The control of microalgae cultivation is approached from passivity-based control perspective. The proposed controller solves set point tracking and stabilizes the control loop by the passivity properties. Moreover, in order to obtain the process state variables from the measurement output, which are used in the control law, a nonlinear Lipschitz observer is proposed. The design of the nonlinear controller and the observer is based on the Droop model, describing the dynamic behaviour of microalgae process in a practical way. Finally, the performance of the nonlinear observer-based controller is indicated by simulation.