Effect of phosphate availability on cyanophycin accumulation in Synechocystis sp. PCC 6803 and the production strain BW86

The cyanobacterial wild-type strain Synechocystis sp. PCC 6803 and its engineered strain BW86 were cultivated under defined conditions in photobioreactors to investigate the effect of phosphate availability on cyanophycin accumulation. Cyanophycin, multi-l-arginyl-poly-l-aspartate, can be deployed as amino acid source or can be chemically converted into polyaspartic acid, a biodegradable polymer. In previous studies it was demonstrated that a single point mutation in the PII signaling protein from the Synechocystis wild type is sufficient to unlock the arginine pathway causing an over accumulation of the biopolymer cyanophycin in BW86. One process strategy to evoke cyanophycin synthesis in Synechocystis is nutrient starvation. Therefore, different phosphate concentrations from 17.5 to 175 μM were tested. Progressive phosphate starvation resulted in an increased cyanophycin accumulation. The highest obtained cyanophycin amounts in g cyanophycin per g cell dry mass were 18% and 40% for Synechocystis sp. PCC 6803 wild type and BW86 respectively, demonstrating that phosphate starvation is an effective route for biotechnological cyanophycin production. By evaluating cyanophycin and phosphor quotas per cell dry mass, it was possible to determine the specific required amount of phosphor to accumulate cyanophycin and to initialize stationary growth phase. Phosphor quotas in the range of 4 to 1 mg phosphor per g cell dry mass triggered cyanophycin biosynthesis while Synechocystis entered the stationary phase at phosphor quotas of 1 mg phosphor per g cell dry mass or less. Additionally, light kinetics was determined. Photon flux densities exceeding 46 μmol photons m−2 s− 1 result in a maximum growth rate of 1.32 d− 1.