The metabolic pathways of carbon assimilation and polyhydroxyalkanoate production by Rhodospirillum rubrum in response to different atmospheric fermentation

The purple nonsulfur bacteria, Rhodospirillum rubrum , is recognized as a potential strain for PHAs bioindustrial processes since they can assimilate a broad range of carbon sources, such as syngas, to allow reduction of the production costs. In this study, we comparatively analyzed the biomass and PHA formation behaviors of R . rubrum under 100% CO and 50% CO gas atmosphere and found that pure CO promoted the PHA synthesis (PHA content up to 23.3% of the CDW). Hydrogen addition facilitated the uptake and utilization rates of CO and elevated 3-HV monomers content (molar proportion of 3-HV up to 9.2% in the presence of 50% H 2 ). To elucidate the genetic events culminating in the CO assimilation process, we performed whole transcriptome analysis of R . rubrum grown under 100% CO or 50% CO using RNA sequencing. Transcriptomic analysis indicated different CO 2 assimilation strategy was triggered by the presence of H 2 , where the CBB played a minor role. An increase in BCAA biosynthesis related gene abundance was observed under 50% CO condition. Furthermore, we detected the α-ketoglutarate (αKG) synthase, converting fumarate to αKG linked to the αKG-derived amino acids synthesis, and series of threonine-dependent isoleucine synthesis enzymes were significantly induced. Collectively, our results suggested that those amino acid synthesis pathways represented a key way for carbon assimilation and redox potential maintenance by R . rubrum growth under syngas condition, which could partly replace the PHA production and affect its monomer composition in copolymers. Finally, a fed-batch fermentation of the R . rubrum in a 3-l bioreactor was carried out and proved H 2 addition indeed increased the PHA accumulation rate, yielding 20% ww -1 PHA production within six days.