Towards high atom economy in whole-cell redox biocatalysis: Up-scaling light-driven cyanobacterial ene-reductions in a flat panel photobioreactor

Light-driven biotransformations in recombinant cyanobacteria benefit from the atom-efficient regeneration of reaction equivalents, such as NADPH, from water and light via oxygenic photosynthesis. However, effective light distribution is hindered by self-shading effects within photosynthetic cells and extended light paths, significantly limiting scalability.

We introduced a flat-panel photobioreactor with a 1-cm optical path length as a scalable system to provide efficient light distribution at high cell densities. Genes encoding five distinct classes of ene-reductases were heterologously expressed in Synechocystis sp. PCC 6803. The resulting strains were evaluated for their efficacy in light-driven ene-reduction reactions across a range of prochiral substrates.

Under standard small-scale reaction conditions, the recombinant strains expressing the ene-reductases OYE3 and mutated TsOYE demonstrated specific activities of up to 150 U g_CDW⁻¹, marking the highest activities recorded for photobiotransformations to date. These strains were chosen for scale-up in a 120-mL flat-panel photobioreactor. The strains achieved volumetric productivities up to 1 g L⁻¹ h⁻¹, surpassing current photosynthesis-driven processes. This setup enabled the conversion of 50 mM 2-methyl maleimide in a fed-batch within approximately 8 hours. The atom economy of this process reached 88% outperforming traditional processes relying on sacrificial cosubstrates such as glucose and formic acid. An E-Factor of 203 indicates that volumetric yield and water consumption for cell cultivation are critical parameters impacting process sustainability.

In summary, we highlight essential factors influencing the sustainability of light-driven whole-cell biotransformations, establishing a robust foundation for future optimization and scale-up efforts in photosynthesis-driven bioproduction