Fed-batch addition of nitrogen for pH control in Chlorella vulgaris . A photoautotrophic Chlorella vulgaris culture was grown in a trickle film photobioreactor on balanced growth media (0.3 gN/L at 36%N-NH4+ and balance as nitrate) under 5% CO2 (v/v) in air and subjected to a diurnal light cycle with a 16-hr photoperiod. The culture was initially grown on potassium nitrate with five ammonium nitrate additions made over the course of the growth period. Online pH was monitored continuously and averaged every minute to demonstrate pH fluctuations due to the ammonium nitrate additions indicated by the arrows and the concentration of ammonium in the reactor following each feeding (A). The red arrow signifies the unexpected pH response following the fifth ammonium nitrate addition. The optical density was measured at 550-nm to monitor biomass growth and converted to culture density using the biological conversion factor of 0.53 gDW/L/OD550 determined experimentally (B). The inset of (B) shows the relative change in proton concentration during ammonium metabolism following the first four NH4NO3 additions.