Microalgae are microscopic plants that exist in an aquatic environment. They are involved in the production of high value compounds and also have applications in energy production. In this work, the regulation of biomass concenteration in a bioreactor is investigated by using an observer-based backstepping control approach. The process is controlled to operate in a constant biomass concentration mode, in order to maintain the culture at a desired concentration and to sustain high biomass productioin levels. Combined with the backstepping controller, a nonlinear Lipschitz observer is proposed. On the basis of the Droop model, which describes the dynamic behavior of the microalgae process and based on the biomass measurements, the stability of the state error dynamics can be guarnateed. Finally, simulations and comparisons with a PI controller are developed to show the performance of the nonlinear observer-based controller in set point tracking and load rejection in the presence of parameter uncertainties.