The ability of the phototrophic bacterium Rhodopseudomonas palustris SP5212 to produce polyhydroxyalkanoates (PHAs), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] in particular was, assessed in acetate medium supplemented with hydroxybutyrate and valerate as cosubstrates. The isolate accumulated the polymer accounting for some 49.06% and 30% of cell dry weight when grown in hydroxybutyrate and valerate, respectively. PHA accumulation as well as 3HV monomer incorporation (30？mol%) was maximum at 0.1% hydroxybutyrate, while valerate at 0.1% and 0.3% was suitable for total polymer accumulation and 3HV monomer incorporation, respectively. Cosupplementation of hydroxybutyrate and valerate in the ratio of 3？:？1 led to the accumulation of PHA accounting for 54% of cell dry weight, which contained more than 50？mol% of 3HV monomer. Moreover, the biphasic cultivation conditions with hydroxybutyrate as cosubstrate have improved the quality as well as quantity of the accumulated copolymer significantly. 1. Introduction Polyhydroxyalkanoates (PHAs) represent a unique class of environmentally biodegradable polymers of commercial importance. They are synthesized and accumulated as intracellular carbon and energy rich reserve materials by a wide variety of bacteria when grown under conditions of carbon excess and at least one of the nutrients becomes limiting [1, 2]. PHAs serve as a sink for reducing equivalents depending on the organisms and the physiological conditions of the cells. Accumulated PHAs also play a significant role in the survival of the producer microorganisms under conditions of environmental stress such as osmotic pressure, desiccation, and UV-irradiation [3]. Moreover, they have received increased attention as alternatives to conventional hydrocarbon based thermoplastics mainly because of their material properties, biodegradability, and biocompatibility. The incorporation of 3-hydroxyvalerate (3HV monomer) into the polymer of 3-hydroxybutyrate (3HB) can result in the synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] with improved thermoplastic properties that are more suitable for commercial application [4]. It is, therefore, apparent that the PHA materials can be tailored to specific applications by varying their chemical structure [5], which could be achieved by variation of producing organisms, carbon sources, cosubstrates and cultural conditions. Altogether, 150 hydroxyalkanoic acids have been identified as constituents of PHAs [6], and these subunits of PHA have been broadly subdivided as short chain length
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