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 NO 3 − fluxes on the biomass production, bioenergy production, and CO 2 fixation. The results show that decreasing the NO 3 − 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 NO 3 − 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 NO 3 − 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 NO 3 − flux level, 4.18 mM/d. In addition, the peak lipid yield, carbohydrate yield, and CO 2 fixation rate were recorded at 482 and 660 mg/L/d, and 3.9 g/L/d, respectively, under the same NO 3 − flux level (4.18 mM/d). Although cultivating TCL-1 under the 4.18 mM/d NO 3 − flux level exhibits great biomass production, CO 2 fixation, and bioenergy production potential, different procedures (the NO 3 − flux decreasing from the N-replete level and the NO 3 − flux increasing from the N-deprived level) could influence the final quantity of bioenergy production and CO 2 fixation. The NO 3 − flux, NO 3 − concentration in the bioreactor, and the NO 3 − flux variation routes are all important factors to determine the nitrogen availability of TCL-1 in continuous cultivation.