Bioprinting of Nanocellulose Hydrogels for Photobiocatalysis Under Continuous Flow

Photosynthetic microorganisms are capable of oxygenic photosynthesis, delivering both oxygen and cofactors to drive enzymatic redox reactions. However, their dependence on visible light limits the tolerable cell densities to achieve high reaction rates. Immobilizing cells within a matrix often increases biocatalyst productivity while allowing facile retainment but also creates mass transfer limitations across the solid-liquid interface. Herein, we address these challenges and present the immobilization of recombinant cyanobacteria in 3D-printed hydrogels of varying geometries. In particular, whole cells of the cyanobacterium Synechocystis sp. PCC 6803, engineered to express the gene of the ene-reductase YqjM, were implemented in biocompatible hydrogels made out of nanofibrillated cellulose and alginate. The hydrogels were 3D-printed via extrusion into different geometries to alleviate light and mass transfer limitations and were applied for the reduction of prochiral 2-methylmaleimide to (R)-2-methylsuccinimide. The obtained reactors exhibit high mechanical stability (620 kPa), efficient flow and mass transfer characteristics, high specific surface area (up to 2129 mm² g^-1), and retention times favorable to achieve high product formation. (R)-2-methylsuccinimide was obtained with a space-time yield of 0.28 g L^-1 h^-1 and a high enantiomeric purity (>99%). The highly atom-efficient chemical process (88%) using only water to provide electrons for NADPH regeneration could be upscaled and can potentially be operated in extended periods to reduce wastewater associated with cell cultivation. Overall, 3D printing of photosynthetic microorganisms embedded in a hydrogel matrix holds significant promise for advancing the development of whole-cell solid-state photosynthetic cell factories. These are important steps toward improved reactor designs and higher efficiencies to improve crucial redox biotransformations.