Environ. Sci. Technol. 2007, 41, 6084-6089
Control of Ferrous Iron Oxidation within Circumneutral Microbial Iron Mats by Cellular Activity and Autocatalysis
Circumneutral FeOB are limited to specific environmental niches where Fe2+ is in abundant supply but chemical Fe2+ oxidation is limited. Chemical oxidation rates within aqueous systems are proportional to oxygen concentration and pH, as described previously (15).
-d[Fe2+]/dt ) kH[Fe2+](pO2)[OH-]2,
where kH ) 8 × 1013 M-2 atm-1 min-1 (1)
,†
JEREMY A. RENTZ,* CHAROENKWAN KRAIYA,‡ GEORGE W. LUTHER III,‡ AND DAVID EMERSON† American Type Culture Collection, Manassas, Virginia 20109, and College of Marine and Earth Sciences, University of Delaware, Lewes, Delaware 19958
Ferrous iron (Fe2+) oxidation by microbial iron mat samples, dominated by helical stalks of Gallionella ferruginea or sheaths of Leptothrix ochracea, was examined. Pseudo-first-order rate constants for the microbial mat samples ranged from 0.029 ( 0.004 to 0.249 ( 0.042 min-1 and correlated well with iron content (R2 ) 0.929). Rate constants for Na azide-treated (1 mM) samples estimated autocatalytic oxidation by iron oxide stalks or sheaths, with values ranging from 0.016 ( 0.008 to 0.062 ( 0.006 min-1. Fe2+ oxidation attributable to cellular activities was variable with respect to sampling location and sampling time, with rate constants from 0.013 ( 0.005 to 0.187 ( 0.037 min-1. Rates of oxidation of the same order of magnitude for cellular processes and autocatalysis suggested that bacteria harnessing Fe2+ as an energy source compete with their own byproducts for growth, not chemical oxidation (under conditions where aqueous oxygen concentrations are less than saturating). The use of cyclic voltammetry within this study for the simultaneous measurement of Fe2+ and oxygen allowed the collection of statistically meaningful and reproducible data, two factors that have limited aerobic, circumneutral, Fe2+-oxidation rate studies.
Introduction Iron bacteria were first described by Ehrenberg in 1836 (1). Since that time, researchers have demonstrated the environmental significance of aerobic, circumneutral, ferrous iron-oxidizing bacteria (FeOB) in engineered and natural systems. Within man-made systems, FeOB can be beneficial, particularly during the biological removal of Fe2+ from drinking water (2-4). However, FeOB can also be nuisance microorganisms. They have long been recognized as contributors of well, pipe, and drain clogging (1, 5), and they also affect steel biocorrosion (6, 7). For natural systems, the contribution of FeOB to Fe cycling has been demonstrated for diverse environments, including Fe seeps (8), groundwater (9), freshwater wetlands (10-12), and marine hydrothermal systems (13, 14). * Corresponding author current address: Department of Civil and Environmental Engineering, Washington State University, Pullman, WA 99164. Phone: 509.335.6411; fax: 509.335.7632; e-mail: rentz@ wsu.edu. † American Type Culture Collection. ‡ University of Delaware. 6084
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 17, 2007
Most circumneutral FeOB are found in environments characterized by low oxygen concentrations, often at oxic/anoxic boundaries where anaerobic, Fe2+-containing water meets the atmosphere (8, 11). In these environments, chemical oxidation rates are slowed and circumneutral FeOB may compete with the chemical oxidation of Fe2+. Alternatively, acidophilic FeOB thrive in low pH environments where chemical oxidation is negligible and aqueous Fe2+ remains very stable (15). Within circumneutral microbial iron mats, several processes contribute to the total iron oxidation rate. Abiotic oxidation includes the chemical oxidation of aqueous Fe2+, as well as the surface-mediated Fe2+ oxidation on abiogenic iron oxides (autocatalysis). Biological Fe2+ oxidation includes catalysis that is mediated directly as a result of lithotrophic metabolism and surface-mediated oxidation on biogenic iron oxides (autocatalysis). These biologically formed iron oxides can be morphologically distinct (e.g., stalks of Gallionella ferruginea and sheaths of Leptothrix ochracea) or amorphous. While this oxidation is not directly linked to bacterial metabolism, the resulting oxidation would not have occurred in the absence of FeOB and may contribute significantly to iron cycling within circumneutral environments (16). Bacterial cells can also passively adsorb Fe2+ (17, 18) and facilitate iron oxide formation (19-21) as a third mechanism that contributes to biological Fe oxidation; this is another form of autocatalysis. The rates at which these different processes occur and the direct role of microbes in mediating them are not well-known. The complexity of these iron oxidation kinetics has presented a challenge demonstrating the specific role that microbes play in iron oxidation at neutral pH, especially in natural systems. To date, laboratory microcosms (16, 22) and field measurements (16, 23) investigating environmental samples have indicated that there is a significant microbial component. However, these investigations did not accurately quantify experimental conditions to confirm the replication of field conditions nor did they measure with adequate temporal resolution to capture the rapid kinetics of natural Fe-oxidation rates. To evaluate environmental Fe2+ oxidation within aerobic, circumneutral iron mats, we examined samples collected from two different field sites. The objectives of this research were to (i) determine Fe2+-oxidation rates for environmental microbial iron mat samples (untreated and azide treated), (ii) estimate oxidation attributable to cellular activity and autocatalysis (occurring on iron oxide surfaces and bacterial surfaces), and (iii) relate microbial iron mat characteristics (chemical, physical, and microbiological) to oxidation rates. A positive-pressure, well-mixed reactor was used to assay Fe2+ oxidation. Cyclic voltammetry was used to measure Fe2+ and O2, resulting in short experimental times (
