Microbial Iron Redox Cycling in a Circumneutral-pH Groundwater Seep

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Abstract
The potential for microbially mediated redox cycling of iron (Fe) in a circumneutral-pH groundwater seep in north central Alabama was studied. Incubation of freshly collected seep material under anoxic conditions with acetate-lactate or H 2 as an electron donor revealed the potential for rapid Fe(III) oxide reduction (ca. 700 to 2,000 μmol liter −1 day −1 ). Fe(III) reduction at lower but significant rates took place in unamended controls (ca. 300 μmol liter −1 day −1 ). Culture-based enumerations (most probable numbers [MPNs]) revealed significant numbers (10 2 to 10 6 cells ml −1 ) of organic carbon- and H 2 -oxidizing dissimilatory Fe(III)-reducing microorganisms. Three isolates with the ability to reduce Fe(III) oxides by dissimilatory or fermentative metabolism were obtained ( Geobacter sp. strain IST-3, Shewanella sp. strain IST-21, and Bacillus sp. strain IST-38). MPN analysis also revealed the presence of microaerophilic Fe(II)-oxidizing microorganisms (10 3 to 10 5 cells ml −1 ). A 16S rRNA gene library from the iron seep was dominated by representatives of the Betaproteobacteria including Gallionella, Leptothrix , and Comamonas species. Aerobic Fe(II)-oxidizing Comamonas sp. strain IST-3 was isolated. The 16S rRNA gene sequence of this organism is 100% similar to the type strain of the betaproteobacterium Comamonas testosteroni (M11224). Testing of the type strain showed no Fe(II) oxidation. Collectively our results suggest that active microbial Fe redox cycling occurred within this habitat and support previous conceptual models for how microbial Fe oxidation and reduction can be coupled in surface and subsurface sedimentary environments.

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