Efficient conversion of acetate or glucose to poly(3-hydroxybutyrate) and glycogen by the single-stage photoheterotrophic cultivation of cyanobacterium Chroococcus hansgirgi TISTR 8561

Microbial poly(3-hydroxybutyrate) (PHB) has been used as bioplastic, while the biopolymer of glucose (glycogen) can be used as efficient substrate for the generation of biofuels. Substantial production of PHB and glycogen by heterotrophic microbes has been constrained by the limited resources of organic substrates. Hence, the heterotrophic cultivation that efficiently converts an organic substrate to the two biopolymers is desirable. However, limited research has been conducted to examine the conversion efficiency of an organic substrate to biopolymers by photoheterotrophic microbes. In this study, the single-stage cultivation of a photoheterotrophic cyanobacterium (Chroococcus hansgirgi TISTR 8561) which accumulates PHB and glycogen was optimized and evaluated for the mass conversion efficiency (CE) of an organic substrate to the two biopolymers. The cells were cultured in BG11 trace element medium with addition of only a single specific organic substrate under light to enable both cellular oxygenic photosynthesis and heterotrophic metabolism, which were evidenced by the oxygen gas evolution and the organic substrate utilization of the cells, respectively. Through optimization of the culture conditions, organic substrate supply, and cultivation time, the mass CE of glucose to both PHB and glycogen of 51 ± 15% (w/w) was obtained, while the mass CE of acetate to these two biopolymers was 71 ± 16% (w/w). These optimized cultures exhibited CE of glucose or acetate to both PHB and glycogen at the higher levels than those of previous reports in photoheterotrophic microbes. This single-stage photoheterotrophic cultivation may be applied for production of biopolymers by other photoheterotrophic microbes.