Interaction of Uranium with Bacterial Cell Surfaces: Inferences from Phosphatase-Mediated Uranium Precipitation

Abstract

Deinococcus radiodurans and Escherichia coli expressing either PhoN, a periplasmic acid phosphatase, or PhoK, an extracellular alkaline phosphatase, were evaluated for uranium (U) bioprecipitation under two specific geochemical conditions (GCs): (i) a carbonate-deficient condition at near-neutral pH (GC1), and (ii) a carbonate-abundant condition at alkaline pH (GC2). Transmission electron microscopy revealed that recombinant cells expressing PhoN/PhoK formed cell-associated uranyl phosphate precipitate under GC1, whereas the same cells displayed extracellular precipitation under GC2. These results implied that the cell-bound or extracellular location of the precipitate was governed by the uranyl species prevalent at that particular GC, rather than the location of phosphatase. MINTEQ modeling predicted the formation of predominantly positively charged uranium hydroxide ions under GC1 and negatively charged uranyl carbonate-hydroxide complexes under GC2. Both microbes adsorbed 6-to 10-fold more U under GC1 than under GC2, suggesting that higher biosorption of U to the bacterial cell surface under GC1 may lead to cell-associated U precipitation. In contrast, at alkaline pH and in the presence of excess carbonate under GC2, poor biosorption of negatively charged uranyl carbonate complexes on the cell surface might have resulted in extracellular precipitation. The toxicity of U observed under GC1 being higher than that under GC2 could also be attributed to the preferential adsorption of U on cell surfaces under GC1. This work provides a vivid description of the interaction of U complexes with bacterial cells. The findings have implications for the toxicity of various U species and for developing biological aqueous effluent waste treatment strategies. IMPORTANCEThe present study provides illustrative insights into the interaction of uranium (U) complexes with recombinant bacterial cells overexpressing phosphatases. This work demonstrates the effects of aqueous speciation of U on the biosorption of U and the localization pattern of uranyl phosphate precipitated as a result of phosphatase action. Transmission electron microscopy revealed that location of uranyl phosphate (cell associated or extracellular) was primarily influenced by aqueous uranyl species present under the given geochemical conditions. The data would be useful for understanding the toxicity of U under different geochemical conditions. Since cell-associated precipitation of metal facilitates easy downstream processing by simple gravitybased settling down of metal-loaded cells, compared to cumbersome separation techniques, the results from this study are of considerable relevance to effluent treatment using such cells. Bioremediation strategies, such as bioreduction (1-3), biosorption (4-8), bioaccumulation (9, 10), and bioprecipitation (5, 11, 12, 13), have been studied for their potential to immobilize U from solutions. There is also a large body of work on microbial interactions with uranium relevant to environmental in situ bioremediation. The...

pH‐dependent speciation and hydrogen (H2) control U(VI) respiration by Desulfovibrio vulgaris

“…Contrary to the conventional wisdom that mono-nuclear complexes are more bioavailable (Bernhard et al, 1998;Croteau et al, 2016;Drobot et al, 2015;Du et al, 2011;Guillauont et al, 2003;Jones et al, 2015;Kulkarni et al, 2016), we found that all of the mono-nuclear, and poly-nuclear species of U(VI) were about equally bioavailable. This means that pH should not affect U(VI) respiration as long as pH itself is not inhibitory.…”

“…The conventional thinking is that free UO 2 2+ and its mono-nuclear complexes with hydroxides and carbonates are bioavailable to microorganisms (Tutem, Apak, Turgut, & Apak, 1991). Poly-nuclear complexes are known to have lesser cell permeability and, hence, may have lower bioavailability (Bernhard et al, 1998;Croteau et al, 2016;Drobot et al, 2015;Guillauont et al, 2003;Jones et al, 2015;Kulkarni, Misra, Gupta, Ballal, & Apte, 2016). In addition, adsorption of U(VI) to colloidal or particulate ligands should render it even less bioavailable (Du et al, 2011).…”

Genomic Characterization of a Mercury Resistant Arthrobacter sp. H-02-3 Reveals the Presence of Heavy Metal and Antibiotic Resistance Determinants

“…in SRS contaminated soils (Jaswal et al, 2019). This is mainly due to the fact that the soil-borne actinomycetes in the Arthrobacter genus are known for their metabolic ability to resist heavy metals including U and Ni (Tsuruta, 2007;Margesin and Schinner, 1996) and degrade a variety of xenobiotic compounds (O'Loughlin et al, 1999;Marx and Aitken, 2000;Juhas et al, 2009;Chen et al, 2016;Kulkarni et al, 2016). In fact, another Arthrobacter strain isolated in our lab from the SRS soils, SRS-W-1-2016, was found to contain a plethora of gene determinants including several gene homologs such as efflux pumps and MFS transporters, known to provide resistance against heavy metal/radionuclides, P-type ATPase translocators and heavy metal-responsive transcriptional regulators (Chauhan et al, 2018).…”

Efek Pelaparan dan Akumulasi Polifosfat terhadap Biopresipitasi Uranium pada Bacillus cereus A66

“…Aktivitas hidrolase organofosfat merupakan proses biomineralisasi yang mengandalkan peran gen PhoN (gen fosfatase asam) dan gen PhoK (gen fosfatase alkali). PhoN merupakan periplasmic acid phosphatase, sedangkan PhoK merupakan extracellular alkaline phosphatase [4]. Aktivitas enzim fosfatase ini juga telah banyak diteliti pada beberapa bakteri seperti Escherichia coli, Pseudomonas aeruginosa, Serratia sp., dan Acinetobacter sp.…”

“…Beberapa penelitian yang dipublikasikan menyatakan bahwa Pi hasil hidrolisis polifosfat akan disekresikan keluar sel sehingga dapat mempresipitasikan logam berat pada permukaan sel [24,14]. Akan tetapi dari hasil penelitian ini terdeteksi dalam larutan uranium sebagian besar Pi masih di dalam sel (Gambar 5a, 5b, 5c,dan 5d).…”