Photohydrogenotrophic Cultivation of Purple Non‐Sulfur Bacteria in an Open Bioreactor: Enhanced Selectivity Through Light Cycling and Ammonium Limitation

ABSTRACT

This work pioneers in photohydrogenotrophic cultivation of Rhodobacter capsulatus in an open, non‐axenic bubble column reactor to maximize biomass productivity and selectivity for purple non‐sulphur bacteria. Using full‐spectrum light and non‐limiting ammonium levels, biomass productivity reached up to 0.98 gTSS L ⁻¹  d ⁻¹ . To enhance microbial selectivity, two operational strategies were tested: (i) alternating of full light spectrum exposure and infrared light which increased pigment production (15 ± 1 mg bacteriochlorophyll gVSS ⁻¹ and 4.7 ± 0.5 mg carotenoids gVSS ⁻¹ ) and protein content (0.78 ± 0.22 g protein gVSS ⁻¹ ) while minimizing microalgae content and (ii) ammonium limitation that stimulated the biological nitrogen fixation capability of purple non‐sulphur bacteria, resulting in the highest purple non‐sulfur bacteria abundance, representing up to 96% of the prokaryotic community, without compromising on productivity. These results demonstrate the feasibility of open culture photohydrogenotrophic cultivation, offering a stepstone to scalable and ecologically aligned alternative for microbial protein production for food and biobased applications.