Functions of PHA
Physiologically, PHA was first associated with the sporulation of bacteria . However, PHA is not always associated with sporulation since not all spore formers make the polymer. The polymer if present, was therefore thought to be a ready source of carbon and energy for the energy demanding process of sporulation . To date it is known that PHA is synthesized by a wide range of microorganisms. For many bacteria, the polymer once accumulated, serves as their carbon and energy source during starvation. PHA constitutes an ideal carbon-energy storage material due to its low solubility and high molecular weight which exerts negligible osmotic pressure to the bacterial cell .
Senior and Dawes  had proposed that PHA also serves as a sink of reducing power and could be regarded as a redox regulator within the cell. It was found that, for members of the Azotobacteriaceae, the reductive step of PHA synthesis appeared to serve as an electron sink for the reducing power which accumulated when electron flow through the electron transfer chain was affected as a consequence of an oxygen limitation . On the other hand, in the symbiotic nitrogen fixation process, PHA accumulation has been implicated as both an energy source as well as to serve as a regulatory role controlling the availability of reducing power for the operation of nitrogenase .