Some cyanobacteria, like S. platensis, can accumulate PHA under phototrophic and/or mixotrophic growth conditions with acetate (Fig. 1c,d). The PHA granules can be stained by Nile blue A (Ostle and Holt 1982) (Fig. 1a) and they appear as discrete electron transparent granules in the cell cytoplasm (Fig. 1b). Cyanobacteria are of particular interest as PHA producers because of their minimal nutrient requirements for growth and capability of accumulating PHA by oxygenic photosynthesis. Like higher plants, cyanobacteria fix CO2 from the atmosphere and turn it into PHA under nitrogen limiting conditions. Most of the known cyanobacteria that are capable of synthesizing PHA usually accumulate PHAs amounting to less than 6 wt% of their cell dry weight (CDW) (Vincenzini et al. 1990; Stal 1992; Arino et al. 1995; Carr 1996). Spirulina platensis and Synechocystis sp. PCC 6803 have been reported to accumulate a maximum of 6 wt% (Campbell et al. 1982) and 7 wt% (Sudesh et al. 2002) of CDW of PHA under mixotrophic conditions. Poly(3-hydroxybutyrate) [P(3HB)] is the most common type of PHA synthesized by most bacteria. P(3HB) is also the most common type of PHA synthesized by cyanobacteria. Vincenzini et al. (1990) found that when Spirulina was cultivated under photoautotrophic conditions, the P(3HB) content was low (0·3 wt% of CDW), while under mixotrophic growth conditions in the presence of acetate, P(3HB) level amounted to about 3 wt% of CDW.