Effects of different nitrogen/phosphorus ratios on the growth and metabolism of microalgae Scenedesmus obliquus cultured in the mixed wastewater from primary settling tank and sludge thickener

Microalgae has great potential in wastewater treatment and resource utilization, but few studies have explored the impact mechanism of pollutants on its biochemical compositions. In this study, the effects of different ratios of nitrogen to phosphorus (N/P) on microalgae Scenedesmus obliquus grown in the mixed wastewater from primary settling tank (PSW) and sludge thickener (STW) were investigated. The optimum N/P ratio for the growth of S. obliquus was determined by nutrient utilization rates and the growth curve. On this basis, the effects of different N/P ratios on its biochemical compositions were explored, and reasons were further analyzed by proteomics. Results showed that at N/P=18, the growth of S. obliquus was optimal (OD680=2.34). And dry weight, lipid production, and chlorophyll-a accumulation were highest (1.70 g·L-1, 0.49 g·L-1, 7.43 mg·L-1), whereas protein production was lowest (0.10 g·L-1). Proteomic results showed that the metabolic activities of S. obliquus cells were the most active at N/P=18. Productivity processes were involved in protein up-regulation, producing more energy. Furthermore, lipid metabolism and fatty acid synthesis pathways were strengthened. The ribosome synthesis activity was significantly enhanced, but the protein content was low, which might attribute to the energy generated by the production activity for other activities.