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The formation of Fe(III) oxides in natural environments occurs in the\udpresence of natural organic matter (OM), resulting in the formation of\udOM–mineral complexes that form through adsorption or coprecipitation\udprocesses. Thus, microbial Fe(III) reduction in natural environments most\udoften occurs in the presence of OM–mineral complexes rather than pure Fe(III)\udminerals. This study investigated to what extent does the content of adsorbed or\udcoprecipitated OM on ferrihydrite influence the rate of Fe(III) reduction by\udShewanella oneidensis MR-1, a model Fe(III)-reducing microorganism,\udin comparison to a microbial consortium extracted from the acidic, Fe-rich\udSchlöppnerbrunnen fen. We found that increased OM content led to\udincreased rates of microbial Fe(III) reduction by S. oneidensis MR-1\udin contrast to earlier findings with the model organism Geobacter\udbremensis. Ferrihydrite–OM coprecipitates were reduced slightly faster than\udferrihydrites with adsorbed OM. Surprisingly, the complex microbial consortia\udstimulated by a mixture of electrons donors (lactate, acetate, and glucose)\udmimics S. oneidensis under the same experimental Fe(III)-reducing\udconditions suggesting similar mechanisms of electron transfer whether or not\udthe OM is adsorbed or coprecipitated to the mineral surfaces. We also\udfollowed potential shifts of the microbial community during the incubation\udvia 16S rRNA gene sequence analyses to determine variations due to the\udpresence of adsorbed or coprecipitated OM–ferrihydrite complexes in contrast\udto pure ferrihydrite. Community profile analyses showed no enrichment of\udtypical model Fe(III)-reducing bacteria, such as Shewanella or\udGeobacter sp., but an enrichment of fermenters (e.g.,\udEnterobacteria) during pure ferrihydrite incubations which are known\udto use Fe(III) as an electron sink. Instead, OM–mineral complexes favored\udthe enrichment of microbes including Desulfobacteria and\udPelosinus sp., both of which can utilize lactate and acetate as an\udelectron donor under Fe(III)-reducing conditions. In summary, this study\udshows that increasing concentrations of OM in OM–mineral complexes determines\udmicrobial Fe(III) reduction rates and shapes the microbial community\udstructure involved in the reductive dissolution of ferrihydrite. Similarities\udobserved between the complex Fe(III)-reducing microbial consortia and the\udmodel Fe(III)-reducer S. oneidensis MR-1 suggest electron-shuttling\udmechanisms dominate in OM-rich environments, including soils, sediments, and\udfens, where natural OM interacts with Fe(III) oxides during mineral\udformation

biogeosciences

Ferrihydrite-associated organic matter (OM) stimulates reduction by &amp;lt;i&amp;gt;Shewanella oneidensis&amp;lt;/i&amp;gt; MR-1 and a complex microbial consortia