Engineering primary metabolism for sustainable production of isoprene in Synechocystis sp. PCC 6803

The imminent climate crisis calls for the development of truly sustainable alternatives in virtually every aspect of human life. The photosynthetic production of chemicals like isoprene, a critical precursor for wide ranging applications, presents a promising alternative to conventional petroleum-based processes. While cyanobacteria can offer the ability for direct CO₂ to isoprene conversion, highly efficient strains are required to reduce costs and resource usage. In this study, we enhanced isoprene productivity in the model cyanobacterium Synechocystis sp. PCC 6803 by engineering the primary metabolism of an isoprene producing strain. We inactivated synthesis of the carbon and energy storage polymer polyhydroxybutyrate (PHB) and increased isoprene synthase copy number in a previously published deletion mutant of PirC (sll0994), a recently discovered regulator involved in the fate of newly fixed carbon during limited nutrient supply. The engineered strains achieved an isoprene production of 278 µg*L^- 1*OD^- 1*h^- 1 and showed elevated productivity under fluctuating nitrogen supply, ultimately resulting in significantly enhanced nitrogen use efficiency for isoprene production. As nitrate represents the ingredient with the highest cost contribution and climate impact in conventionally used cyanobacterial growth media, the presented strategy could reduce cost and environmental footprint without impeding performance for a wide array of compounds sustainably produced with cyanobacteria.