Abstract
Purple non-sulfur bacteria (PNSBs) are well known for their metabolic versatility. Among them, Rhodospirillum rubrum can assimilate a broad range of carbon sources, including volatile fatty acids (VFAs), such as acetate, propionate or butyrate. These carbon sources are gaining increasing interest in bioindustrial processes since they allow reduction of the production costs. Recently, our lab discovered that, after long term cultivation with acetate as unique carbon source, Rs. rubrum got acclimated to this carbon source which resulted in a drastic reduction of the lag phase. This acclimation was characterized by the amplification of the genomic region containing, among others, genes belonging to the ethylmalonyl-CoA (EMC) pathway, which has been demonstrated to be required for acetate assimilation in Rs. rubrum. In this paper, we combined bacterial growth analysis with proteomic (SWATH -Sequential Windowed Acquisition of All Theoretical Fragment Ion Mass Spectra-processing) investigation to better understand the bacterial response to a sudden increase of the light intensity. We compared the impact of suddenly increasing light intensity on the WT strain to that on the newly described acetate-competent strain in the presence of acetate. Contrary to what was observed with the WT strain, we observed that the acetate-competent strain was tolerant to the light stress. Proteomic analysis revealed that increasing light intensity had a significant impact on the photosynthetic apparatus, especially in the wild-type strain cultivated in the presence of acetate and low concentration of HCO3–. This phenomenon was accompanied by a relatively higher abundance of certain stress related proteins. Our results suggested that the production of PHA, but also potentially of branched chain amino acids synthesis, could be part of the mechanism used by Rs. rubrum to adapt to the light stress and the redox imbalance it triggered.
Highlights
Rhodospirillum rubrum (Rs. rubrum) is a purple non-sulfur bacterium (PNSB) belonging to the α-proteobacteria class and is well known for its metabolic versatility performing either autotrophic or heterotrophic metabolism
It is well known that the assimilation of acetate, propionate, butyrate or valerate as a sole source of carbon in anaerobic environments is associated with the consumption of HCO3− (Rinne et al, 1965; Porter and Merrett, 1972; Ehrenreich and Widdel, 1994; Laguna et al, 2011)
Bicarbonate concentration can help bacterial growth start up, since we demonstrated that a reduced lag phase could be obtained by inoculating the culture at a high OD680 nm (Leroy et al, 2015)
Summary
Rhodospirillum rubrum (Rs. rubrum) is a purple non-sulfur bacterium (PNSB) belonging to the α-proteobacteria class and is well known for its metabolic versatility performing either autotrophic or heterotrophic metabolism. In case of sudden light increase, the resulting excess in proton motive force has been shown to increase the reverse flux through NADH dehydrogenase, reducing NAD+ to NADH (Herter et al, 1998; Golomysova et al, 2010) Supporting this redox imbalance hypothesis, a reduction in the lag phase could be obtained using a higher concentration of bicarbonate ions (50 mM) in the medium (De Meur et al, 2018), suggesting that the bicarbonate fixation, which act as an electron sink, helped cells to start growing. Proteomic data showed lower relative abundance of the Ribulose Bisphosphate Carboxylase/Oxygenase (RuBisCO), key enzyme of the CalvinBenson-Bassham- (CBB) cycle in the acetate condition compared to the succinate condition, ruling out involvement of CBB cycle in electron sinking in this case This observation was explained by the fact that the ethylmalonyl-CoA pathway offers another HCO3−-consuming pathway, probably helping to maintain the redox homeostasis. As PHAs have been proposed as an electron sink in purple bacteria and as they represent an important biotechnological application, we analyzed the production of PHAs in response to the sudden increase in the light intensity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
