Air-grown cultures demonstrate pH control through stoichiometry-balanced nitrogen feed prior to carbon limitation. A photoautotrophic Chlamydomonas reinhardtii culture was grown on balanced growth media under 5% CO2 (v/v) in baffled shake flasks and subjected to a diurnal light cycle with a 16-hr photoperiod. The culture was started on potassium nitrate followed by ammonium nitrate additions (indicated by arrows and given in mgN-NH4+/L) began after 5-hrs of growth with a total of 0.3 gN/L added over the 48-hr growth period. The culture was allowed to grow past the onset of carbon limitation as indicated by the dotted red line (~21 hrs). The optical density was measured at 550-nm to monitor culture growth (A). The pH was measured online to monitor the proton imbalance from nitrogen metabolism (B). The inset of (B) shows the change in proton concentration per ammonium assimilated for the second to fifth NH4NO3 additions prior to carbon limitation.