Fundamental aspects of sucrose metabolism reveal a trophic link between <i>Rhodospirillum rubrum</i> and <i>Rhodobacter capsulatus</i>

Purple non-sulfur bacteria (PNSB) are well known to have an exceptional metabolic versatility. However, while the growth of PNSB on sugar-rich streams has been extensively explored, their ability to metabolize sugars is poorly understood. Here, we explore the metabolic mechanisms of sucrose, glucose, and fructose utilization in two phototrophic PNSB, <i>Rhodospirillum rubrum</i> and <i>Rhodobacter capsulatus</i>. Our findings demonstrate distinct carbohydrate assimilation capacities, as well as the use of different metabolic strategies for each species. Moreover, a trophic link was identified between the two species during co-cultivation, resulting from the production of fermentation by-products by <i>Rh. capsulatus</i>, which are then reassimilated by <i>Rs. rubrum</i>. Finally, we demonstrate that the synergy observed between <i>Rs. rubrum</i> and <i>Rh. capsulatus</i> can be successfully scaled up in a photobioreactor system. Our study highlights how fundamental knowledge of metabolism and the establishment of a trophic link between two PNSB species might be useful for the development of biobased economy and resource recovery strategies.IMPORTANCEThe diverse metabolic capacities found in microbial communities expand the possibilities of microbial biotechnological exploitation. In this study, we demonstrate that <i>Rhodospirillum rubrum</i> and <i>Rhodobacter capsulatus</i>, two purple non-sulfur bacteria, adopt different metabolic strategies for sugar assimilation. These differences allow them to benefit from each other, resulting in enhanced carbon yield and productivity compared to pure cultures. We also showed that the trophic link between both species can be scaled up in a photobioreactor system. Understanding these interactions expands the potential for designing microbial consortia optimized for the valorization of carbohydrate-rich waste streams using purple non-sulfur bacteria.