Unraveling the quorum sensing system of the photosynthetic bacterium Rhodospirillum rubrum S1H under light anaerobic conditions

MELiSSA (Micro-Ecological Life Support System Alternative) is a regenerative recycling system that produces oxygen, water and food for long haul space flights, which includes 5 compartments of microorganisms and plants. Rhodospirillum rubrum S1H colonizes compartment 2 and has a key role in the degradation of the organic waste, metabolizing mainly volatile fatty acids coming from compartment 1. Previous studies reported that continuous culture of the bacterium in the MELiSSA photobioreactor could lead to unwanted thick biofilm formation, shading the culture from the incoming light. It was suggested that cell-to-cell communication, also called quorum sensing (QS), could play a role in this phenomenon. In fact, QS is a common behavior in photosynthetic bacteria where cell-to-cell communication is mainly based on production of acyl homoserine lactones (AHLs) as chemical messenger molecules. In nature, QS helps these bacteria to overcome environmental fluctuations such as light availability, nutrient supply and temperature change. Through QS, bacteria can control expression of target genes and mount a co-operative response like biofilm formation. Our aim was to investigate the unknown QS system of R. rubrum S1H specifically under MELiSSA relevant culture conditions, meaning with light and anaerobic, using acetate as carbon source. Bioinformatics analysis revealed genes homologous to the well-known QS system luxIR, in the genome of R. rubrum. Also a second potential AHLs synthase that belongs to the HdtS family, which is not related to LuxI protein family, was identified. The gene Rru_A3396 (named rruI), coding for a putative AHLs synthase enzyme, was knocked out giving rise to a QS-silent mutant named M68. Phenotypic, proteomic and transcriptomic analysis of R. rubrum S1H (wild type, WT) and M68 showed that the QS system controls the biosynthesis of the photosynthetic pigments such as carotenoids and bacteriochlorophyll, the swimming motility and chemotaxis, the carbon metabolism and adaptation to growth on acetate as carbon source and other cellular functions such as amino acids uptake. Regarding the relationship between QS and biofilm formation, our results showed that QS promotes biofilm formation under a low shear environment. Altogether these results have shown the functionality and key role (direct or indirect) of QS in R. rubrum. Currently the use of Genetically Modified Organisms within the MELiSSA loop is prohibited. Nevertheless, the use of a QS-silent mutant in the MELiSSA loop might be a better choice compared to the addition of external biofilm-disrupting compounds that could have a negative impact on the following bioreactors including compartment 3 where the nitrifying community needs to grow as a biofilm. Further investigations are necessary to characterize the QS-silent mutant growing under bioreactor conditions through analysis of the growth kinetics and metabolic output, i.e. the effluent produced by M68 in comparison to WT to insure that such a QS-silent mutant would still perform adequately in the MELiSSA system.