Genetic mechanisms for Se(VI) reduction and synthesis of trigonal 1-D nanostructures in Stenotrophomonas bentonitica: Perspectives in eco-friendly nanomaterial production and bioremediation

Identification of a multi-modal mechanism for Se(VI) reduction and Se(0) allotropic transition by Stenotrophomonas bentonitica

Microorganisms can play a key role in selenium (Se) bioremediation and the fabrication of Se-based nanomaterials by reducing toxic forms (Se(VI) and Se(IV)) into Se(0). In recent years, omics have become a useful tool in understanding the metabolic pathways involved in the reduction process. This paper aims to elucidate the specific molecular mechanisms involved in Se(VI) reduction by the bacterium Stenotrophomonas bentonitica. Both cytoplasmic and membrane fractions were able to reduce Se(VI) to Se(0) nanoparticles (NPs) with different morphologies (nanospheres and nanorods) and allotropes (amorphous, monoclinic, and trigonal). Proteomic analyses indicated an adaptive response against Se(VI) through the alteration of several metabolic pathways including those related to energy acquisition, synthesis of proteins and nucleic acids, and transport systems. Whilst the thioredoxin system and the Painter reactions were identified to play a crucial role in Se reduction, flagellin may also be involved in the allotropic transformation of Se. These findings suggest a multi-modal reduction mechanism is involved, providing new insights for developing novel strategies in bioremediation and nanoparticle synthesis for the recovery of critical materials within the concept of circular economy.

Reduction of selenite to selenium nanoparticles by highly selenite-tolerant bacteria isolated from seleniferous soil

The microbial reduction of selenite to elemental selenium nanoparticles (SeNPs) is thought to be an effective detoxification process of selenite for many bacteria. In this study, Metasolibacillus sp. ES129 and Oceanobacillus sp. ES111 with high selenite reduction efficiency or tolerance were selected for systematic and comparative studies on their performance in selenite removal and valuable SeNPs recovery. The kinetic monitoring of selenite reduction showed that the highest transformation efficiency of selenite to SeNPs was achieved at a concentration of 4.24 mM for ES129 and 4.88 mM for ES111. Ultramicroscopic analysis suggested that the SeNPs produced by ES111 and ES129 had been formed in cytoplasm and subsequently released to extracellular space through cell lysis process. Furthermore, the transcriptome analysis indicated that the expression of genes involved in bacillithiol biosynthesis, selenocompound metabolism and proline metabolism were significantly up-regulated during selenite reduction, suggesting that the transformation of selenite to Se0 may involve multiple pathways. Besides, the up-regulation of genes associated with nucleotide excision repair and antioxidation-related enzymes may enhance the tolerance of bacteria to selenite. Generally, the exploration of selenite reduction and tolerance mechanisms of the highly selenite-tolerant bacteria is of great significance for the effective utilization of microorganisms for environmental remediation.

Amorphous structure and crystal stability determine the bioavailability of selenium nanoparticles

Microorganisms play a critical role in the biogeochemical cycling of selenium, often reducing selenite/selenate to elemental selenium nanoparticles (SeNPs). These SeNPs typically exist in an amorphous structure but can transform into a trigonal allotrope. However, the crystal structural transition process and its impact on selenium bioavailability have not been well studied. To shed light on this, we prepared chemosynthetic and biogenic SeNPs and investigated the stability of their crystal structure. We found that biogenic SeNPs exhibited a highly stable amorphous structure in various conditions, such as lyophilization, washing, and laser irradiation, whereas chemosynthetic SeNPs transformed into a trigonal structure in the same conditions. Additionally, a core-shell structure was observed in biogenic SeNPs after electron beam irradiation. Further analysis revealed that biogenic SeNPs showed a coordination reaction between Se atoms and surface binding biomacromolecules, indicating that the outer layer of Se-biomacromolecules complex prevented the SeNPs from crystallizing. We also investigated the effects of SeNPs crystal structures on the bioavailability in bacteria, yeast, and plants, finding that the amorphous structure of SeNPs determined Se bioavailability.

Simultaneous bioreduction of tellurite and selenite by Yarrowia lipolytica, Trichosporon cutaneum, and their co-culture along with characterization of biosynthesized Te-Se nanoparticles

BACKGROUND

Natural and anthropogenic activities, such as weathering of rocks and industrial processes, result in the release of toxic oxyanions such as selenium (Se) and tellurium (Te) into the environment. Due to the high toxicity of these compounds, their removal from the environment is vital.

RESULTS

In this study, two yeast strains, Yarrowia lipolytica and Trichosporon cutaneum, were selected as the superior strains for the bioremediation of tellurium and selenium. The reduction analyses showed that exposure to selenite induced more detrimental effects on the strains compared to tellurite. In addition, co-reduction of pollutants displayed almost the same results in selenite reduction and more than ~ 20% higher tellurite reduction in 50 h, which shows that selenite triggered higher tellurite reduction in both strains. The selenite and tellurite kinetics of removal were consistent with the first-order model because of their inhibitory behavior. The result of several characterization experiments, such as FE-SEM (Field emission scanning electron microscopy), dynamic light scattering (DLS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffractometer (XRD), and dispersive X-ray (EDX) on Te-Se nanoparticles (NPs) revealed that the separated Te-Se NPs were needle-like, spherical, and amorphous, consisted of Te-Se NPs ranging from 25 to 171 nm in size, and their surface was covered with different biomolecules.

CONCLUSIONS

Remarkably, this work shows, for the first time, the simultaneous bioreduction of tellurite and selenite and the production of Te-Se NPs using yeast strains, indicating their potential in this area, which may be applied to the nanotechnology industry and environmental remediation.