Capillary biofilm reactors (CBRs) are attractive for growing photoautotrophic bacteria as they allow high cell-density cultivation. Here, we evaluated the CBR system's suitability to grow an artificial consortium composed of Synechocystis sp. PCC 6803 and Pseudomonas sp. VBL120. The impact of reactor material, flow rate, pH, O₂, and medium composition on biomass development and long-term biofilm stability at different reactor scales was studied. Silicone was superior over other materials like glass or PVC due to its excellent O₂ permeability. High flow rates of 520 μL min⁻¹ prevented biofilm sloughing in 1 m capillary reactors, leading to a 54% higher biomass dry weight combined with the lowest O₂ concentration inside the reactor compared to standard operating conditions. Further increase in reactor length to 5 m revealed a limitation in trace elements. Increasing trace elements by a factor of five allowed for complete surface coverage with a biomass dry weight of 36.8 g m⁻² and, thus, a successful CBR scale-up by a factor of 25.

Practical application: Cyanobacteria use light energy to upgrade CO₂, thereby holding the potential for carbon-neutral production processes. One of the persisting challenges is low cell density due to light limitations and O₂ accumulation often occurring in established flat panel or tubular photobioreactors. Compared to planktonic cultures, much higher cell densities (factor 10 to 100) can be obtained in cyanobacterial biofilms. The capillary biofilm reactor (CBR) offers good growth conditions for cyanobacterial biofilms, but its applicability has been shown only on the laboratory scale. Here, a first scale-up study based on sizing up was performed, testing the feasibility of this system for large-scale applications. We demonstrate that by optimizing nutrient supply and flow conditions, the system could be enlarged by factor 25 by enhancing the length of the reactor. This reactor concept, combined with cyanobacterial biofilms and numbering up, holds the potential to be applied as a flexible, carbon-neutral production platform for value-added compounds.