The mechanism in cyanobacteria cells can increase PHB accumulation under nitrogen limitation (Balaji et al, 2012). The biochemical pathway of the PHB synthesis, cyanobacteria synthesize PHB by substrate as acetyl-CoA. Acetate will be chemically transformed into acetyl-CoA under presence of free CoA; as a result, acetyl-CoA can be transformed into PHB (Poirier 2002; Abed et al. 2009). However, supplying excess carbon can cause accumulation of PHB or by-product (i.e. fatty acid, glycogen). And nitrogen limitation is predicted to block amino acid synthesis in tricarboxylic acid cycle (TCA cycle), resulting in the increase of the substrate of PHB synthesis such as the nicotinamide adenine dinucleotide phosphate (NADPH). As a result, the PHB synthesis increases (Balaji et al, 2012). Thus, NADPH is likely to be response in enhanced PHB accumulation in nitrogen-limiting cells. For an effect of phosphorus limitation on PHB synthesis, it might involve in the increase allocation of carbon-type substrates to the PHB synthetic pathway but does not relate to the enzyme activities in PHB synthesis (Gonzalez-Fernandez and Ballesteros, 2012).