A reactor includes a tank intended to contain a mass to be treated, for example formed from microorganisms, an assembly that rotates about an axis to ensure stirring of this mass, and a plurality of illumination sources to assist the treatment of this mass. The tank has an inner wall with plates fixed thereto, whose planes are oriented towards the axis of the rotating assembly and parallel to same, so as to prevent the formation of a vortex within the mass under the action of the rotating assembly; the illumination sources are carried by the plates, being encapsulated, with at least the part of the plates that carries them, in a material compatible with the mass treated and in a thickness that allows light to be diffused into the tank. This configuration may be scaled, while keeping the ratio of the illumination surface relative to the tank volume approximately constant.