Dissimilatory reduction of Fe(III) and other electron acceptors by a Thermus isolate

Abstract

A thermophilic bacterium that can use O2, NO3-, Fe(III), and S0 as terminal electron acceptors for growth was isolated from groundwater sampled at a 3.2-km depth in a South African gold mine. This organism, designated SA-01, clustered most closely with members of the genus Thermus, as determined by 16S rRNA gene (rDNA) sequence analysis. The 16S rDNA sequence of SA-01 was >98% similar to that of Thermus strain NMX2 A.1, which was previously isolated by other investigators from a thermal spring in New Mexico. Strain NMX2 A.1 was also able to reduce Fe(III) and other electron acceptors. Neither SA-01 nor NMX2 A.1 grew fermentatively, i.e., addition of an external electron acceptor was required for anaerobic growth. Thermus strain SA-01 reduced soluble Fe(III) complexed with citrate or nitrilotriacetic acid (NTA); however, it could reduce only relatively small quantities (0.5 mM) of hydrous ferric oxide except when the humic acid analog 2,6-anthraquinone disulfonate was added as an electron shuttle, in which case 10 mM Fe(III) was reduced. Fe(III)-NTA was reduced quantitatively to Fe(II); reduction of Fe(III)-NTA was coupled to the oxidation of lactate and supported growth through three consecutive transfers. Suspensions of Thermus strain SA-01 cells also reduced Mn(IV), Co(III)-EDTA, Cr(VI), and U(VI). Mn(IV)-oxide was reduced in the presence of either lactate or H2. Both strains were also able to mineralize NTA to CO2 and to couple its oxidation to Fe(III) reduction and growth. The optimum temperature for growth and Fe(III) reduction by Thermus strains SA-01 and NMX2 A.1 is approximately 65 degrees C; their optimum pH is 6.5 to 7.0. This is the first report of a Thermus sp. being able to couple the oxidation of organic compounds to the reduction of Fe, Mn, or S.

FIG. 1

16S rDNA-based molecular phylogeny (maximum-likelihood method) of various Thermus strains, including metal-reducing SA-01 and NMX2 A.1 and also various non-Thermus outgroup species. The phylogeny was constructed with sequences corresponding to E. coli positions 49 to 71, 102 to 180, 221 to 451, and 481 to 1259. The tree shows the close phylogenetic relationship of these metal-reducing strains within the genus Thermus. The scale bar shows the expected number of changes per sequence position.

FIG. 2

Reduction of Fe(III)-NTA coupled to lactate oxidation and growth by Thermus strain SA-01. Fe(II) concentration (a), lactate concentration (b), and cell density (c) versus time are shown. Error bars show 1 standard deviation (n = 3).

FIG. 3

Fe(II) concentrations (a) and cell densities (b) of Thermus strain SA-01 during repeated transfers into fresh basal medium. Error bars show 1 standard deviation (n = 3).

FIG. 4

Mineralization of ¹⁴C-labeled lactate to ¹⁴CO₂ by Thermus strain SA-01 in the presence and absence of Fe(III)-NTA (a) and concomitant reduction of Fe(III)-NTA to Fe(II) in the presence of lactate (b). Error bars show 1 standard deviation (n = 2).

FIG. 5

Reduction of HFO and growth by Thermus strain SA-01 with lactate as the electron donor. Fe(II) concentrations without AQDS (a) and in the presence of 0.1 mM AQDS (b) and cell density (c) versus time are shown. Error bars show 1 standard deviation (n = 2).

FIG. 6

Reduction of various electron acceptors by suspensions of Thermus strain SA-01 cells in media containing lactate, acetate, and/or H₂ as the potential electron donor(s) (as described in Materials and Methods). Levels of reduction of Fe(III)-NTA (a), Fe(III)-citrate (b), HFO (c), Co(III)-EDTA (d), Cr(VI) (e), U(VI) (f), and Mn(IV) (g and h) are shown. Electron donors for Mn reduction were lactate (g) and H₂ (h). Filled circles show results for experimental treatments (with cells); open circles show results for controls (no cells). Error bars show 1 standard deviation (n = 3).

FIG. 7

Growth rate (μ) and Fe(III) reduction rate versus temperature for Thermus strains SA-01 and NMX2 A.1 cultured with lactate as the electron donor and Fe(III)-NTA as the electron acceptor. Filled circles, μ; open circles, Fe(III)-reduction rate.

FIG. 8

Percent mineralization of ¹⁴C-labeled NTA by Thermus strains SA-01 and NMX2 A.1. Error bars show 1 standard deviation (n = 2).