Efficient utilization of xylose requires CO fixation in Synechococcus elongatus PCC 7942.

Cyanobacteria show great promise as autotrophic hosts for the renewable biosynthesis of useful chemicals from CO and light. While they can efficiently fix CO, cyanobacteria are generally outperformed by heterotrophic production hosts in terms of productivity and titer. Photomixotrophy, or co-utilization of sugars and CO 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 CO. Untargeted metabolomics implied that RuBisCO was a significant bottleneck, especially at ambient CO levels, restricting the supply of lower glycolysis metabolites needed for 2,3-butanediol production. The dependence of the strain on elevated CO levels suggests some practical limitations on how xylose photomixotrophy can be efficiently carried out in S. elongatus.