A sun-tracking light distributor for a photosynthetic culture in an aqueous liquid, comprising: two light distribution walls made of a transparent material, creating an elongated V-shaped channel adapted to receive the sunlight and to be partly immersed in the aqueous liquid, the sun-tracking light distributor has a center of buoyancy, a vertical passing through the center of buoyancy defines an axis of flotation, and a pivot axis of the sun-tracking light distributor is offset relative to the axis of flotation. Owing to the offset pivot axis, the orientation of the light distributor is changed by varying the level of aqueous liquid, thus allowing a tracking of the sun's altitude.