Fe(III) reduction and U(VI) immobilization by Paenibacillus sp. strain 300A, isolated from Hanford 300A subsurface sediments

Abstract

A facultative iron-reducing [Fe(III)-reducing] Paenibacillus sp. strain was isolated from Hanford 300A subsurface sediment biofilms that was capable of reducing soluble Fe(III) complexes [Fe(III)-nitrilotriacetic acid and Fe(III)-citrate] but unable to reduce poorly crystalline ferrihydrite (Fh). However, Paenibacillus sp. 300A was capable of reducing Fh in the presence of low concentrations (2 μM) of either of the electron transfer mediators (ETMs) flavin mononucleotide (FMN) or anthraquinone-2,6-disulfonate (AQDS). Maximum initial Fh reduction rates were observed at catalytic concentrations (<10 μM) of either FMN or AQDS. Higher FMN concentrations inhibited Fh reduction, while increased AQDS concentrations did not. We also found that Paenibacillus sp. 300A could reduce Fh in the presence of natural ETMs from Hanford 300A subsurface sediments. In the absence of ETMs, Paenibacillus sp. 300A was capable of immobilizing U(VI) through both reduction and adsorption. The relative contributions of adsorption and microbial reduction to U(VI) removal from the aqueous phase were ∼7:3 in PIPES [piperazine-N,N'-bis(2-ethanesulfonic acid)] and ∼1:4 in bicarbonate buffer. Our study demonstrated that Paenibacillus sp. 300A catalyzes Fe(III) reduction and U(VI) immobilization and that these reactions benefit from externally added or naturally existing ETMs in 300A subsurface sediments.

Fig 1

(A) Fe(III) reduction [shown as Fe(II) production] by Paenibacillus sp. 300A. (B) Fh reduction [shown as Fe(II) production] by Paenibacillus sp. 300A in the presence of 2 μM FMN or AQDS. The dashed lines are cell-free abiotic controls. The error bars represent ± standard deviations (n = 3).

Fig 2

Initial Fh reduction rate by Paenibacillus sp. 300A in the presence of various FMN and AQDS concentrations.

Fig 3

Fh reduction [shown as Fe(II) production] by Paenibacillus sp. 300A in the presence of 300A sediment slurry (slurry plus cells plus Fh). Slurry plus Fh, slurry plus cells, and slurry were used as controls. The error bars represent the standard deviations (n = 3).

Fig 4

(A) U(VI) immobilization by Paenibacillus sp. 300A in bicarbonate-buffered non-growth medium. U(VI) immobilization lacking cells and immobilization with heat-killed cells were used as abiotic controls. The initial U(VI) concentration used in the immobilization study was 100 μM. The error bars represent the standard deviations (n = 3). (B and C) Transmission electron microscopy (TEM) photomicrographs of unstained thin sections. (D) High-resolution TEM micrograph of U precipitates. (E) Selected-area electron diffraction (SAED) (top) and an energy-dispersive X-ray spectrum of the U particles (bottom).

Fig 5

U(VI) immobilization by Paenibacillus sp. 300A in PIPES-buffered non-growth medium. U(VI) immobilization without cells and immobilization with heat-killed cells were used as the abiotic controls. The initial U(VI) concentration used in the immobilization study was 100 μM. The error bars represent the standard deviations (n = 3).

Fig 6

U(VI) immobilization by Paenibacillus sp. 300A in the presence of 300A sediment slurry. U(VI) immobilization lacking cells was used as the abiotic control. The initial U(VI) concentration used in the immobilization study was 100 μM. The error bars represent the standard deviations (n = 3).