Formate-driven photoautotrophic growth and biopolymer storage in anaerobic purple bacteria

Using formate as a CO₂ proxy offers key advantages for autotrophic polyhydroxyalkanoate (PHA) production by lowering substrate costs and avoiding the limitations of gas-fed CO₂ systems. This study aimed to enrich photoautotrophic PHA-accumulating purple phototrophic bacteria (PPB) using formate, with an acetate-fed culture as a photoheterotrophic control. Light promoted growth in the formate-fed reactor, reaching soluble chemical oxygen demand (COD) consumption rates and biomass content of 14 ± 2 mgCOD L^-1h^-1 and 1100 ± 500 mgVSS L^-1 at 50 W m^-2. However, PHA accumulation on formate was independent of light and substrate loading, with yields of 0.04 ± 0.03 mgCOD mgCOD^-1 and PHA contents of 2.1-6.5 wt%. Cereibacter consistently dominated the formate-fed reactor. This study characterizes, for the first time, photoautotrophic growth and PHA accumulation by anaerobic PPB-enriched mixed cultures driven by the direct feeding of formate and identifies challenges linked to enrichment strategies and metabolic acetyl-CoA regulation.