Microalgae are a promising alternative for biofuel production since they can synthesize and accumulate considerable amounts of lipids. In this paper, we propose an interval observer for the estimation of the microalgal lipid quota. The observer is based on a change of coordinate that involves a time varying gain. We introduce a dynamics for the gain, whose trajectory converges toward a predefined optimal value (which maximizes the convergence rate of the observer). The observer performance is illustrated with experimental data of Isochrysis affinis galbana (clone T-iso) cultures.