Effects of nitrogen availability on the bioenergy production potential and CO₂ fixation of Thermosynechococcus CL-1 under continuous cultivation

Nitrogen availability directly affects the microalgal metabolism. A thermophilic cyanobacterium named Thermosynechococcus CL-1 was cultivated in a continuous system to evaluate the effects of NO₃^- fluxes on the biomass production, bioenergy production, and CO₂ fixation. The results show that decreasing the NO₃^- flux to a N-deprived level (1.01 mM/d) enhances the carbohydrate content in TCL-1 to 45%, accompanied by a decrease in the lipid content. However, increasing the NO₃^- flux from a N-deprived level (1.01 mM/d) decreases the carbohydrate content dramatically, accompanied by a slight increase in the lipid content. No matter whether the NO₃^- flux decreases from a N-replete level (8.35 mM/d) to a N-deprived one (1.01 mM/d), or increases from a N-deprived level to higher one, the peak biomass yield occurs at the same NO₃^- flux level, 4.18 mM/d. In addition, the peak lipid yield, carbohydrate yield, and CO₂ fixation rate were recorded at 482 and 660 mg/L/d, and 3.9 g/L/d, respectively, under the same NO₃^- flux level (4.18 mM/d). Although cultivating TCL-1 under the 4.18 mM/d NO₃^- flux level exhibits great biomass production, CO₂ fixation, and bioenergy production potential, different procedures (the NO₃^- flux decreasing from the N-replete level and the NO₃^- flux increasing from the N-deprived level) could influence the final quantity of bioenergy production and CO₂ fixation. The NO₃^- flux, NO₃^- concentration in the bioreactor, and the NO₃^- flux variation routes are all important factors to determine the nitrogen availability of TCL-1 in continuous cultivation.