pH control maintained during nitrogen limitation in Chlamydomonas reinhardtii . A photoautotrophic Chlamydomonas reinhardtii culture was grown on balanced growth media under 5% CO2 (v/v) in air in a trickle film bioreactor and subjected to a diurnal light cycle with a 16-hr photoperiod. The culture was initially started on potassium nitrate until ammonium nitrate additions began after 10 hours of growth with a total of 0.6 gN/L added over the 75-hr growth period. The pH was measured online to monitor the proton imbalance from nitrogen metabolism before and after nitrogen depletion (indicated by red line at ~25 hrs) with black arrows indicating the time of each NH4NO3 addition, and the red arrow indicating the addition of NH4NO3 supplemented with a balance of non-nitrogen nutrient salts (A). The optical density was measured at 550-nm and converted to biomass density using the conversion factor of 0.51 gDW/L/OD550 determined experimentally (B). Also shown in (B inset) is the change in proton concentration per utilized nitrate.