Metabolic Regulation in <i>Scenedesmus quadricauda</i> Cells Improved Antioxidant Capacity and Oxygen Release under Nitrous Oxide Stress

To improve the microalgal tolerance to nitrous oxide (N2O) impurity in the flue gas of circulating fluidized bed boilers in power plants, metabolic pathways in Scenedesmus quadricauda cells were regulated to improve antioxidant capacity and oxygen release under N2O stress. The biomass dry weight increased by 13.8% with a N2O concentration of 80 ppm, which effectively increased the photosynthetic electron transfer efficiency and oxygen release rate. Superoxide dismutase activity in microalgal cells increased 2.21-fold, which significantly enhanced cellular antioxidant capacity. N2O restricted glycolysis and the tricarboxylic acid cycle to synthesize more starch from intermediate products of photosynthetic carbon fixation, resulting in lower oxygen consumption in microalgal respiration. N2O decreased the cellular carbon-to-nitrogen ratio and extracellular polymers, but expression of metabolites such as methyl gallate was upregulated to enhance cellular anti-inflammatory and antibacterial capacity. Chlorophyll fluorescence quenching was reduced with an N2O concentration of 300 ppm, resulting in cellular photoinhibition and increased reactive oxygen species, which led to oxidative damage and a 1.45-fold increase in the malondialdehyde content.