phosphoketolase pathway contributes to carbon metabolism in cyanobacteria central carbon metabolism in cyanobacteria comprises the calvinbensonbassham cbb cycle glycolysis the pentose phosphate pp pathway and the tricarboxylic acid tca cycle redundancy in this complex metabolic network renders the rational engineering of cyanobacterial metabolism for the generation of biomass biofuels and chemicals a challenge here we report the presence of a functional phosphoketolase pathway which splits xylulose5phosphate or fructose6phosphate to acetate precursor acetyl phosphate in an engineered strain of the model cyanobacterium synechocystis glgcxylab in which glycogen synthesis is blocked and xylose catabolism enabled through the introduction of xylose isomerase and xylulokinase we show that this mutant strain is able to metabolise xylose to acetate on nitrogen starvation to see whether acetate production in the mutant is linked to the activity of phosphoketolase we disrupted a putative phosphoketolase gene slr0453 in the glgcxylab strain and monitored metabolic flux using 13c labelling acetate and 2oxoglutarate production was reduced in the light a metabolic flux analysis based on isotopic data suggests that the phosphoketolase pathway metabolises over 30 of the carbon consumed by glgcxylab during photomixotrophic growth on xylose and co2 disruption of the putative phosphoketolase gene in wildtype synechocystis also led to a deficiency in acetate production in the dark indicative of a contribution of the phosphoketolase pathway to heterotrophic metabolism we suggest that the phosphoketolase pathway previously uncharacterized in photosynthetic organisms confers flexibility in energy and carbon metabolism in cyanobacteria and could be exploited to increase the efficiency of cyanobacterial carbon metabolism and photosynthetic productivity