Impaired glycogen synthesis causes metabolic overflow reactions and affects stress responses in the cyanobacterium Synechocystis sp. PCC 6803.

The biosynthesis of glycogen or starch is one of the main strategies developed by living organisms for the intracellular storage of carbon and energy. In phototrophic organisms, such polyglucans accumulate due to carbon fixation during photosynthesis and are used to provide maintenance energy for cell integrity, function and viability in dark periods. Moreover, it is assumed that glycogen enables cyanobacteria to cope with transient starvation conditions, as observed in most micro-organisms. Here, glycogen accumulates when an appropriate carbon source is available in sufficient amounts but growth is inhibited by lack of other nutrients. In this study, the role of glycogen in energy and carbon metabolism of phototrophic cyanobacteria was first analysed via a comparative physiological and metabolic characterization of knockout mutants defective in glycogen synthesis. We first proved the role of glycogen as a respiratory substrate in periods of darkness, the role of glycogen as a reserve to survive starvation periods such as nitrogen depletion and the role of glycogen synthesis as an ameliorator of carbon excess conditions in the model organism Synechocystis sp. PCC 6803. We provide striking new insights into the complex carbon and nitrogen metabolism of non-diazotrophic cyanobacteria: a phenotype of sensitivity to photomixotrophic conditions and of reduced glucose uptake, a non-bleaching phenotype based on an impaired acclimation response to nitrogen depletion and furthermore a phenotype of energy spilling. This study shows that the analysis of deficiencies in glycogen metabolism is a valuable tool for the identification of metabolic regulatory principles and signals.