Photosynthetic microorganisms have enormous potential to produce fuels and value-added compounds sustainably. Efficient cultivation concepts that enable optimal light and CO₂ supply are necessary for the realization of high cell densities (HCDs), and subsequently for process implementation. We introduce capillary biofilm reactors with a high surface to volume ratio, and thus enhanced light availability, enabling HCDs of photo-autotrophic microorganisms. However, oxygenic photosynthesis leads to O₂ accumulation in such systems, impairing biofilm growth. We combined O₂ producing Synechocystis with O₂ respiring Pseudomonas using proto-cooperation to achieve HCDs of up to 51.8 g_BDW L⁻¹. This concept was coupled to the challenging C-H oxyfunctionalization of cyclohexane to cyclohexanol with a remarkable conversion of >98% and selectivity of 100% (KA oil). High photoautotrophic biocatalyst concentrations were established and resulted in a productivity of 3.76 g_cyclohexanol m⁻² day⁻¹, which was maintained for at least one month.