The present invention provides transgenic microalgae expressing at least one exogenous biologically active protein. The protein-expressing microalgae are used for the oral delivery of the biologically active protein to the target organism in its intact and functional form. The exogenous protein, expressed in algae, is characterized by being biologically active, and exerting at least one specific activity having a beneficial effect on the subject consuming the algae. Particularly, the transgenic microalgae are used as animal food for aquatic or land animals welfare or as food supplement for human healthcare.