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The presence of Fe-oxidizing bacteria in the rhizosphere of four different species of wetland plants was investigated in a diverse wetland environment that had Fe(II) concentrations ranging from tens to hundreds of micromoles per liter and a pH range of 3.5 to 6.8. Enrichments for neutrophilic, putatively lithotrophic Fe-oxidizing bacteria were successful on roots from all four species; acidophilic Fe-oxidizing bacteria were enriched only on roots from plants whose root systems were exposed to soil solutions with a pH of <4. InSagittaria australis there was a positive correlation (P < 0.01) between cell numbers and the total amount of Fe present; the same correlation was not found for Leersia oryzoides. These results present the first evidence for culturable Fe-oxidizing bacteria associated with Fe-plaque in the rhizosphere.

We compared the reactivity and microbial reduction potential of Fe(III) minerals in the rhizosphere and non-rhizosphere soil to test the hypothesis that rapid Fe(III) reduction rates in wetland soils are explained by rhizosphere processes. The rhizosphere was defined as the area immediately adjacent to a root encrusted with Fe(III)-oxides or Fe plaque, and non-rhizosphere soil was >0.5 cm from the root surface. The rhizosphere had a significantly higher percentage of poorly crystalline Fe (66+/-7%) than non-rhizosphere soil (23+/-7%); conversely, non-rhizosphere soil had a significantly higher proportion of crystalline Fe (50+/-7%) than the rhizosphere (18+/-7%, P<0.05 in all cases). The percentage of poorly crystalline Fe(III) was significantly correlated with the percentage of FeRB (r=0.76), reflecting the fact that poorly crystalline Fe(III) minerals are labile with respect to microbial reduction. Abiotic reductive dissolution consumed about 75% of the rhizosphere Fe(III)-oxide pool in 4 h compared to 23% of the soil Fe(III)-oxide pool. Similarly, microbial reduction consumed 75-80% of the rhizosphere pool in 10 days compared to 30-40% of the non-rhizosphere soil pool. Differences between the two pools persisted when samples were amended with an electron-shuttling compound (AQDS), an Fe(III)-reducing bacterium (Geobacter metallireducens), and organic carbon. Thus, Fe(III)-oxide mineralogy contributed strongly to differences in the Fe(III) reduction potential of the two pools. Higher amounts of poorly crystalline Fe(III) and possibly humic substances, and a higher Fe(III) reduction potential in the rhizosphere compared to the non-rhizosphere soil, suggested the rhizosphere is a site of unusually active microbial Fe cycling. The results were consistent with previous speculation that rapid Fe cycling in wetlands is due to the activity of wetland plant roots.

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soil and rates of radial oxygen loss. Radial O 2 loss is in turn influenced by plant activity (Bedford et al., 1991; Iron (III) plaque forms on the roots of wetland plants from the Kuehn and Suberkropp, 1998) and morphological charreaction of Fe(II) with O 2 released by roots. Recent laboratory studies acteristics such as suberized and lignified roots (Armhave shown that Fe plaque is more rapidly reduced than non-rhizostrong et al., 2000). Under anaerobic conditions, the sphere Fe(III) oxides. The goals of the current study were to determine in situ rates of: (i) Fe(III) reduction of root plaque and soil Fe(III) amorphous Fe(III) oxides in root plaque serve as an exoxides, (ii) root Fe(III) deposition, and (iii) root and soil organic cellent substrate for FeRB that mediate Fe(III) reduction matter decomposition. Iron (III) reduction was investigated using a in freshwater environments (Lovley, 2000). Iron(III) novel buried-bag technique in which roots and soil were buried in reducing bacteria have been successfully enriched from heat-sealed membrane bags (Versapor 200 membrane, pore size ϭ the rhizosphere (King and Garey, 1999) where they 0.2 m) in late fall following plant senescence. Bags were retrieved can account for up to 12% of all bacterial cells (Weiss at monthly intervals for 1 yr to assess changes in total C and Fe mass, et al., 2003). Fe mineralogy, Fe(II)/Fe(III) ratio, and the abundances of Fe(II)-The juxtaposition of oxic and anoxic conditions in the oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB). The rhizosphere, separated either temporally or spatially, soil C and Fe pools did not change significantly throughout the year, results in a rhizosphere Fe cycle in which Fe plaque is but root C and total root Fe mass decreased by 40 and 70%, respectively. When total Fe losses were adjusted for changes in the ratio of alternately deposited and then reduced. Previous studies Fe(II)/Fe(III), over 95% of the Fe(III) in the plaque was reduced have reported a higher Fe(III) reduction potential in during the 12-mo study, at a peak rate of 0.6 mg Fe(III) g dry weight Ϫ1 the rhizosphere Fe(III) pool than the non-rhizosphere d Ϫ1 (gdw Ϫ1 d Ϫ1). These estimates exceed the crude estimate of Fe(III) soil pool. For example, we recently reported that Ͼ75% accumulation [0.3 mg Fe(III) g dry weight Ϫ1 d Ϫ1 ] on bare-root plants of the Fe plaque is reduced in 10 d vs. Ͻ40% of the soil that were transplanted into the wetland for a growing season. We Fe(III) oxide pool (Weiss et al., 2004). Results from anconcluded that root plaque has the potential to be reduced as rapidly other short-term (7 d) experiment examining rates of as it is deposited under field conditions. Fe(III) reduction in salt marshes support the idea of more rapid Fe(III) reduction in rhizosphere than nonrhizophere soils (Gribsholt et al., 2003). However, both radial O 2 loss and Fe(II) oxidation penetrate to some extent

In order to evaluate natural attenuation in contaminated aquifers, there has been a recent recognition that a multidisciplinary approach, incorporating microbial and molecular methods, is required. Observed decreases in contaminant mass and identified footprints of biogeochemical reactions are often used as evidence of intrinsic bioremediation, but characterizing the structure and function of the microbial populations at contaminated sites is needed. In this paper, we review the experimental approaches and microbial methods that are available as tools to evaluate the controls on microbially mediated degradation processes in contaminated aquifers. We discuss the emerging technologies used in biogeochemical studies and present a synthesis of recent studies that serve as models of integrating microbiological approaches with more traditional geochemical and hydrogeologic approaches in order to address important biogeochemical questions about contaminant fate.

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