Microbial production of poly-D-3-hydroxybutyrate from CO₂

Abstract.

This short review covers the biotechnological aspects of the production of poly-D-3-hydroxybutyric acid, P(3HB), from H₂, O₂ and CO₂ by autotrophic culture of the hydrogen-oxidizing bacterium, Ralstonia eutropha. Considering the efficiency of utilization of a gas mixture as substrate, a practical fermentation process using R. eutropha for the mass production of P(3HB) from CO₂ should be designed on the basis of a recycled-gas, closed-circuit culture system. Also, maintaining the O₂ concentration in a gas phase lower than 6.9% (v/v) is essential to prevent the gas mixture from exploding. Our study, using an explosion-proof fermentation bench plant and a two-stage culture system with a newly designed air-lift fermenter, demonstrated that very high P(3HB) yield and productivity could be obtained while the O₂ concentration was maintained below 6.9%. However, a study on the continuous production of P(3HB) from CO₂ by chemostat culture of R. eutropha revealed that the productivity and content of P(3HB) in the cells was considerably lower than by fed-batch culture. It is deduced that the use of the hydrogen-oxidizing bacterium, Alcaligenes latus, which accumulates P(3HB) even in the exponential growth phase, will be useful for the effective production of P(3HB) from CO₂.