Efficient utilization of xylose requires CO₂ fixation in Synechococcus elongatus PCC 7942

Cyanobacteria show great promise as autotrophic hosts for the renewable biosynthesis of useful chemicals from CO₂ and light. While they can efficiently fix CO₂, cyanobacteria are generally outperformed by heterotrophic production hosts in terms of productivity and titer. Photomixotrophy, or co-utilization of sugars and CO₂ as carbon feedstocks, has been implemented in cyanobacteria to greatly improve productivity and titers of several chemical products. We introduced xylose photomixotrophy to a 2,3-butanediol producing strain of Synechococcus elongatus PCC 7942 and characterized the effect of gene knockouts, changing pathway expression levels, and changing growth conditions on chemical production. Interestingly, 2,3-butanediol production was almost completely inhibited in the absence of added CO₂. Untargeted metabolomics implied that RuBisCO was a significant bottleneck, especially at ambient CO₂ levels, restricting the supply of lower glycolysis metabolites needed for 2,3-butanediol production. The dependence of the strain on elevated CO₂ levels suggests some practical limitations on how xylose photomixotrophy can be efficiently carried out in S. elongatus.