Ethanol, glycogen and glucosylglycerol represent competing carbon pools in ethanol-producing cells of Synechocystis sp.  PCC 6803 under high-salt conditions. 
Cyanobacteria are photoautotrophic micro-organisms, which are increasingly being used as microbial cell factories to produce, for example, ethanol directly from solar energy and CO2.  Here, we analysed the effects of different salt concentrations on an ethanol-producing strain of Synechocystis sp.  PCC 6803 that overexpresses the pyruvate decarboxylase (pdc) from Zymomonas mobilis and the native alcohol dehydrogenase (adhA).  Moderate salinities of 2 % NaCl had no negative impact on ethanol production, whereas the addition of 4 % NaCl resulted in significantly decreased ethanol yields compared to low-salt conditions.  Proteomic analysis identified a defined set of proteins with increased abundances in ethanol-producing cells.  Among them, we found strong up-regulation of α-1,4 glucan phosphorylase (GlgP, Slr1367) in the producer strain, which consistently resulted in a massive depletion of glycogen pools in these cells regardless of the salinity.  The salt-induced accumulation of the compatible solute glucosylglycerol was not affected by the ethanol production.  Glycogen and probably compatible solutes could present competing pools with respect to organic carbon, explaining the decreased ethanol production at the highest salinity.