Electron transfer from humic substances to biogenic and abiogenic Fe(III) oxyhydroxide minerals

Microbial humic substance (HS) reduction and subsequent abiotic electron transfer from reduced HS to poorly soluble Fe(III) (oxyhydr)oxides, a process named electron shuttling, significantly increases microbial Fe(III) mineral reduction rates. However, the importance of electron shuttling in nature and notably the electron transfer from HS to biogenic Fe(III) (oxyhydr)oxides have thus far not been determined. In this study, we have quantified the rate and extent of electron transfer from reduced and nonreduced Pahokee Peat humic acids (PPHA) and fresh soil organic matter (SOM) extracts to both synthetic and environmentally relevant biogenic Fe(III) (oxyhydr)oxides. We found that biogenic Fe(III) minerals were reduced faster and to an equal or higher degree than their abiogenic counterparts. Differences were attributed to differences in crystallinity and the association of bacterial biomass with biogenic minerals. Compared to purified PPHA, SOM extract transferred fewer electrons per milligram of carbon and electron transfer was observed only to poorly crystalline ferrihydrite but not to more crystalline goethite. This indicates a difference in redox potential distribution of the redox-active functional groups in extracted SOM relative to the purified PPHA. Our results suggest that HS electron shuttling can also contribute to iron redox processes in environments where biogenic Fe(III) minerals are present.