.5, 1 and 1.5 g L^-1 NaNO₃ for 4 days. Then, the physiological and biochemical changes were measured. It was shown that the total lipid contents, neutral lipid ratios as well as their related genes (accD and DGAT) increased obviously while the polar lipid ratios, degrees of unsaturation as well as their related genes (PGP and desC) decreased significantly in nitrogen stress groups. The obvious correlations supported that gene expressions should be the necessary pathways to regulate the lipid changes in C. pyrenoidosa under nitrogen stress. The changes in ROS and Ca²⁺ signaling as well as their significant correlations with corresponding genes and lipid parameters were analyzed. The results suggested that ROS and Ca²⁺ may regulate these gene expressions and lipid changes in C. pyrenoidosa under nitrogen stress conditions. This was verified by the subordinate tests with an ROS inhibitor and calcium reagents. It also uncovered the clues of mutual interference between ROS and Ca²⁺ signaling. To summarize, this study revealed the signaling pathways of important lipid changes in microalgae under N stress.