Dinitrogen fixation by open purple non-sulfur bacteria cultures for protein production: Diazotrophy boosts photoheterotrophic uptake rates

Purple non-sulfur bacteria (PNSB) offer a sustainable alternative to current inefficient protein production systems thanks to their high yields. This study explored the potential of specialized diazotrophic PNSB open cultures for protein production, benchmarking their performance against ammonium-grown PNSB and other diazotrophs. While diazotrophic yields (0.85-0.93 gCOD_biomass·gCOD_substrate^-1; COD being chemical oxygen demand) were slightly lower than non-diazotrophic (∼1.0), they were over double those of heterotrophic-diazotrophic rhizobacteria, with full N recovery as biomass (∼1.0 gN_biomass·gN_fixed^-1). Photoheterotrophic-diazotrophic uptake rates were the fastest ever reported for PNSB and any other diazotroph (e.g., 5.20 ± 0.83 vs. 2.64 ± 0.34 gCOD_substrate·gCOD_biomass^-1·d^-1 for PNSB on NH₄⁺). Optimal rates required high light intensities, aligning with diazotrophic energy demands. Photoheterotrophic-diazotrophic conditions were highly selective, enriching a specialized Rhodopseudomonas palustris strain. Biomass protein contents and essential amino acid metrics confirmed nutritional suitability for humans. This work lays the background for exploiting PNSB's potential to address global protein demands through sustainable nitrogen fixation.