This paper addresses a study of the regulation of the biomass density in a closed microalgal photobioreactor by using a linearizing control approach. The photobioreactor system was programmed to operate in a constant biomass density mode, in order to maintain the culture at the optimal population density and sustaining high biomass production levels. The designed control law uses an input-output linearizing control in an inner loop, a proportional integral Derivative (P.I.D.) regulator has been added to cancel steady errors. In addition, this PID-regulator is supplied with an anti-windup compensator. Thus, the proposed strategy takes the internal dynamics into account and stabilizes them simultaneously with the control of the input-output behaviour of the system. Commonly, the output of the system is the biomass concentration. However, in this study, we propose to consider a new output: the ratio between two state variables, which is equivalent to the total inorganic carbon available quantity per cell. This new quantity is shown to lead to best regulation performance. We illustrate our approach with numerical simulations and show its benefits for the cell cultivations, mainly for ensuring achievement of the culture with regards to classical experiments problems.