Background: Poly-β-hydroxybutyrate (PHB) from cyanobacteria is considered to be a potentially viable source for degradable plastic. Yet, to date, only a modest level of research has been reported in this area. PHB has variety of properties such as natural origin, biocompatibility, biodegradability, piezoelectricity, streospecificity, thermoplasticity and optical activity, which make it suitable for various applications in health industry. Methods: This article elucidates comprehensive information and knowledge with regard to several stress conditions (e.g., salt stress, heat and chilling stresses, heavy metal supplementation, limitation of gas exchange and nutrient deficiency) to enhance PHB production from a diazotrophic cyanobacterium, Aulosira fertilissima CCC 444. Results: The results showed that cultures reached a maximum biomass concentration of 631 mg l-1, regardless of initial inoculum levels. However, with an initial inoculum of 80 mg dry cell weight (dcw) l-1, maximum PHB accumulation was achieved under regular light-dark cycles. A. fertilissima supplemented with heavy metals showed enhancement in PHB accumulation up to 18.7% of dcw in 1.0 mg l-1 Ni-supplemented medium for 7 days, followed by 17.6% (dcw) in 1.5 mg l-1 Cu-supplemented medium. NaCl stress, heat and chilling stresses and deficiencies of potassium, calcium, and magnesium had negative effects on PHB accumulation. Interestingly, supplementation of 0.5% acetate at the initiation of gas exchange limitation boosted the accumulation of PHB to 48.7% (dcw) on 14 days of incubation, a value 7-fold higher compared to the control.
Conclusion: Gas exchange limitation with carbon supplementation found to be an appropriate condition for enhanced PHB accumulation in cyanobacteria. Thus, using cyanobacteria for PHB production is one of the most promising ecofriendly ways as these are oxygen evolving photoautotrophic organisms; require minimal inorganic nutrients for growth and due to aquatic in nature, cyanobacteria do not compete for arable land for their cultivation. Currently, intensive research is essential in this area for ingenious utilization of cyanobacterial biomass to produce environmentally affordable bioplastic.