Zero-valent iron as an alternative electron donor for extracellular electron uptake linked to CO₂ fixation in Rhodopseudomonas palustris

Anoxygenic phototrophs oxidize both organic and inorganic electron donors for phototrophic fixation of CO₂ without O₂ generation, playing important roles in global carbon cycles. However, it remains unknown whether and how they can fix CO₂ using zero-valent iron (ZVI) as solid-phase electron donor. This study investigated the feasibility of Fe⁰-driven CO₂ fixation by the model bacteria Rhodopseudomonas palustris using nano- (nZVI) and micron-ZVI (mZVI). The results showed that ZVI could empower photoautotrophic and photoheterotrophic growth of R. palustris through iron biocorrosion, with CO₂ fixation increased by up to 15%. The ZVI-driven CO₂ fixation was attributed to H₂-mediated extracellular electron uptake (EEU) and Fe(II) oxidation. The genes encoding EEU-associated pathways were up-regulated in the presence of ZVI, indicating that ZVI promoted CO₂ fixation through direct transfer. However, ZVI cannot enable dark CO₂ fixation. These findings highlighted the potential of ZVI as a solid electron donor for enhanced microbial CO₂ fixation.