Metabolic Profile of Chlamydomonas reinhardtii in Alkaline Environment and Role in Sodic Soil Improvement and Rice Seedlings in Response to Alkaline Stress

The western part of north-eastern China is mostly covered by soda soil, which has a significant impact on the growth, breeding and production of crops. The present study investigates the physiological responses and metabolic adaptation of Chlamydomonas reinhardtii P.A.Dangeard (C. reinhardtii) under different types of alkaline stress and the potential application to soda soil improvement and plants in response to alkaline stress. The soda–saline soil was treated with C. reinhardtii for 5 and 120 days, respectively, and the chemical properties of the soil were determined. The microalga was cultured for a period of 12 days, with and without an alkaline medium, and then the metabolic profiles of the microalgae were then analyzed by metabolomics. Results showed that C. reinhardtii exhibited a high level of tolerance to 40 mM NaHCO3, pH 8.3, and pH 10.3 with an increase in incubation time. Moreover, C. reinhardtii has been demonstrated to ameliorate selected physicochemical properties of soda–saline soil by reducing soil pH, electrical conductivity (EC), and the contents of CO32− and HCO3− by 40–100% (w/w, wet biomass) addition for 120 cultivation days, respectively. It was demonstrated that rice seedlings were less affected by alkaline stress in the presence of C. reinhardtii. Furthermore, C. reinhardtii has the capacity to adapt to alkaline environments by regulating key pathways such as cofactor biosynthesis, glycerophospholipid metabolism, amino acid biosynthesis, purine metabolism, carbon metabolism, tropane, piperidine, and pyridine alkaloid biosynthesis. This adaptation is characterized by the accumulation of metabolites, including saccharides, amino acids and organic acids. These interact with high pH, CO32− and HCO3− ions. This study elucidates that C. reinhardtii has the capacity to improve soda–saline soil and respond well to diverse alkaline stresses, attributable to its intricate metabolic profile.