Inhibition of nitrate assimilation by ammonium in Chlorella vulgaris. Photoautotrophic Chlorella vulgaris cultures were grown in 1.5-L loop air-lift photobioreactors on 0.0135 gN-NH4+/L with chloride and nitrate as the counter-ions. The reactor was supplemented with 5% CO2 (v/v) in air. The optical density was measured at 550-nm at 3 to 4-hr intervals and was converted to biomass density using a ratio of 0.52 gDW/L/OD550 (A). The pH was measured continuously online and averaged at 30-min intervals to track the proton imbalance due to nitrogen utilization (B).