Bacterial strains found in the soils of a municipal solid waste dumping site facilitated phosphate solubilization along with cadmium remediation

Pathway to industrial application of heterotrophic organisms in critical metals recycling from e-waste

The transition to renewable energies and electric vehicles has triggered an unprecedented demand for metals. Sustainable development of these technologies relies on effectively managing the lifecycle of critical raw materials, including their responsible sourcing, efficient use, and recycling. Metal recycling from electronic waste (e-waste) is of paramount importance owing to ore-exceeding amounts of critical elements and high toxicity of heavy metals and organic pollutants in e-waste to the natural ecosystem and human body. Heterotrophic microbes secrete numerous metal-binding biomolecules such as organic acids, amino acids, cyanide, siderophores, peptides, and biosurfactants which can be utilized for eco-friendly and profitable metal recycling. In this review paper, we presented a critical review of heterotrophic organisms in biomining, and current barriers hampering the industrial application of organic acid bioleaching and biocyanide leaching. We also discussed how these challenges can be surmounted with simple methods (e.g., culture media optimization, separation of microbial growth and metal extraction process) and state-of-the-art biological approaches (e.g., artificial microbial community, synthetic biology, metabolic engineering, advanced fermentation strategies, and biofilm engineering). Lastly, we showcased emerging technologies (e.g., artificially synthesized peptides, siderophores, and biosurfactants) derived from heterotrophs with the potential for inexpensive, low-impact, selective and advanced metal recovery from bioleaching solutions.

Development and testing of environment friendly nanohybrid coatings for sustainable agriculture technologies

Less than 50% of the applied urea fertilizer is taken up by plants due to poor nitrogen (N) use efficiency which affects overall agricultural productivity and leads to serious environmental and economic problems. Additionally, soils with high salinity might limit zinc (Zn) availability. Low Zn use efficiency (<30%) when applied as synthetic salts, e.g., zinc sulfate has therefore minimized their applicability. Within the past two decades, nanotechnology has gained a lot of interest in the development of effective nano fertilizers with high nutrient use efficiency (NUE). In this perspective, the approach of coating conventional fertilizers with nano materials especially, the ones which are essential nutrients has researched because of their high use efficiency and reduced losses. In this work, a novel and innovative formulation of hybrid nano fertilizer has been prepared for the sustainable release of nutrients. Zinc oxide nanoparticles (ZnO-NPs <50 nm) were incorporated into the biodegradable polymer (gelatin) and coated on urea using a fluidized bed coater. Among all the formulations, GZnSNPs (1.5% gelatin+0.5% elemental Zn as ZnO-NPs) showed a significant delay in urea release (<80 %) after 120 min). The sand column experiment showed sustainable Zn release for GZnSNPs i.e., 2.7 ppm vs. 3.5 ppm (GZnS) after the 6th day. Moreover, a substantial increase in wheat grain yield (6500 kg/ha), N uptake (46.5 kg/ha) and Zn uptake (21.64 g/ha) were observed for fields amended with GZnSNPs. The composition of GZnSNPs was valuable since this attracted the highest return relative to the other treatments. Gelatin supplied small N-containing molecules, resulting in extra value addition with ZnO-NPs thus increasing yield and fertilizer properties more relative to the same amount of elemental Zn given via bulk salt. Therefore, the findings of the current study recommend the use of ZnO-NPs in the agricultural sector without any negative effects on yield and NUE.

Phosphate-Solubilizing Bacteria: Advances in Their Physiology, Molecular Mechanisms and Microbial Community Effects

Phosphorus is an essential nutrient for all life on earth and has a major impact on plant growth and crop yield. The forms of phosphorus that can be directly absorbed and utilized by plants are mainly HPO42- and H2PO4-, which are known as usable phosphorus. At present, the total phosphorus content of soils worldwide is 400-1000 mg/kg, of which only 1.00-2.50% is plant-available, which seriously affects the growth of plants and the development of agriculture, resulting in a high level of total phosphorus in soils and a scarcity of available phosphorus. Traditional methods of applying phosphorus fertilizer cannot address phosphorus deficiency problems; they harm the environment and the ore material is a nonrenewable natural resource. Therefore, it is imperative to find alternative environmentally compatible and economically viable strategies to address phosphorus scarcity. Phosphorus-solubilizing bacteria (PSB) can convert insoluble phosphorus in the soil into usable phosphorus that can be directly absorbed by plants, thus improving the uptake and utilization of phosphorus by plants. However, there is no clear and systematic report on the mechanism of action of PSB. Therefore, this paper summarizes the discovery process, species, and distribution of PSB, focusing on the physiological mechanisms outlining the processes of acidolysis, enzymolysis, chelation and complexation reactions of PSB. The related genes regulating PSB acidolysis and enzymatic action as well as genes related to phosphate transport and the molecular direction mechanism of its pathway are examined. The effects of PSB on the structure and abundance of microbial communities in soil are also described, illustrating the mechanism of how PSB interact with microorganisms in soil and indirectly increase the amount of available phosphorus in soil. And three perspectives are considered in further exploring the PSB mechanism in utilizing a synergistic multi-omics approach, exploring PSB-related regulatory genes in different phosphorus levels and investigating the application of PSB as a microbial fungicide. This paper aims to provide theoretical support for improving the utilization of soil insoluble phosphorus and providing optimal management of elemental phosphorus in the future.

Global Situation of Bioremediation of Leachate-Contaminated Soils by Treatment with Microorganisms: A Systematic Review

This systematic review presents the current state of research in the last five years on contaminants in soils, especially in leachates from solid waste landfills, with emphasis on biological remediation. In this work, the pollutants that can be treated by microorganisms and the results obtained worldwide were studied. All the data obtained were compiled, integrated, and analyzed by soil type, pollutant type, bacterial type, and the countries where these studies were carried out. This review provides reliable data on the contamination of soils worldwide, especially soils contaminated by leachate from municipal landfills. The extent of contamination, treatment objectives, site characteristics, cost, type of microorganisms to be used, and time must be considered when selecting a viable remediation strategy. The results of this study can help develop innovative and applicable methods for evaluating the overall contamination of soil with different contaminants and soil types. These findings can help develop innovative, applicable, and economically feasible methods for the sustainable management of contaminated soils, whether from landfill leachate or other soil types, to reduce or eliminate risk to the environment and human health, and to achieve greater greenery and functionality on the planet.

Development and testing of zinc sulfate and zinc oxide nanoparticle-coated urea fertilizer to improve N and Zn use efficiency

Nitrogen (N) losses from conventional fertilizers in agricultural systems are very high, which can lead to serious environmental pollution with economic loss. In this study, innovative slow-release fertilizers were prepared using zinc (Zn) [nanoparticles (NPs) or in bulk], using molasses as an environmentally friendly coating. Several treatments were prepared using Zn in different concentrations (i.e., 0.25%, 0.5%, and 4% elemental Zn). The zinc oxide nanoparticles (ZnO-NPs) were prepared from zinc sulfate heptahydrate (ZnSO₄·7H₂O), and were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Furthermore, the Zn-loaded urea samples were tested for urea N release rate, leaching of water from soil, and crushing strength to assess the impact of coating on the final finished product. Pot experiments were conducted simultaneously to check the agronomic effects of Zn-coated slow-release urea on the growth and development of wheat (Triticum aestivum L.). The laboratory and pot results confirmed that the ZnO-NP treatments boost wheat growth and yield as a result of reduced N and Zn release. UZnNPs2 (urea coated with 0.5% ZnO-NPs and 5% molasses) demonstrated the best results among all the treatments in terms of slow nutrient release, N and Zn uptake, and grain yield. The UZnNPs2 treatment increased plant yield by 34% (i.e., 4,515 vs. 3,345 kg ha^-1) relative to the uncoated prill-treated crop because of the slower release of Zn and N.

Potential of mineral-solubilizing bacteria for physiology and growth promotion of Chenopodium quinoa Willd

Nutrient deficiency in wild plant species, including quinoa (Chenopodium quinoa Willd), can be overcome by applying mineral-solubilizing bacteria. Quinoa is a gluten-free, nutritious food crop with unique protein content. The present study aimed to characterize mineral-solubilizing rhizobacterial strains and to evaluate their plant growth-promoting potential in quinoa seedlings. More than sixty rhizobacterial strains were isolated from the quinoa rhizosphere and found eighteen strains to be strong phosphate solubilizers. Most of these bacterial strains showed zinc solubilization, and more than 80% of strains could solubilize manganese. The selected strains were identified as Bacillus altitudinis Cq-3, Pseudomonas flexibilis Cq-32, Bacillus pumilus Cq-35, Pseudomonas furukawaii Cq-40, Pontibacter lucknowensis Cq-48, and Ensifer sp. Cq-51 through 16S rRNA partial gene sequencing. Mainly, these strains showed the production of organic acids, including malic, gluconic, tartaric, ascorbic, lactic, and oxalic acids in insoluble phosphorus amended broth. All strains showed production of gluconic acids, while half of the strains could produce malic, ascorbic, lactic, and oxalic acids. These strains demonstrated the production of indole-3-acetic acid in the presence as well as in the absence of L-tryptophan. The bacterial strains also demonstrated their ability to promote growth and yield attributes, including shoot length, root length, leave numbers, root and shoot dry biomass, spike length, and spikes numbers of quinoa in pots and field trials. Increased physiological attributes, including relative humidity, quantum flux, diffusive resistance, and transpiration rate, were observed due to inoculation with mineral solubilizing bacterial strains under field conditions. P. lucknowensis Cq-48, followed by P. flexibilis Cq-32, and P. furukawaii Cq-40 showed promising results to promote growth, yield, and physiological attributes. The multi-traits characteristics and plant growth-promoting ability in the tested bacterial strains could provide an opportunity for formulating biofertilizers that could promote wild quinoa growth and physiology.

Facile coating of micronutrient zinc for slow release urea and its agronomic effects on field grown wheat (Triticum aestivum L.)

Slow release urea has been widely tested in recent past as an effective method to enhance the crop productivity with fewer environmental concerns. However, very few research studies have been performed using micronutrients as a source of slow release of urea nitrogen. A laboratory and field study were carried out to check the agronomic effects of zinc oxide nanoparticles and its bulk salt coatings on urea prills on wheat (Triticum aestivum L.). Different concentrations of zinc oxide nanoparticles (0.25, 0.5 and 4% elemental zinc) were coated on urea prills to slow down the release rate. Bulk zinc oxide salt (ZnO) with similar concentrations was also used in parallel to make a comparison between nano and bulk salt. The SEM of zinc oxide nanoparticles clearly depicted zinc oxide nanoparticles size within a range of 50-90 nm. The XRD and FTIR spectrums also showed its characteristics peak at designated positions. Field study revealed than 0.5% zinc oxide nanoparticles coated urea boosted the crop growth and yield in comparison to the bulk zinc oxide coated urea having similar zinc concentrations, i.e., 0.25%, 0.5% and 4% elemental zinc. The plant parameters like plant height, root length, root volume, grain yield and dry matter weight were significantly increased due to application of zinc oxide nanoparticles.

A Potential Biofertilizer—Siderophilic Bacteria Isolated From the Rhizosphere of Paris polyphylla var. yunnanensis

The increasing demands for crop production have become a great challenge while people also realizing the significance of reductions in synthetic chemical fertilizer use. Plant growth-promoting rhizobacteria (PGPR) are proven biofertilizers for increasing crop yields by promoting plant growth via various direct or indirect mechanisms. Siderophilic bacteria, as an important type of PGPR, can secrete siderophores to chelate unusable Fe³⁺ in the soil for plant growth. Siderophilic bacteria have been shown to play vital roles in preventing diseases and enhancing the growth of plants. Paris polyphylla var. yunnanensis (PPVY) is an important traditional Chinese herb. However, reports about its siderophilic bacteria are still rare. This study firstly isolated siderophilic bacteria from the rhizosphere soil of PPVY, identified by morphological and physio-biochemical characteristics as well as 16S rRNA sequence analysis. The dominant genus in the rhizobacteria of PPVY was Bacillus. Among 22 isolates, 21 isolates produced siderophores. The relative amount of siderophores ranged from 4 to 41%. Most of the isolates produced hydroxamate siderophores and some produced catechol. Four isolates belonging to Enterobacter produced the catechol type, and none of them produced carboxylate siderophores. Intriguingly, 16 strains could produce substances that have inhibitory activity against Candida albicans only in an iron-limited medium (SA medium). The effects of different concentrations of Fe³⁺ and three types of synthetic chemical fertilizers on AS19 growth, siderophore production, and swimming motility were first evaluated from multiple aspects. The study also found that the cell-free supernatant (CFS) with high siderophore units (SUs) of AS19 strain could significantly promote the germination of pepper and maize seeds and the development of the shoots and leaves of Gynura divaricata (Linn.). The bacterial solution of AS19 strain could significantly promote the elongation of the roots of G. divaricata (Linn.). Due to its combined traits promoting plant growth and seed germination, the AS19 has the potential to become a bioinoculant. This study will broaden the application prospects of the siderophilic bacteria-AS19 as biofertilizers for future sustainable agriculture.