Greening the Production of Indigo Blue Exploiting Light and a Recombinant <i>Synechocystis</i> sp. <scp>PCC6803</scp> Strain Expressing the Enzyme <scp>mFMO</scp>

Cyanobacteria are emerging as interesting cell factories, offering the significant advantage of their in-built photosynthetic machinery, which generates NADPH to support redox biocatalysis. In this study, we assessed the potential of the cyanobacterium Synechocystis sp. PCC6803 in producing the dye indigo by light-driven whole-cell biotransformation using indole as a starting compound. A stable transgenic strain expressing a flavin-containing monooxygenase from Methylophaga aminisulfidivorans (mFMO) was engineered, enabling light-dependent indigo production. Upon optimising conditions, effective biotransformations could be performed, resulting in 112 mg/L indigo (86% conversion of the furnished indole). Additionally, we present a method for the recovery of the secreted dye directly from the growth medium through solid-phase absorption on polyamide nets. Overall, the effectiveness and sustainability of the biotransformation in Synechocystis sp. PCC6803 performed at the laboratory scale provide a strong basis for further exploring the applicability of the process.