Enhanced polyhydroxybutyrate (PHB) accumulation in heterotrophically grown Arthrospira platensis under nitrogen deprivation

Abstract

The non-heterocystous cyanobacterium Arthrospira platensis was investigated for the conditions under which high accumulation of polyhydroxybutyrate (PHB) occurred. Cells grown photoautotrophically under nitrogen-deprived conditions had high accumulation of PHB whose granules were clearly visualized using Nile-Red staining. A further increase of PHB was observed in cells grown photoheterotrophically for 3 days under nitrogen-deprived condition with 0.50%(w/v) acetate supplementation (−N +Act) showing the maximum PHB contents of 19.2 ± 0.5%(w/w DW). This was significantly higher than that in cells grown under the same condition but with either 0.1%(w/v) butyrate, or 0.75%(w/v) glucose, or 0.1%(w/v) propionate supplementation (P < 0.05). RT-PCR analysis revealed that photoheterotrophically grown cells under −N+ Act condition had a significant increase of phaB and phaC transcript levels (P < 0.05) compared with those supplemented with other organic carbons. The representative FTIR spectra of the extracted polymer from A. platensis showed prominent peaks at 1721 cm−1 and 1279 cm−1 for the carbonyl (C=O) stretching of the ester group and asymmetric C–O–C stretching vibration, respectively. This spectral pattern is comparable with that of the standard PHB spectra, thus confirming the chemical nature of the extract. The overall results demonstrated that the enhanced PHB accumulation in A. platensis was attributed to the increased expression of phaB and phaC responsible for increased PHB biosynthesis when cells were grown photoheterotrophically in nitrogen-deprived medium containing organic carbon compounds.