Valorising organic waste for the production of high-value molecules by purple non-sulfur bacteria

Biomethanation has become a frequently used method to treat organic waste, producing biogas, containing around 50% methane and 50% CO2, and digestate which contains ammonium and volatile fatty acids (VFA). Digestate is currently used as a fertiliser due to its high nitrogen content. However, an incomplete transformation of the organic waste into biogas increases the VFA contents, which can be utilised by purple non-sulfur bacteria (PNSB). Through their versatile metabolism, VFA could be valorised into single-cell proteins, vitamin B12, bioplastics in the form of polyhydroxyalkanoates (PHA), carotenoids or hydrogen. Here, two types of cultures of purple bacteria were explored: Monocultures of Rhodospirillum rubrum, the centre of interest for the studies of purple bacteria in our laboratory, and co-cultures containing Rs. rubrum, Rhodobacter capsulatus and Cereibacter sphaeroides. To monitor the ability of these bacteria to assimilate VFA, they were cultivated in a culture medium containing 40% acetate, 20% propionate, 15% butyrate, 5% isobutyrate, 10% valerate and 10% isovalerate as carbon sources (synthetic digestate). In the monocultures of Rs. rubrum, acetate and propionate were assimilated first, (iso)butanoate and valerate next and finally isovaleric acid (Figure 1.A). Furthermore, the resistance to ammonium of the Rs. rubrum monocultures and the co-cultures of the three strains was evaluated, as digestate contains high levels of ammonium. In synthetic digestate medium containing 70 mM or 140 mM of NH4Cl, the growth of Rs. rubrum was not hindered (Figure 1.B). However, statistically significant reduction of growth of Rs. rubrum was observed in culture medium containing 210 mM or 350 mM NH4Cl. A similar trend was observed in the co-cultures which were inhibited by the presence of 350 mM NH4Cl as well. Finally, although the initial inoculants for the co-cultures contained a similar number of bacteria, Rs. rubrum became the most prominent strain by the stationary phase. This was confirmed by the plate-count method, 16S rDNA sequencing and mass spectrometry. In the next phase, digestate from an anaerobic digestion using kitchen waste will be used to assess the effects on the bacterial growth while optimising the production of PHA and carotenoids.