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 photoautotrophic microorganisms. However, oxygenic photosynthesis leads to O-2 accumulation in such systems, impairing biofilm growth. We combined O-2 producing Synechocystis with O-2 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.