Dynamic responses of Rhodobacter capsulatus to light and oxygen cultivated on H2 and CO2: Effects on the productivity of biomass, protein, pigments, coenzyme Q10, and antioxidant capacity

Hydrogenotrophic autotrophic growth of purple non-sulphur bacteria, on H 2 and CO 2 , provides a route to produce proteins and high-value chemicals such as pigments and coenzyme Q10 while avoiding inputs derived from fossil fuels and agriculture. While heterotrophic production of these compounds is established, production under hydrogenotrophic conditions and systematic research on the influence of light and oxygen on these compounds remains underexplored. In this study, Rhodobacter capsulatus was cultivated in a bioreactor under photohydrogenotrophic (with light), chemohydrogenotrophic (with O 2 ), and mixohydrogenotrophic (with both light and O 2 ) conditions. The study focused on volumetric productivity and biomass composition (proteins, pigments, coenzyme Q10, and antioxidant capacity). Highest productivities of biomass (1.20 ± 0.35 gTSS L −1 d −1 ) and protein (0.65 ± 0.14 g protein L −1 d −1 ) were achieved mixohydrogenotrophically. Photohydrogenotrophic conditions exhibited the highest bacteriochlorophyll (27.0 ± 1.8 mg gTSS −1 ) and carotenoid (7.5 ± 0.4 mg gTSS −1 ) content, which was dependent on biomass concentration in the reactor. The elevated pigment content also resulted in the highest antioxidant capacity of the biomass. In contrast, chemohydrogenotrophy resulted in the lowest volumetric productivity (0.61 gTSS L −1 d −1 ) and reduction of pigment content, although coenzyme Q10 concentration remained similar to mixohydrogenotrophic conditions (2.3 to 2.6 mg gTSS −1 ). These findings demonstrate that the hydrogenotrophic metabolism of Rb. capsulatus can be strategically tuned to prioritise volumetric productivity or high-value compounds for microbial food or feed applications.