A dynamic model for the photoautotrophic growth of microalgae in photobioreactor that describes the main variables of the system and allows the precise prediction of the pH in the culture was proposed and validated. The dynamic behavior of the biological system was expressed through a multistate model in continuous-time formulation, based on mass-balance equations and local photosynthetic responses of the anisotropic medium, further associated with a set of algebraic equations that describes the thermodynamic properties of the ammoniacarbon dioxidewater ternary solute system. The global photoautotrophic growth model was validated on experimental data acquired from a torus reactor inoculated with Chlamydomonas reinhardtii cells. The model response was studied in simulation for all identified input variables (dilution rate, incident light intensity, temperature, and flow rates of input gases). (c) 2013 American Institute of Chemical Engineers AIChE J 60: 585-599, 2014