Behavioral model of nitrogen utilization which facilities photobioreactor pH control. This schematic represents the control elements that give rise to the observed pH response that can be controlled with incremental addition of ammonium. The presence of ammonium inhibits transport of nitrate into the algal cell and results in a net efflux of protons to the culture media. Following depletion of ammonium from the media, nitrate is transported into the cell and reduced to ammonium prior to assimilation, resulting in a net influx of protons from the culture media. Notations: Amt = ammonium transporter, Nit = nitrate transporter, NR = nitrate reductase.