Evidence for microbial Fe(III) reduction in anoxic, mining-impacted lake sediments (Lake Coeur d'Alene, Idaho)

Abstract

Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg(-1), suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter(-1)) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe(3)O(4)), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg(-1). Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 x 10(5) cells g (dry weight) of sediment(-1). Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined.

FIG. 1

Map of Lake Coeur d'Alene, Idaho, and the CDAR delta (inset), showing the sampling sites used in this study. The CDAR is the source of metal contamination in the sediments.

FIG. 2

E_h and pH as a function of depth in the CDAR delta sediments. The E_h decreases beneath the surface, indicating that there is a lack of oxygen and reducing conditions.

FIG. 3

Vertical distribution of iron phases in CDAR delta sediments. (A) Total iron. (B) Weak-acid-soluble Fe(II). (C) Ratio of weak-acid-soluble Fe(II) to total iron. (D) Fe(II) dissolved in pore water. (E) Mass of magnetic minerals. The error bars represent standard deviations from the means (n = 3) for weak-acid-soluble Fe(II) data. Symbols without error bars show data from single determinations.

FIG. 4

Raman spectra of mineral grains from the magnetic fraction of CDAR delta sediments (A) and natural mineral standards (B).

FIG. 5

Growth coupled to Fe(II) generation by strains CdA-2 (A) and CdA-3 (B). The error bars represent standard deviations from the means (n = 3). Symbols: ●, cell density in the presence of acetate; ○, cell density in the absence of acetate; ▴, amount of Fe(II) produced in the presence of acetate; ▵, amount of Fe(II) produced in the absence of acetate.

FIG. 6

Phylogenetic tree based on 16S rRNA gene sequences, showing the positions of newly isolated strains CdA-2 and CdA-3 in the family Geobacteraceae (29). The Geobacter cluster in this group is indicated by boldface lines. Phylogenetic distances were calculated as described by Felsenstein (20). The sequence of Escherichia coli was used as an outgroup. Bar = 10% estimated sequence divergence.