Effects of Mn(II) on UO₂ Dissolution under Anoxic and Oxic Conditions

Groundwater composition and coupled redox cycles can affect the long-term stability of U­(IV) products from bioremediation. The effects of Mn­(II), a redox active cation present at uranium-contaminated sites, on UO₂ dissolution in both oxic and anoxic systems were investigated using batch and continuous-flow reactors. Under anoxic conditions Mn­(II) inhibited UO₂ dissolution, which was probably due to adsorption of Mn­(II) and precipitation of MnCO₃ that decreased exposure of U­(IV) surface sites to oxidants. In contrast, Mn­(II) promoted UO₂ dissolution under oxic conditions through Mn redox cycling. Oxidation of Mn­(II) by O₂ produced reactive Mn species, possibly short-lived Mn­(III) in solution or at the surface, that oxidatively dissolved the UO₂ more rapidly than could the O₂ alone. At pH 8 the Mn cycling was such that there was no measurable accumulation of particulate Mn oxides. At pH 9 Mn oxides could be produced and accumulate, while they were continuously reduced by UO₂, with Mn­(II) returning to the aqueous phase. With the rapid turnover of Mn in the redox cycle, concentrations of Mn as low as 10 μM could maintain an enhanced UO₂ dissolution rate. The presence of the siderophore desferrioxamine B (a strong Mn­(III)-complexing ligand) effectively decoupled the redox interactions of uranium and manganese to suppress the promotional effect of Mn­(II)

Impact of Microbial Mn Oxidation on the Remobilization of Bioreduced U(IV)

Effects of Mn redox cycling on the stability of bioreduced U­(IV) are evaluated here. U­(VI) can be biologically reduced to less soluble U­(IV) species and the stimulation of biological activity to that end is a salient remediation strategy; however, the stability of these materials in the subsurface environments where they form remains unproven. Manganese oxides are capable of rapidly oxidizing U­(IV) to U­(VI) in mixed batch systems where the two solid phases are in direct contact. However, it is unknown whether the same oxidation would take place in a porous medium. To probe that question, U­(IV) immobilized in agarose gels was exposed to conditions allowing biological Mn­(II) oxidation (HEPES buffer, Mn­(II), 5% O₂ and <i>Bacillus</i> sp. SG-1 spores). Results show the oxidation of U­(IV) to U­(VI) is due primarily to O₂ rather than to MnO₂. U­(VI) produced is retained within the gel to a greater extent when Mn oxides are present, suggesting the formation of strong surface complexes. The implication for the long-term stability of U in a bioremediated site is that, in the absence of competing ligands, biological Mn­(II) oxidation may promote the immobilization of U­(VI) produced by the oxidation of U­(IV)

Ubiquitous Dissolved Inorganic Carbon Assimilation by Marine Bacteria in the Pacific Northwest Coastal Ocean as Determined by Stable Isotope Probing

<div><p>In order to identify bacteria that assimilate dissolved inorganic carbon (DIC) in the northeast Pacific Ocean, stable isotope probing (SIP) experiments were conducted on water collected from 3 different sites off the Oregon and Washington coasts in May 2010, and one site off the Oregon Coast in September 2008 and March 2009. Samples were incubated in the dark with 2 mM ¹³C-NaHCO₃, doubling the average concentration of DIC typically found in the ocean. Our results revealed a surprising diversity of marine bacteria actively assimilating DIC in the dark within the Pacific Northwest coastal waters, indicating that DIC fixation is relevant for the metabolism of different marine bacterial lineages, including putatively heterotrophic taxa. Furthermore, dark DIC-assimilating assemblages were widespread among diverse bacterial classes. Alphaproteobacteria, Gammaproteobacteria, and Bacteroidetes dominated the active DIC-assimilating communities across the samples. Actinobacteria, Betaproteobacteria, Deltaproteobacteria, Planctomycetes, and Verrucomicrobia were also implicated in DIC assimilation. <em>Alteromonadales</em> and <em>Oceanospirillales</em> contributed significantly to the DIC-assimilating Gammaproteobacteria within May 2010 clone libraries. 16S rRNA gene sequences related to the sulfur-oxidizing symbionts Arctic96BD-19 were observed in all active DIC assimilating clone libraries. Among the Alphaproteobacteria, clones related to the ubiquitous SAR11 clade were found actively assimilating DIC in all samples. Although not a dominant contributor to our active clone libraries, Betaproteobacteria, when identified, were predominantly comprised of <em>Burkholderia</em>. DIC-assimilating bacteria among <em>Deltaproteobacteria</em> included members of the SAR324 cluster. Our research suggests that DIC assimilation is ubiquitous among many bacterial groups in the coastal waters of the Pacific Northwest marine environment and may represent a significant metabolic process.</p> </div

DataSheet1.DOCX

<p>The observation of significant concentrations of soluble Mn(III) complexes in oxic, suboxic, and some anoxic waters has triggered a re-evaluation of the previous Mn paradigm which focused on the cycling between soluble Mn(II) and insoluble Mn(III,IV) species as operationally defined by filtration. Though Mn(II) oxidation in aquatic environments is primarily bacterially-mediated, little is known about the effect of Mn(III)-binding ligands on Mn(II) oxidation nor on the formation and removal of Mn(III). Pseudomonas putida GB-1 is one of the most extensively investigated of all Mn(II) oxidizing bacteria, encoding genes for three Mn oxidases (McoA, MnxG, and MopA). P. putida GB-1 and associated Mn oxidase mutants were tested alongside environmental isolates Pseudomonas hunanensis GSL-007 and Pseudomonas sp. GSL-010 for their ability to both directly oxidize weakly and strongly bound Mn(III), and to form these complexes through the oxidation of Mn(II). Using Mn(III)-citrate (weak complex) and Mn(III)-DFOB (strong complex), it was observed that P. putida GB-1, P. hunanensis GSL-007 and Pseudomonas sp. GSL-010 and mutants expressing only MnxG and McoA were able to directly oxidize both species at varying levels; however, no oxidation was detected in cultures of a P. putida mutant expressing only MopA. During cultivation in the presence of Mn(II) and citrate or DFOB, P. putida GB-1, P. hunanensis GSL-007 and Pseudomonas sp. GSL-010 formed Mn(III) complexes transiently as an intermediate before forming Mn(III/IV) oxides with the overall rates and extents of Mn(III,IV) oxide formation being greater for Mn(III)-citrate than for Mn(III)-DFOB. These data highlight the role of bacteria in the oxidative portion of the Mn cycle and suggest that the oxidation of strong Mn(III) complexes can occur through enzymatic mechanisms involving multicopper oxidases. The results support the observations from field studies and further emphasize the complexity of the geochemical cycling of manganese.</p

Rooted neighbor-joining phylogenetic tree of <i>Bacteroidetes</i> involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10, CR-20, and LP-6 in May 2010.

<p>High similar OTUS were collapsed for clarity.</p

Rooted neighbor-joining phylogenetic tree of <i>Alphaproteobacteria</i> species actively involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10 in September 2008, March 2009, and May 2010. Clones representing highly similar OTUS were collapsed for clarity.

<p>Rooted neighbor-joining phylogenetic tree of <i>Alphaproteobacteria</i> species actively involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10 in September 2008, March 2009, and May 2010. Clones representing highly similar OTUS were collapsed for clarity.</p

Class distributions of 16S rRNA gene clone libraries of ¹³C fractions from all ¹³C-NaHCO₃ incubations at NH-10 in September 2008, March 2009, and May 2010, as well as at LP-6 and CR-20 in May 2010.

<p>Class distributions of 16S rRNA gene clone libraries of ¹³C fractions from all ¹³C-NaHCO₃ incubations at NH-10 in September 2008, March 2009, and May 2010, as well as at LP-6 and CR-20 in May 2010.</p

Distribution of bacterial orders observed in 16S rRNA gene clone libraries of ¹²C vs. ¹³C fractions from ¹³C-NaHCO₃ incubations at NH-10 in September 2008 and March 2009.

<p>Distribution of bacterial orders observed in 16S rRNA gene clone libraries of ¹²C vs. ¹³C fractions from ¹³C-NaHCO₃ incubations at NH-10 in September 2008 and March 2009.</p

Rooted neighbor-joining phylogenetic tree of <i>Alphaproteobacteria</i> involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10, CR-20, and LP-6 in May 2010. Clones representing highly similar OTUS were collapsed for clarity.

<p>Rooted neighbor-joining phylogenetic tree of <i>Alphaproteobacteria</i> involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10, CR-20, and LP-6 in May 2010. Clones representing highly similar OTUS were collapsed for clarity.</p

A. Rooted neighbor-joining phylogenetic tree of <i>Gammaproteobacteria</i> involved in DIC assimilation obtained from 16S rRNA gene clone libraries of ¹³C fractions from SIP experiments conducted at NH-10, CR-20, and LP-6 in May 2010.

<p>Highly similar OTUS were collapsed for clarity. Shaded clades represent those identified by Swan et al., 2012 to contain RubisCo and possibly participate in dark carbon fixation.</p