[en] Rhodospirillum rubrum S1H is a α-proteobacteria that has the ability to metabolize volatile fatty acids through photoheterotrophic metabolism. R. rubrum was consequently selected by the European Space Agency to colonize the second compartment of its bioregenerative life support system (i.e. MELiSSA loop) and to remove VFA from the effluents produced by the first compartment. Recently we have highlighted new pathways in acetate assimilation in R. rubrum. Here we report, based on bacterial growth and proteomic analysis as well as targeted mutagenesis, a better view of the photoheterotrophic metabolism of butyrate. We also bring new insights in the long-term adaptation of bacterial strains to specific culture conditions.
According to quantitative SWATH-MS proteomic analysis, butyrate is first converted into crotonyl-CoA and then into acetyl-coA. Because R. rubrum is an isocitrate lyase lacking organism, an alternative anaplerotic pathway to the well-known glyoxylate cycle should be used to replenish the TCA cycle in intermediates for biosynthesis. We suggest here that the ethylmalonyl-CoA (EMC) pathway and the valine biosynthesis and degradation (VbSD) pathways could be used to convert butyrate into succinate.
Our bacterial growth analysis showed that assimilation of butyrate depends on the amounts of carbonate supplied. In accordance with the observed upregulation of RuBisCO, CO2 fixation is most probably used as a redox balancing reaction necessary to sustain butyrate assimilation.
Targeted mutagenesis experiments confirmed the crucial role played by the EMC pathway in the assimilation of acetate, and supported the hypothesis of a second anaplerotic pathway in the butyrate assimilation. These experiments also lead us to consider the presence of multiple copies of the targeted gene, especially in strains maintained in the presence of acetate as carbon source, depicting a probable genetic adaptation to constant specific culture conditions.
Altogether our data allows better understanding of metabolic pathways involved in VFA photoassimilation in R. rubrum. Consequently, key metabolites or enzymes could be monitored to ensure the stability and the efficiency of the MELiSSA loop during long term space flight.
This research was supported by the European Space Agency and BELSPO via the PRODEX and GSTP budgets.
