Illumination systems, devices, and methods for cultivating biomasses. A bioreactor system is operable for growing photosynthetic organisms. The bioreactor system includes a bioreactor and an illumination system. The illumination system includes one more optical waveguides configured to light at least some of a plurality of photosynthetic organisms retained in the bioreactor. In some embodiments, the one or more optical waveguides include a plurality of structures configured to direct light energy from a solar energy collector, and a plurality of artificial light sources, along the interior of the waveguide. In some embodiments, the one more optical waveguides include a plurality of light-diffusing structures configured to guide at least a portion of the light from the solar energy collector and a plurality of artificial light sources directed along the interior of the waveguide, to the exterior of the waveguide.