Metabolic pathways of Chlorella sp. cells induced by exogenous spermidine against nitric oxide damage from coal-fired flue gas

To protect microalgae that are used for photosynthetic CO2 fixation against high NO concentrations from coal-fired flue gas, 500 mu M exogenous spermidine was added into Chlorella sp. solution resulting in an elevation of biomass yield by 30.5% under 327 ppm NO. Metabolomics, proteomics and enzyme activities were analyzed, revealing three effects of spermidine on Chlorella sp. resistance to NO stress. First, spermidine induced NO fixation in amino acids and their metabolites, mainly in form of 5-oxoproline (1.51-fold), which occurred through intracellular conversion reactions between citrulline and arginine. Accordingly, cellular respiration was strengthened along with a weakened NO inhibition, which enhanced active transport with ATP consumption. Second, spermidine guarded Chlorella sp. against peroxidation damage by improving activity of antioxidant enzymes. Finally, it protected the photosynthetic system of Chlorella sp. by increasing abundance of related enzymes to enhance carbon fixation. Thus exogenous spermidine improved biomass production against NO environment.