Abstract: Embodiments of the present disclosure describe methods, systems, and compositions for the production of bioplastic from a gaseous substrate containing a carbon source by a two-stage fermentation process. Stage I includes introducing a gaseous substrate comprising CO2 and H2 to a primary bioreactor containing a culture of an acetogenic microorganism, anaerobically fermenting the gaseous substrate under elevated pressure to produce an acid-containing culture medium, and stage II includes aerobically fermenting the acid of the culture medium in a secondary bioreactor containing a culture of a PHA-producing microorganism to produce a polyhydroxyalkanoate (PHA). The culture medium can be suitable for both the first and second stages and enable the primary and secondary bioreactors to be connected in a sustainable continuous system.