pH instability from nitrogen metabolism in Chlamydomonas reinhardtii . Photoautotrophic Chlamydomonas reinhardtii cultures were grown in 1.5-L loop air-lift photobioreactors on 5% CO2 (v/v) in air and 0-9% nitrogen from ammonium (0 to 0.027 gN-NH4+/L) with the balance nitrogen as nitrate. The optical density was measured at 550-nm to monitor culture growth and converted to biomass density using the conversion factor of 0.52 gDW/L/OD550 determined experimentally (A). The culture pH was monitored online continuously with samples averaged every 5 minutes to monitor nitrogen utilization (B). The nitrate concentration was measured offline using an ion selective electrode (C).