The aerobic metabolism of a mixture of acetate and starch was studied with main emphasis on their interaction and the effect on their storage as PHB and glycogen, respectively. Pulse feeding strongly increased the storage of both substrates; however, the presence of starch decreased PHB storage whereas the presence of acetate did not affect glycogen storage. Indeed, C-13 NMR isotopomer analysis suggested an increase of acetate utilization towards TCA cycle, due to an increased request of ATP production for glycogen biosynthesis regulated by ADP-GlcPPase. This in turn influenced the partition flux for pyruvate synthesis between TCA cataplerosis and glyoxylate shunt. The corresponding reduction of PHB synthesis was in agreement with the competition for HS-CoA between KGDH activity and acetyl-CoA for PHB synthesis pathway. As a practical consequence, bioprocesses for PHA production from volatile fatty acids could be negatively affected from other carbon sources, such as unfermented carbohydrates. (C) 2012 Elsevier Ltd. All rights reserved.