Isolation and identification of selenite reducing archaea from Tuz (salt) Lake In Turkey

Multi-pathways-mediated mechanisms of selenite reduction and elemental selenium nanoparticles biogenesis in the yeast-like fungus Aureobasidium melanogenum I15

Selenium (Se) plays a critical role in diverse biological processes and is widely used across manufacturing industries. However, the contamination of Se oxyanions also poses a major public health concern. Microbial transformation is a promising approach to detoxify Se oxyanions and produce elemental selenium nanoparticles (SeNPs) with versatile industrial potential. Yeast-like fungi are an important group of environmental microorganisms, but their mechanisms for Se oxyanions reduction remain unknown. In this study, we found that Aureobasidium melanogenum I15 can reduce 1.0 mM selenite by over 90% within 48 h and efficiently form intracellular or extracellular spherical SeNPs. Metabolomic and proteomic analyses disclosed that A. melanogenum I15 evolves a complicated selenite reduction mechanism involving multiple metabolic pathways, including the glutathione/glutathione reductase pathway, the thioredoxin/thioredoxin reductase pathway, the siderophore-mediated pathway, and multiple oxidoreductase-mediated pathways. This study provides the first report on the mechanism of selenite reduction and SeNPs biogenesis in yeast-like fungi and paves an alternative avenue for the bioremediation of selenite contamination and the production of functional organic selenium compounds.

Toxic heavy metal/oxyanion tolerance in haloarchaea from some saline and hypersaline ecosystems

Toxic heavy metal/oxyanion contamination has increased severely through the last decades. In this study, 169 native haloarchaeal strains were isolated from different saline and hypersaline econiches of Iran. After providing pure culture and performing morphological, physiological, and biochemical tests, haloarchaea resistance toward arsenate, selenite, chromate, cadmium, zinc, lead, copper, and mercury were surveyed using an agar dilution method. On the basis of minimum inhibitory concentrations (MICs), the least toxicities were found with selenite and arsenate, while the haloarchaeal strains revealed the highest sensitivity for mercury. On the other hand, the majority of haloarchaeal strains exhibited similar responses to chromate and zinc, whereas the resistance level of the isolates to lead, cadmium, and copper was very heterogeneous. 16 S ribosomal RNA (rRNA) gene sequence analysis revealed that most haloarchaeal strains belong to the Halorubrum and Natrinema genera. The obtained results from this study showed that among the identified isolates, Halococcus morrhuae strain 498 had an exceptional resistance toward selenite and cadmium (64 and 16 mM, respectively). Also, Halovarius luteus strain DA5 exhibited a remarkable tolerance against copper (32 mM). Moreover, strain Salt5, identified as Haloarcula sp., was the only strain that could tolerate all eight tested heavy metals/oxyanions and had a significant tolerance of mercury (1.5 mM).

Allotropy of selenium nanoparticles: Colourful transition, synthesis, and biotechnological applications

Elemental selenium (Se⁰ ) nanomaterials undergo allotropic transition from thermodynamically-unstable to more stable phases. This process is significantly different when Se⁰ nanoparticles (NPs) are produced via physico-chemical and biological pathways. While the allotropic transition of physico-chemically synthesized Se⁰ is fast (minutes to hours), the biogenic Se⁰ takes months to complete. The biopolymer layer covering biogenic Se⁰ NPs might be the main factor controlling this retardation, but this still remains an open question. Phylogenetically-diverse bacteria reduce selenium oxyanions to red amorphous Se⁰ allotrope, which has low market value. Then, red Se⁰ undergoes allotropic transition to trigonal (metallic grey) allotrope, the end product having important industrial applications (e.g. semiconductors, alloys). Is it not yet clear whether biogenic Se⁰ presents any biological function, or it is mainly a detoxification and respiratory by-product. The better understanding of this transition would benefit the recovery of Se⁰ NPs from secondary resources and its targeted utilization with respect to each allotropic stage. This review article presents and critically discusses the main physico-chemical methods and biosynthetic pathways of Se⁰ (bio)mineralization. In addition, the article proposes a conceptual model for the resource recovery potential of trigonal selenium nanomaterials in the context of circular economy.

Conversion of selenite by Haloferax alexandrinus GUSF-1 (KF796625) to pentagonal selenium nanoforms which in vitro modulates the formation of calcium oxalate crystals

AIM

To investigate the ability of Haloferax alexandrinus GUSF-1 (KF796625) to biosynthesize non-toxic elemental selenium (Se⁰ ) and check their capacity in in vitro crystal structure modulation of calcium oxalate, which are implicated in the development of renal calculi.

METHODS AND RESULTS

Haloferax alexandrinus GUSF-1 (KF796625) during growth in the presence of 5 mmol L^-1 of selenite formed insoluble brick-red particles. Se⁰ formed was monitored spectrophotometrically using a combination of two assays; the ascorbic acid reduction and sodium sulphide solubilization assay. After 168 h of growth, 2.89 mmol L^-1 of Se⁰ was formed from 4.9 mmol L^-1 of selenite. Absorption bands at 1.5, 11.2 and 12.5 keV in EDX spectroscopy confirmed that the brick-red particulate matter was Se⁰ . Furthermore, these selenium nanoparticles (SeNPs) were pentagonal in shape in transmission electron microscopy imaging. The peak positions in X-ray diffractogram at 2θ values of 23.40°, 29.66°, 41.26°, 43.68°, 45.24°, 51.62°, 55.93° and 61.47° and the relative intensities further confirmed the formation of Se⁰ . In vitro addition of 50 and 100 µg ml^-1 of these SeNPs to the mixture of sodium chloride, calcium chloride and sodium oxalate affected and modulated the shape and size of rectangular-shaped calcium oxalate crystals (average area of 1.23 ± 0.2 µm² ) to smaller rectangular-shaped crystals (average area of 0.54 ± 0.2 µm² ) and spherical-shaped crystals (average area 0.13 ± 0.005 µm² ).

CONCLUSION

Haloferax alexandrinus GUSF-1 (KF796625) transformed selenite to Se⁰ pentagonal nanoforms that modulated in vitro the formation of crystal shape and size of calcium oxalate.

SIGNIFICANCE AND IMPACT OF STUDY

There are no reports on conversion of selenite to Se⁰ among the Haloferax genera, and this study involving the formation of pentagonal SeNPs with capacity to modulate the formation of calcium oxalate crystals in haloarchaea is recorded as the first report and of significance in pharmaceutical research related to formulations abetting urinary calculi.

Respiratory Selenite Reductase from Bacillus selenitireducens Strain MLS10

The putative respiratory selenite [Se(IV)] reductase (Srr) from Bacillus selenitireducens MLS10 has been identified through a polyphasic approach involving genomics, proteomics, and enzymology. Nondenaturing gel assays were used to identify Srr in cell fractions, and the active band was shown to contain a single protein of 80 kDa. The protein was identified through liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a homolog of the catalytic subunit of polysulfide reductase (PsrA). It was found to be encoded as part of an operon that contains six genes that we designated srrE, srrA, srrB, srrC, srrD, and srrF SrrA is the catalytic subunit (80 kDa), with a twin-arginine translocation (TAT) leader sequence indicative of a periplasmic protein and one putative 4Fe-4S binding site. SrrB is a small subunit (17 kDa) with four putative 4Fe-4S binding sites, SrrC (43 kDa) is an anchoring subunit, and SrrD (24 kDa) is a chaperon protein. Both SrrE (38 kDa) and SrrF (45 kDa) were annotated as rhodanese domain-containing proteins. Phylogenetic analysis revealed that SrrA belonged to the PsrA/PhsA clade but that it did not define a distinct subgroup, based on the putative homologs that were subsequently identified from other known selenite-respiring bacteria (e.g., Desulfurispirillum indicum and Pyrobaculum aerophilum). The enzyme appeared to be specific for Se(IV), showing no activity with selenate, arsenate, or thiosulfate, with a K_m of 145 ± 53 μM, a V _max of 23 ± 2.5 μM min^-1, and a k _cat of 23 ± 2.68 s^-1 These results further our understanding of the mechanisms of selenium biotransformation and its biogeochemical cycle.IMPORTANCE Selenium is an essential element for life, with Se(IV) reduction a key step in its biogeochemical cycle. This report identifies for the first time a dissimilatory Se(IV) reductase, Srr, from a known selenite-respiring bacterium, the haloalkalophilic Bacillus selenitireducens strain MLS10. The work extends the versatility of the complex iron-sulfur molybdoenzyme (CISM) superfamily in electron transfer involving chalcogen substrates with different redox potentials. Further, it underscores the importance of biochemical and enzymological approaches in establishing the functionality of these enzymes.

Two selenium tolerant Lysinibacillus sp. strains are capable of reducing selenite to elemental Se efficiently under aerobic conditions

Microbes play important roles in the transport and transformation of selenium (Se) in the environment, thereby influencing plant resistance to Se and Se accumulation in plant. The objectives are to characterize the bacteria with high Se tolerance and reduction capacity and explore the significance of microbial origins on their Se tolerance, reduction rate and efficiency. Two bacterial strains were isolated from a naturally occurred Se-rich soil at tea orchard in southern Anhui Province, China. The reduction kinetics of selenite was investigated and the reducing product was characterized using scanning electron microscopy and transmission electron microscopy-energy dispersive spectroscopy. The bacteria were identified as Lysinibacillus xylanilyticus and Lysinibacillus macrolides, respectively, using morphological, physiological and molecular methods. The results showed that the minimal inhibitory concentrations (MICs) of selenite for L. xylanilyticus and L. macrolides were 120 and 220 mmol/L, respectively, while MICs of selenate for L. xylanilyticus and L. macrolides were 800 and 700 mmol/L, respectively. Both strains aerobically reduced selenite with an initial concentration of 1.0 mmol/L to elemental Se nanoparticles (SeNPs) completely within 36 hr. Biogenic SeNPs were observed both inside and outside the cells suggesting either an intra- or extracellular reduction process. Our study implied that the microbes from Se-rich environments were more tolerant to Se and generally quicker and more efficient than those from Se-free habitats in the reduction of Se oxyanions. The bacterial strains with high Se reduction capacity and the biological synthesized SeNPs would have potential applications in agriculture, food, environment and medicine.

Microbial Transformations of Selenium Species of Relevance to Bioremediation

Selenium species, particularly the oxyanions selenite (SeO3 (2-)) and selenate (SeO4 (2-)), are significant pollutants in the environment that leach from rocks and are released by anthropogenic activities. Selenium is also an essential micronutrient for organisms across the tree of life, including microorganisms and human beings, particularly because of its presence in the 21st genetically encoded amino acid, selenocysteine. Environmental microorganisms are known to be capable of a range of transformations of selenium species, including reduction, methylation, oxidation, and demethylation. Assimilatory reduction of selenium species is necessary for the synthesis of selenoproteins. Dissimilatory reduction of selenate is known to support the anaerobic respiration of a number of microorganisms, and the dissimilatory reduction of soluble selenate and selenite to nanoparticulate elemental selenium greatly reduces the toxicity and bioavailability of selenium and has a major role in bioremediation and potentially in the production of selenium nanospheres for technological applications. Also, microbial methylation after reduction of Se oxyanions is another potentially effective detoxification process if limitations with low reaction rates and capture of the volatile methylated selenium species can be overcome. This review discusses microbial transformations of different forms of Se in an environmental context, with special emphasis on bioremediation of Se pollution.