Naturally-occurring bacteriogenic iron oxides as an unaccounted for component of aqueous carbon cycling

Iron (oxyhydr)oxides are highly reactive, environmentally ubiquitous organic carbon (OC) sinks that play a critical role in terrestrial and aquatic carbon cycling. However, the current understanding of environmental iron (Fe) oxide affinity for OC is limited primarily to synthetic or abiogenic oxides. Bacteriogenic Fe oxides (BIOs), commonly observed in quiescent waterways and inundated rhizospheres, are physically and chemically unique from synthetic analogues and possess high reactivity for oxyanions of arsenic and chromium, suggesting BIOs may be an effective contributor to OC sequestration. Using sorption and desorption batch reactions, paired with Fourier transform ion cyclotron resonance mass spectrometry and Fourier transform infrared spectroscopy, this work demonstrates (1) the carbon sequestration potential of BIOs, and (2) distinct molecular fractionation at the BIO interface. Bacteriogenic Fe oxides were found to sorb OC to an extent equivalent to 2-line ferrihydrite (when normalized by surface area) and sorption complexes are chemically resilient to desorption, indicating BIOs may significantly contribute to aqueous OC cycling. Interestingly, high-resolution spectroscopic analysis provides direct evidence for the preferential sorption of carboxylic-containing species and concurrent release of aliphatic groups unassociated with carboxylic acids, providing insight into exchange processes between BIOs and surrounding dissolved organic matter. Therefore, we conclude that bacteriogenic iron oxides are critical, yet under-studied, participants in the cycling of aqueous OC that may enhance current carbon cycling models and act as a buffer from organic contaminants.