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

Anoxygenic phototrophs oxidize both organic and inorganic electron donors for phototrophic fixation of CO without O generation, playing important roles in global carbon cycles. However, it remains unknown whether and how they can fix CO using zero-valent iron (ZVI) as solid-phase electron donor. This study investigated the feasibility of Fe-driven CO 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 CO fixation increased by up to 15%. The ZVI-driven CO fixation was attributed to H-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 CO fixation through direct transfer. However, ZVI cannot enable dark CO fixation. These findings highlighted the potential of ZVI as a solid electron donor for enhanced microbial CO fixation.