The removal of selenite and cadmium by immobilized biospheres: Efficiency, mechanisms and bacterial community

Transcriptome sequencing analysis of gene expression in phosphate-solubilizing bacterium 'N3' and grafted watermelon plants coping with toxicity induced by cadmium

Cadmium (Cd) is a harmful metal in soil, and reducing Cd accumulation in plants has become a vital prerequisite for maintaining food safety. Phosphate-solubilizing bacteria (PSB) can not only improve plant growth but also inhibit the transportation of metals to roots. However, data on gene expression in PSB Burkholderia sp. strain 'N3' and grafted watermelon plants dealing with Cd remain to be elucidated. In this study, core genes and metabolic pathways of strain 'N3' and grafted plants were analyzed by Illumina sequencing. Results showed that 356 and 2527 genes were upregulated in 'N3' and grafted watermelon plants, respectively, whereas 514 and 1540 genes were downregulated in 'N3' and grafted watermelon plants, respectively. Gene ontology enrichment analysis showed that signal transduction, inorganic ion transport, cell motility, amino acid transport, and metabolism pathways were marked in 'N3'. However, pathways such as secondary metabolite biosynthesis, oxidation-reduction process, electron transfer activity, and channel regulator activity were marked in the grafted plants. Six genes related to pentose phosphate, glycolysis, and gluconeogenesis metabolism were upregulated in the grafted plants. This study paves the way for developing potential strategies to improve plant growth under Cd toxicity.

Multifaceted synergistic facilitation mechanism of conductive polymers in promoting selenite bioreduction and biological detoxification

Here, polypyrrole (PPY) was first used to the bioreduction of toxic selenite, while the acceleration effect and mechanism were explored. Experiment results suggested that PPY could enhance the selenite bioreduction from 0.42 to 1.04 mg/(L·h). The tests of electrochemical analysis and cytochrome c (cyt-c) content confirmed that PPY promoted the intracellular/intracellular electron transfer of Shewanella oneidensis·MR-1 in selenite bioreduction process. The enhancement of metabolic activity by PPY contributed to biological detoxification, which was manifested in the increased extracellular polymeric substances (EPS), adenosine triphosphate (ATP), electron transfer system activity (ETSA), membrane permeability and enzyme activity. Transcriptome analysis of DEGs, KEGG pathway enrichment and GO functional classification verified that the environmental adaptability of Shewanella oneidensis·MR-1 was enhanced with the addition of PPY. The transmission electron microscopy (TEM) images indicated that PPY promoted the biosynthesis of selenium nanoparticles (SeNPs), which was beneficial to reduce cell damage. Combined with the above results, a multifaceted synergistic facilitation mechanism based on "conductive cross-linking network" was elaborated from electron transfer, microbial metabolism and environmental adaptability. This study shed light the effect of conductive polymers (CPs) on selenite bioreduction and provided new insights into the bioremediation of toxic pollutants.

Effect of goethite-loaded montmorillonite on immobilization of metal(loid)s and the micro-ecological soil response in non-ferrous metal smelting areas

In this work, the immobilization stabilization and mechanism of heavy metal(loid)s by goethite loaded montmorillonite (GMt) were investigated, and the soil microbial response was explored. The simulated acid rain leaching experiment showed that GMt had a higher acid tolerance and the more stable heavy metal(loid)s fixation ability. The soil incubation demonstrated that GMt significantly decreased the available Cd, Zn, Pb and As concentration. Interestingly, higher immobilization of heavy metals was observed by GMt in highly acid leached and acidic soils. The richness and diversity of bacterial communities improved after the addition of GMt. GMt induced the enrichment of the excellent functional bacteria of the phylum Proteobacteria as well as the genus Massilia and Sphingomonas. The main immobilization mechanisms of heavy metal(loid)s by GMt include electrostatic interaction, complexation, precipitation and oxidation. The addition of the GMt also optimizes the soil bacterial community structure, which further facilitates the immobilization of heavy metal(loid)s. Our results confirm that the novel GMt has a promising application in the immobilization and stabilization of heavy metal(loid)s contaminated soils in non-ferrous metal smelting areas.

Soil addition of MnSO4 reduces wheat Cd accumulation by simultaneously increasing labile Mn and decreasing labile Cd concentrations in calcareous soil: A two-year pot study

Cadmium (Cd) pollution of wheat fields is a serious environmental and health problem that warrants attention. Manganese (Mn)-containing materials are considered effective for inhibiting Cd accumulation in Cd-contaminated acidic soils. However, information on the long-term remediation effects of Mn fertilizers on Cd accumulation in wheat and on the microbial community in calcareous soils remain limited. Here, a two-year pot experiment was conducted to assess the performance of 0.05-0.2% MnSO₄ addition in Cd-contaminated calcareous soils (total Cd concentration: 3.65 mg/kg) on Cd accumulation in wheat as well as on the soil bacterial community. The formation of Mn oxides and transformation of exchangeable Cd to stable Cd fractions confirmed that the application of MnSO₄ significantly decreased CaCl₂-extractable Cd concentrations in soil (0-47.08%). In addition, MnSO₄ addition improved the antagonistic effect of Cd and Mn ions in the wheat rhizosphere by increasing the available Mn concentration in the soil (1.04-3.52 times), thereby significantly reducing wheat Cd accumulation by 24.66-54.70%. Notably, the addition of MnSO₄ did not affect the richness and diversity (P > 0.05) but altered the composition and function of bacterial communities, especially those involved in metabolism and genetic information processing. Importantly, the effects of MnSO₄ on Cd immobilization in soil (10.66-47.08%) and the inhibition of Cd accumulation in wheat (12.13-54.30%) can last for two years after one addition. Furthermore, the maximum decrease in Cd concentration in grains was found in the low-Cd wheat cultivar, with values of 31.39-54.70% and 19.94-54.30% in the first and second years, respectively. Based on the present findings, the combination of MnSO₄ with a low-Cd wheat cultivar is effective for the safe utilization of Cd-contaminated calcareous soils.

Selenite elimination via zero-valent iron modified biochar synthesized from tobacco straw and copper slag: Mechanisms and agro-industrial practicality

Cost-effectively improving the performance of biochar is essential for its large-scale practical application. In this work, the agro-industrial by-products copper slag and tobacco straw were employed for the preparation of modified biochar (CSBC). The obtained CSBC exhibited satisfactory capacity on Se(IV) immobilization of 190.53 mg/g, with surface interactions determined by the monolayer and mainly chemisorption. The removal mechanisms included chemical reduction, electrostatic attraction, co-precipitation, and formation of complexations. Interestingly, the existence of Cu2Se structure after adsorption indicated the involvement of Cu species within Se(IV) elimination. Moreover, the industrial agricultural practicality of CSBC was evaluated by regeneration tests, economic assessment, and pot experiments. The results demonstrate that iron species-modified biochar prepared from two agro-industrial by-products is a promising and feasible candidate for selenite removal from wastewater.

Genomic insights from Paraclostridium bifermentans HD0315_2: General features and pathogenic potential

Background

Paraclostridium bifermentans is the most diverse distributed species of Paraclostridium and can cause fatal human infections under rare conditions. However, its pathogenic mechanisms and adaptation ability behind infections remain unclear. Herein, we reported the complete genome sequence of P. bifermentans HD0315_2 isolated from the feces of a patient with Crohn's disease. Then, we performed genomic analyses to understand its pathogenic mechanisms and adaptation ability.

Results

The de novo assembly revealed that the HD0315_2 strain carried a circular chromosome of 3.27 Mb and six circular plasmids (19.41 to 139.50 kb). The phylogenomic analysis assigned the HD0315_2 strain as P. bifermentans and reclassified some previously non-P. bifermentans strains into this clade. The general genomic features showed that this species harbored a flexible genomic pool characterized by variable genome length and multiple plasmids. Then, the HD0315_2 strain was predicted as a human pathogen with high probability, and Listeria LIPI-1 virulence proteins were identified on its genome. Besides, abundant antibiotics/metal/stress resistant genes, such as asrABCH, cat, mccF, macB, entS, albA, bcrA, and tetB, were carried by either the genome or the plasmids. Furthermore, we proposed that transposase-directed horizontal gene transfer was responsible for the distribution of multiple copies of the hin gene in the plasmids.

Conclusion

The flexible genomic pool of P. bifermentans encodes abundant functions for antimicrobial or oxidative stress resistance, helping it successfully inhabit and adapt to diverse environments. Moreover, P. bifermentans HD0315_2 might infect hosts via a Listeria LIPI-1-like cycle, with the help of a plasmid expressing the Hin DNA invertase to evade host immune responses.