Carbon monoxide fermentation to bioplastic: the effect of substrate adaptation on Rhodospirillum rubrum

Abstract

Rhodospirillum rubrum is a gram-negative bacterium that naturally takes advantage of CO and which, in the presence of acetate, accumulates carbon and energy units as polyhydroxybutyrate (PHB). Since the conversion of CO depends on a large protein membrane complex that is expressed after the exposure to carbon monoxide, this study presents the effects of a CO-based acclimation in R. rubrum on the growth trend and on the production of PHB. The strain was cultured in two consecutive fermentation cycles on 15% of CO, and the behaviour of this species, in the presence of acetate or a reducing sugar, such as fructose, was compared. The exposure of R. rubrum to CO during the first adaptation phase led to the development of a metabolically active population characterised by a greater biomass growth. The supply of fructose ensured a shorter lag-phase and a higher biomass titre, but it also determined a decrease in the biopolymer accumulation. However, R. rubrum showed the best carbon utilisation in the absence of fructose, with a growth molar yield of 48 mg mol−1, compared to the 12 mg mol−1 obtained for fructose feeding.