According to embodiments of the present invention, a bioreactor for growing photoautotrophic organisms is provided. The bioreactor includes a vessel configured to receive the photoautotrophic organisms, the vessel having a longitudinal axis, which a circumferential wall extends around said axis, wherein said circumferential wall is translucent so as to enable light to enter the vessel from the outside for acting on the photoautotrophic organisms, wherein a device for providing an uneven distribution of the light intensity within said vessel along said axis is provided.