This study investigates a real-time optimization strategy for micro-algae continuous processes. The objective is to determine the optimal dilution rate maximizing the biomass productivity on the basis of the on-line measurement of biomass concentration and minimum prior process knowledge. Two different micro-algae strains (with different growth rates) are considered in this study: Dunaliela tertiolecta and Isochyris galbana. The extremum seeking control shows good performances in both cases. Practical tuning guidelines can be derived based on the strain growth dynamics.