The role of Fe(III) bioreduction by methanogens in the preservation of organic matter in smectite

Abstract

Organic matter in sediments is largely associated with clay minerals and can be preserved over geological time. However, microbial activity can possibly influence this association and release organic matter from clay minerals via reductive or oxidative dissolution of clay minerals. In this study, the relationship between bioreduction of structural Fe(III) in smectite and organic matter release from smectite structure was investigated. A model organic compound, 12-aminolauric acid (ALA) was intercalated into the interlayer region of an iron-rich smectite (nontronite, NAu-2). Two methanogens: mesophilic Methanosarcina mazei and thermophilic Methanothermobacter thermautotrophicus were selected to reduce structural Fe(III) in ALA-intercalated nontronite. As a comparison, sodium dithionite was used to chemically reduce structural Fe(III) in the same mineral. The results showed that the intercalation of ALA into the nontronite interlayer decreased both the rate and the extent of Fe(III) bioreduction. Furthermore, methanogenesis was more inhibited by the presence of intercalated ALA in the nontronite structure relative to pure nontronite. After the bioreduction, the intercalated ALA was partially released, and the extent of release was positively correlated with the extent of Fe(III) reduction. A low reduction extent of bioreduction (b 30%) resulted in little ALA release, whereas a nearly complete chemical reduction by sodium dithionite released all intercalated ALA. SEM observations and aqueous chemistry data suggested that reductive dissolution was a main mechanism for the observed ALA release. Because naturally prevalent biological reduction is a slow process with a low reduction extent relative to chemical reduction, the results of this study demonstrated that organic matter preserved within smectite structure should not be released by the mechanism of iron reduction.

Mutual Interactions between Reduced Fe-Bearing Clay Minerals and Humic Acids under Dark, Oxygenated Conditions: Hydroxyl Radical Generation and Humic Acid Transformation

“…26 Nontronite contains 21.2% total Fe, 98% of which is Fe(III). 26 Most of the structural Fe in smectite minerals is redoxactive. 27 Reduced montmorillonite and nontronite are capable of producing abundant •OH upon oxygenation, and the yield of • OH is positively correlated with oxidation of structural Fe(II).…”

“…Two smectite minerals, montmorillonite (SWy-2) and nontronite (NAu-2), were chosen in this study. Montmorillonite is widely distributed in soils and sediments, and it contains 2.3% total Fe, 97% of which is Fe­(III) . Nontronite contains 21.2% total Fe, 98% of which is Fe­(III) .…”

“…Montmorillonite is widely distributed in soils and sediments, and it contains 2.3% total Fe, 97% of which is Fe­(III) . Nontronite contains 21.2% total Fe, 98% of which is Fe­(III) . Most of the structural Fe in smectite minerals is redox-active .…”

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

“…This correlation suggests that the expandable internal surface of smectite (i.e., interlayer region) can sequester large amounts of molecular-scaled OM (Keil and Mayer, 2014; Kennedy and Wagner, 2011; however, see discussion in Mahamat Ahmat et al, 2016). Indeed, it has been suggested that enhanced organic matter preservation in sediments is largely due to increased smectite abundance in sedimentary record through geological time (Kennedy et al, 2006): organic matter, once associated with smectite-rich sediments, would be physically protected against microbial oxidation (see details in Keil and Mayer, 2014;Mahamat Ahmat et al, 2016Zhang et al, 2014). However, as reminded above, smectite can also bring reactive iron, which will impact OM preservation: first, iron oxi-hydroxides are electron acceptors used by bacteria during OM remineralization; an input of reactive iron may thus fuel OM-consuming bacteria.…”

Molecular Determination of Organic Adsorption Sites on Smectite during Fe Redox Processes Using ToF-SIMS Analysis

“…The enrichment of Al in NAu-2 after Fe(III) bioreduction was frequently reported in previous studies but was empirically understood as mainly being integrated into the NAu-2 crystal (mainly substitutes the tetrahedral Si sheet). 56,66,67 However, if Al only stayed within the tetrahedral sheet as Si did, both Al and Si signals would show a relatively high intensity in the ToF-SIMS pattern of BR-NAu-2 (i.e., similar to the situation of Figure 1a), which was not observed in either positive or negative ion spectra, suggesting that an Al-rich layer with some organic matter stays on NAu-2 surfaces over the Si layer (similar to Figure 1f). Considering that no external Al and K sources were added to the experiment, thus either a dissolution/recrystallization or a partial dissolution/redeposition process occurred in NAu-2 particles, leading to the formation of an Al-rich layer on basal surfaces.…”