Exploring the accumulation of poly-β-hydroxybutyrate in the marine cyanobacterium Nostoc sp. BDU 00591: A comprehensive study on extraction, stimulation and characterisation

Abstract

Poly-β-hydroxybutyrate (PHB) is a biopolymer with significant potential in biotechnology and biomedicine due to its biodegradability and biocompatibility nature. Although marine cyanobacteria outperform other resources in terms of water footprint, there are few reports on PHB accumulation in marine cyanobacteria. Thus, the present study aimed to evaluate the PHB production in a marine cyanobacterium, Nostoc sp. BDU 00591, from which 54.8 mg L^-1 PHB yield with a content of 6.8% of dry cell weight (dcw), was obtained on the 21st day of incubation under control laboratory conditions. Subsequently, the impact of varying initial culture density, phosphate (P) starvation, and supplementation with organic carbon sources like citrate and acetate at varied concentrations (0.2–0.6% w/v) were evaluated for PHB accumulation, where 4.3-fold higher content under P-starved conditions and ∼2-fold rise in yield under 0.4% acetate and 0.6% acetate supplementations than the control condition was recorded. Therefore, to simultaneously augment PHB yield and content in Nostoc sp., combined studies were performed incorporating P-starved conditions along with 0.4% acetate and 0.6% citrate separately, where the combination of 0.4% acetate supplementation + P-starved conditions increased PHB yield by 5.6-fold and content by 5.9-fold compared to the control. In addition, the incubation period was reduced to 14 days, resulting in an 8.5-fold higher PHB productivity in the test cyanobacterium. Furthermore, characterisation of the obtained PHB by gas chromatography confirmed the presence of homopolymer β-hydroxybutyrate, while thermal analysis through differential scanning calorimetry revealed the melting and crystallisation temperatures as 173.2 °C and 85.8 °C, respectively, thus exhibiting its flexibility and rigidity in different applications.