pH “toxicity” from nitrogen metabolism in Chlorella vulgaris . Photoautotrophic Chlorella vulgaris cultures were grown in shake flasks with 5% CO2 (v/v) in air on 0.3 gN/L with 0 to 36% as ammonium (0 to 0.108 gN-NH4+/L) and the remaining as nitrate (0.3 to 0.192 gN-NO3-/L). The highest ammonium concentration corresponded to the nitrogen composition of our stoichiometrically-balanced photoautotrophic growth media. The optical density was measured at 550-nm to monitor culture growth as a proxy for dry weight (A). Offline pH samples were taken at 3-hr intervals, degassed, and measured in anticipation of a proton imbalance due to nitrogen metabolism (B).