Biotechnological processes aiming at producing bio-compounds from microalgae have not yet reached economic sustainability because of the high costs of downstream processing. To improve economic profitability, microalgae immobilization and co-culture with nonphotosynthetic organisms exchanging nutrients and metabolites constitutes an advantageous alternative to bulk mono-cultivation because of the higher productivity. However, the equilibrium between a symbiotic and competitive relationship is delicate in bacteria-microalgae co-cultures. The challenge remains in immobilizing and co-culturing them in separate compartments, thereby preserving the advantages of co-culture while eliminating the potential for competition. To achieve this objective, we propose an innovative strategy based on the immobilization of microalgae and bacteria in microstructured hydrogels. A triphasic poly(ethylene oxide) (PEO)-based hydrogel containing dextran and gelatin phases was created and characterized by confocal microscopy and mechanical tests. Several physicochemical parameters were tested for obtaining a core-corona structure that could host separately microalgae and bacteria, thus allowing the co-culture of both microorganisms. The hydrogel was used as a micro-bioreactor allowing immobilized microalgae and bacteria to be co-cultured in separated compartments for at least 7 days and kept alive over 5 weeks despite the degradation of the gelatin phase. Microalgae and bacteria multiplied in their respective compartments until they filled all the available space. The fitness of the microalgae was affected by the immobilization before returning to normal performance. Physiological measurements have qualified a corecorona structure within hydrogels as a good environment for co-culture and immobilization of microalgae, thus validating the proof-of-concept and opening up possibilities for future applications in biotechnology such as the production of biomolecules.