Microalgae are photosynthetic organism that are cultivated in photobioreactors. One of the most important factors affecting the growth rate of these organisms is the light intensity, where lower intensity drives photolimitation, where the amount of photons that they receive is not enough to trigger the photosynthetic machinery, and, on the other side higher intensity induces damage in key proteins of the photosynthetic center of the cells. Due to mixing of the photobioreactor, the growth of the cells is affected by the hydrodynamics of the photobioreactor. We use the mechanistic model of Han for photoinhibition and photolimitation, and we compute the average in time of the growth rate for a single cell for any continuous light signal, we analyze these computations, and we illustrate how mixing influences the average growth rate for simply periodic functions. Then, we discuss the challenge of computing the growth rate in photobioreactors simulated via computational fluid dynamics, considering a single-phase and incompressible fluid, and we finish our analysis presenting the case of the raceway pond.