Organic Acid Association Promotes Long-Distance Electron Transfer by Iron

Iron-mediated electron transfer links elemental cycling, microbial respiration, and contaminant transformation, but Fe(III) precipitation at circumneutral pH restricts the mobility of Fe-associated reducing equivalents. Here, we show that low-molecular-weight organic acids promote Fe-associated reducing-equivalent transport by suppressing localized Fe precipitation. Using a spatially separated reactor inoculated with Shewanella oneidensis MR-1, we quantified electron transfer by mediated electrochemical oxidation and visualized its distance using Ag+ trapping and photothermal imaging. Citric acid (CA) enabled transfer over 9.5 ± 0.4 mm within 24 h and delivered 0.06 ± 0.01 μmol e- across 1 cm over 72 h, whereas transfer was negligible without CA. Spectroscopic and Fe-speciation analyses indicated that CA limited precipitation and maintained a diffusible, redox-active Fe fraction. The effect extended to multiple organic acids, with efficiency decreasing from citric to tartaric, oxalic, and acetic acid. Upon aeration, Fe-CA increased remote hydroxyl radical production 5.5-fold and phenol degradation 2.4-fold relative to Fe-only. These results demonstrate that organic acids extend the spatial reach of microbially derived extracellular electrons by maintaining mobile, redox-active Fe, thereby supporting remote redox coupling between spatially separated reducing and oxidizing zones and potentially influencing contaminant transformation in subsurface environments.