Purple sulfur phototrophic bacteria accumulate various storage materials, such as sulfur globules, glycogen or polyhydroxy alkanoates (PHAs) under appropriate conditions. The formation of these materials requires reducing power which might be recovered upon their breakdown. This work aims at the understanding of the metabolism of PHA and its link to the nitrogenase mediated in vivo H-2 evolution in Thiocapsa roseopersicina BBS. The strain could accumulate 30% of the dry cell weight in the form of PHAs. Analysis of the genome sequence revealed the loci involved in PHAs synthesis and degradation. Phylogenetic analysis indicated independent evolution of the anabolic and catabolic proteins. A mutant carrying deleted PHA biosynthesis genes has been created in a host containing nitrogenase but none of the hydrogenases. Determination of the H-2 evolving capacity of the mutant revealed significantly reduced H-2 production in PHA deficient cells. Addition of excess electron sources such as thiosulfate stimulated the H-2 production via multiple effects. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.