Photoautotrophic polyhydroxyalkanoate (PHA) accumulation in mixed purple bacteria using formate, carbon dioxide and carbon monoxide

Single-carbon (C1) substrates are promising feedstock alternatives for polyhydroxyalkanoate (PHA) production, as they shift biopolymer production away from costly organic carbons and add value to syngas from industrial waste streams. This field is broadly studied in monocultures, while mixed cultures have received limited attention. This study examined mixed purple phototrophic bacteria (PPB) cultures as a platform to convert C1 substrates (formate, CO₂, CO and H₂) into PHA under different nitrogen and carbon compositions in a two-stage batch operation. In single-substrate experiments, high PHA contents were obtained with CO₂/H₂ as the carbon and electron donors (27 ± 9 wt% PHA in 890 ± 130 mgVSS L^-1), while more moderate PHA levels were achieved with formate (17 ± 2 wt% in 780 ± 80 mgVSS L^-1 biomass) and CO/H₂ (13 ± 3 wt% in 540 ± 50 mgVSS L^-1 biomass). These differences were linked to their respective substrate assimilation pathways, with CO₂ being more readily incorporated through the Calvin-Benson-Bassham cycle. Additionally, supplementing formate with CO₂/H₂ improved PHA yields (32 ± 7 wt% in 890 ± 40 mgVSS L^-1 biomass) by increasing electron loading and maintaining reducing conditions. Overall, the study demonstrates PPB-enriched cultures as competitive photoautotrophic platforms to produce PHA, presenting potential strategies for high-yield biopolymer synthesis from syngas.