Photobioreactors provide controlled light, CO₂, and mixing for photosynthetic microalgae (e.g. Chlorella, Spirulina) and cyanobacteria. Products include β-carotene, astaxanthin, and biomass. Key design challenges include: photoinhibition at high light intensities, self-shading as cell density increases, and CO₂ mass transfer. Tubular loop, flat panel, and helical designs maximise surface-area-to-volume ratio for light penetration.