Volutin occurs in numerous microorganisms as spherical granules ranging from 0.01 to 2 µm in diameter and displaying the special property of metachromacity when stained with basic dyes such as methylene blue and toluidine blue. The main compounds occurring in the granules from Acinetobacter calcoacetieus were found to be polyphosphate and poly-β-hydroxybutyrate (PHB) together with nucleic acids, metal cations and some protein. Two types of granule have been found based on the predominant compound and have been denoted as P- and L-volutin granules. P-volutin granules are phosphorus-rich while L-volutin granules consist largely of PHB. L-volutin was found to be formed in a medium supplying sufficient nutrients, while P-volutin formed after induction of the phosphorus-accumulating apparatus by phosphorus-starvation. In biological phosphorus removal, PHB is synthesized under anaerobic conditions and degraded during aeration, while phosphorus accumulation follows a complimentary pattern. The exact significance of the formation of the two types of granules is not clear, although their formation under different conditions in Acinetobacter suggests a similar mechanism to that operational in activated sludge.

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