Poly-β-hydroxybutyrate (PHB) production by Synechocystis PCC6803 from CO2: Model development

The biosynthesis of poly-beta-hydroxybutyrate (PHB) by bioconversion of CO2 is a sustainable alternative to the non-renewable, petroleum-based polymer production. Indeed, the PHB production by conversion of CO2 contributes to the reduction of the greenhouse-gas concentration in the atmosphere. A kinetic dynamic model of PHB production by autotrophic cultures of Synechocystis PCC6803 was proposed and developed by means of the biochemical networks simulator COPASI. Two classes of cells were assumed to be present in the broth: growing cells, PHB producing cells. The model included the two classes of cells and their nitrogen and phosphate internal quota. The dynamics of the cell growth and PHB production were described taking into account: cellular growth rate; lysis rate; nitrate and phosphate utilization rate; PHB production rate. The assessment of the kinetic parameters and of the yields (model calibration) was carried out by the regression of experimental data. Tests were carried out in photo-bioreactors under dynamic light system (light/dark cycle) using media characterized by initial nitrate concentration ranging between 0 and 1.5 g/L. The developed model was validated with respect to independent experimental set. The proposed model successfully reproduced the experimental data (cell concentration, nitrogen and phosphate concentration and PHB content): the square correlation coefficient of the investigated variable concentrations ranged between 0.81 and 0.99. Parameter sensitivity analysis was carried out to assess the role of the selected parameters on cell growth and PHB accumulation. The dynamics of cellular growth were not significantly affected by a +/- 20% variation of maximum specific growth rate, of velocity of conversion to PHB producing cells, and of maximum uptake rate of nitrate. The PHB accumulation dynamics were particularly sensitive to the variation of the value of the investigated parameters. The proposed model may support the design and the optimization of a PHB production process by means of autotrophic cultures.