Aerobic uranium (VI) bioprecipitation by metal‐resistant bacteria isolated from radionuclide‐ and metal‐contaminated subsurface soils

Caulobacter crescentus is known to tolerate high levels of uranium [U(VI)], but its detoxification mechanism is poorly understood. Here we show that C. crescentus is able to facilitate U(VI) biomineralization through the formation of U-P i precipitates via its native alkaline phosphatase activity. The U-P i precipitates, deposited on the cell surface in the form of meta-autunite structures, have a lower U/P i ratio than do chemically produced precipitates. The enzyme that is responsible for the phosphatase activity and thus the biomineralization process is identified as PhoY, a periplasmic alkaline phosphatase with broad substrate specificity. Furthermore, PhoY is shown to confer a survival advantage on C. crescentus toward U(VI) under both growth and nongrowth conditions. Results obtained in this study thus highlight U(VI) biomineralization as a resistance mechanism in microbes, which not only improves our understanding of bacterium-mineral interactions but also aids in defining potential ecological niches for metal-resistant bacteria.

Section: Resultsmentioning

confidence: 99%

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confidence: 99%

Biomineralization of Uranium by PhoY Phosphatase Activity Aids Cell Survival in Caulobacter crescentus

Yung

Jiao

2014

“…In particular, extracellular polysaccharides (EPS) secreted by many microorganisms contain side groups with negatively charged ligands such as hydroxyl, carboxyl, or phosphate, which attract cations like U(VI) and reduce their mobility (8,9). Additionally, phosphatase enzymes have been reported to mediate U(VI) precipitation through the release of inorganic phosphate from organophosphates (10)(11)(12)(13)(14).…”

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confidence: 99%

Modulation of Medium pH by Caulobacter crescentus Facilitates Recovery from Uranium-Induced Growth Arrest

Park

Jiao

2014

The oxidized form of uranium [U(VI)] predominates in oxic environments and poses a major threat to ecosystems. Due to its ability to mineralize U(VI), the oligotroph Caulobacter crescentus is an attractive candidate for U(VI) bioremediation. However, the physiological basis for U(VI) tolerance is unclear. Here we demonstrated that U(VI) caused a temporary growth arrest in C. crescentus and three other bacterial species, although the duration of growth arrest was significantly shorter for C. crescentus. During the majority of the growth arrest period, cell morphology was unaltered and DNA replication initiation was inhibited. However, during the transition from growth arrest to exponential phase, cells with shorter stalks were observed, suggesting a decoupling between stalk development and the cell cycle. Upon recovery from growth arrest, C. crescentus proliferated with a growth rate comparable to that of a control without U(VI), although a fraction of these cells appeared filamentous with multiple replication start sites. Normal cell morphology was restored by the end of exponential phase. Cells did not accumulate U(VI) resistance mutations during the prolonged growth arrest, but rather, a reduction in U(VI) toxicity occurred concomitantly with an increase in medium pH. Together, these data suggest that C. crescentus recovers from U(VI)-induced growth arrest by reducing U(VI) toxicity through pH modulation. Our finding represents a unique U(VI) detoxification strategy and provides insight into how microbes cope with U(VI) under nongrowing conditions, a metabolic state that is prevalent in natural environments.

“…The overexpression of the polyphosphate kinase (ppk) gene in Pseudomonas aeruginosa results in the significant accumulation of polyphosphates which degrade under carbon-starved conditions, and the phosphates released therefrom precipitate uranyl out of the solutions (24). Several environmental strains, such as Cellulomonas, Arthrobacter, Rahnella, and Bacillus, isolated from contaminated subsurface soils of the Department of Energy's Field Research Centre (ORFRC) were shown to immobilize uranium as a biogenic phosphate mineral resulting from polyphosphate metabolism or organophosphate hydrolase activity (25)(26)(27).…”

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confidence: 99%

“…Previous studies on uranium bioremediation/biomineralization have focused on the bacterial reduction of U(VI) to uraninite or monomeric U(IV) under anoxic conditions or on enzymatic bioprecipitation in the presence of exogenous organic phosphate substrate under oxic conditions (15,25,(27)(28)(29) and anoxic conditions (30) below a circumneutral pH. However, much of the uranyl contamination found in groundwater/ aquatic systems exists above a circumneutral pH where uranyl mobility is predominantly controlled by carbonates through the formation of highly soluble and stable uranyl carbonate complexes (31).…”

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confidence: 99%

Unusual Versatility of the Filamentous, Diazotrophic Cyanobacterium Anabaena torulosa Revealed for Its Survival during Prolonged Uranium Exposure

Acharya

Chandwadkar

Nayak

2017

Reports on interactions between cyanobacteria and uranyl carbonate are rare. Here, we present an interesting succession of the metabolic responses employed by a marine, filamentous, diazotrophic cyanobacterium, Anabaena torulosa for its survival following prolonged exposure to uranyl carbonate extending up to 384 h at pH 7.8 under phosphate-limited conditions. The cells sequestered uranium (U) within polyphosphates on initial exposure to 100 M uranyl carbonate for 24 to 28 h. Further incubation until 120 h resulted in (i) significant degradation of cellular polyphosphates causing extensive chlorosis and cell lysis, (ii) akinete differentiation followed by (iii) extracellular uranyl precipitation. X-ray diffraction (XRD) analysis, fluorescence spectroscopy, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopy established the identity of the bioprecipitated uranium as a U(VI) autunite-type mineral, which settled at the bottom of the vessel. Surprisingly, A. torulosa cells resurfaced as small green flakes typical of actively growing colonies on top of the test solutions within 192 to 240 h of U exposure. A consolidated investigation using kinetics, microscopy, and physiological and biochemical analyses suggested a role of inducible alkaline phosphatase activity of cell aggregates/akinetes in facilitating the germination of akinetes leading to substantial regeneration of A. torulosa by 384 h of uranyl incubation. The biomineralized uranium appeared to be stable following cell regeneration. Altogether, our results reveal novel insights into the survival mechanism adopted by A. torulosa to resist sustained uranium toxicity under phosphate-limited oxic conditions. IMPORTANCE Long-term effects of uranyl exposure in cyanobacteria under oxic phosphate-limited conditions have been inadequately explored. We conducted a comprehensive examination of the metabolic responses displayed by a marine cyanobacterium, Anabaena torulosa, to cope with prolonged exposure to uranyl carbonate at pH 7.8 under phosphate limitation. Our results highlight distinct adaptive mechanisms harbored by this cyanobacterium that enabled its natural regeneration following extensive cell lysis and uranium biomineralization under sustained uranium exposure. Such complex interactions between environmental microbes such as Anabaena torulosa and uranium over a broader time range advance our understanding on the impact of microbial processes on uranium biogeochemistry.KEYWORDS biomineralization, cyanobacteria, phosphatase, polyphosphates, regeneration, uranium M icroorganisms have existed on earth for at least 3.5 billion years (1) and are the most successful life forms to date (2). They have been exposed to their immediate environments comprising essential elements or complex mixtures of naturally occur-