Harnessing plant-beneficial bacterial encapsulation: A sustainable strategy for facilitating cadmium bioaccumulation in Medicago sativa.

Plant-beneficial bacteria (PBB) have emerged as a promising approach for assisting phytoremediation of heavy metal (HM)-contaminated soils. However, their colonization efficiency is often challenged by complex soil environments. In this study, we screened one rhizobacterium (Klebsiella variicola Y38) and one endophytic bacterium (Serratia surfactantfaciens Y15) isolated from HM-contaminated soils and plants for their high resistance to Cd and strong growth-promoting abilities. These strains were encapsulated individually or in combination with alginate and applied with Medicago sativa in Cd-contaminated soil pot experiments. The effectiveness of different bacterial formulations in promoting plant growth and enhancing Cd bioconcentration in M. sativa was evaluated. Results showed that PBB application enhanced plant growth and antioxidant capacity while reducing oxidative damage. Encapsulated formulations outperformed unencapsulated ones, with combined formulations yielding superior results to individual applications. Quantitative PCR indicated enhanced PBB colonization in Cd-contaminated soils with alginate encapsulation, potentially explaining the higher efficacy of alginate-encapsulated PBB. Additionally, the bacterial agents modified Cd speciation in soils, resulting in increased Cd bioaccumulation in M. sativa by 217-337 %. The alginate-encapsulated mixed bacterial agent demonstrated optimal effectiveness, increasing the Cd transfer coefficient by 3.2-fold. Structural equation modeling and correlation analysis elucidated that K. variicola Y38 promoted Cd bioaccumulation in M. sativa roots by reducing oxidative damage and enhancing root growth, while S. surfactantfaciens Y15 facilitated Cd translocation to shoots, promoting shoot growth. The combined application of these bacteria leveraged the benefits of both strains. These findings contribute to diversifying strategies for effectively and sustainably remediating Cd-contaminated soils, while laying a foundation for future investigations into bacteria-assisted phytoremediation.

Acid-tolerant Pseudomonas citronellolis YN-21 exhibits a high heterotrophic nitrification capacity independent of the amo and hao genes.

Heterotrophic nitrifying bacteria are found to be promising candidates for implementation in wastewater treatment systems due to their tolerance to extreme environments. A novel acid-resistant bacterium, Pseudomonas citronellolis YN-21, was isolated and reported to have exceptional heterotrophic nitrification capabilities in acidic condition. At pH 5, the highest NH4+ removal rate of 7.84 mg/L/h was displayed by YN-21, which was significantly higher than the NH4+ removal rates of other strains in neutral and alkaline environments. Remarkably, a distinct accumulation of NH2OH and NO3- was observed during NH4+ removal by strain YN-21, while traditional amo and hao genes were not detected in the genome, suggesting the possible presence of alternative nitrifying genes. Moreover, excellent nitrogen removal performance was displayed by YN-21 even under high concentrations of metal ion stress. Consequently, a broad application prospect in the treatment of leather wastewater and mine tailwater is offered by YN-21.

Mitigating heavy metal accumulation in tobacco: Strategies, mechanisms, and global initiatives.

The threat of heavy metal (HM) pollution looms large over plant growth and human health, with tobacco emerging as a highly vulnerable plant due to its exceptional absorption capacity. The widespread cultivation of tobacco intensifies these concerns, posing increased risks to human health as HMs become more pervasive in tobacco-growing soils globally. The absorption of these metals not only impedes tobacco growth and quality but also amplifies health hazards through smoking. Implementing proactive strategies to minimize HM absorption in tobacco is of paramount importance. Various approaches, encompassing chemical immobilization, transgenic modification, agronomic adjustments, and microbial interventions, have proven effective in curbing HM accumulation and mitigating associated adverse effects. However, a comprehensive review elucidating these control strategies and their mechanisms remains notably absent. This paper seeks to fill this void by examining the deleterious effects of HM exposure on tobacco plants and human health through tobacco consumption. Additionally, it provides a thorough exploration of the mechanisms responsible for reducing HM content in tobacco. The review consolidates and synthesizes recent domestic and international initiatives aimed at mitigating HM content in tobacco, delivering a comprehensive overview of their current status, benefits, and limitations.

Phosphate-solubilizing bacteria: Their agroecological function and optimistic application for enhancing agro-productivity.

Phosphorus (P) is a limiting nutrient in the soil-plant nutrient cycling. Although the exogenous application of chemical P fertilizers can satisfy crop P requirements during critical growth phases. While excessive P fertilizers use results in low phosphorus acquisition efficiency (PAE), it has serious environmental consequences and hastens the depletion of P mineral reserves. Phosphate-solubilizing bacteria (PSB) have the potential to make insoluble phosphate available to plants through solubilization and mineralization, increasing crop yields while maintaining environmental sustainability. Existing reviews mainly focus on the beneficial effects of PSB on crop performance and related mechanisms, while few of them elucidate the action mechanisms of PSB in soil-microbe-plant interactions for crop cultivation with high yield efficiency. Hence, this study provides a comprehensive review of the physicochemical and molecular mechanisms (e.g., root exudates, extracellular polysaccharides, organic acids, phosphatases, and phosphate-specific transport systems) of PSB to facilitate the P cycle in the soil-plant systems. Further, the potential of commercial applications of PSB (e.g., genetic engineering, seed priming and coating) are also discussed in order to highlight their contribution to sustainable agriculture. Finally, existing challenges and future prospects in agricultural applications are proposed. In conclusion, we firmly believe that PSB represent a highly significant biotechnological tool for enhancing agricultural productivity and offers a wide range of extensive potential applications.

Acidovorax PSJ13, a novel, efficient polyacrylamide-degrading bacterium by cleaving the main carbon chain skeleton without the production of acrylamide.

Given the environmental challenge caused by the wide use of polyacrylamide (PAM), an environmental-friendly treatment method is required. This study demonstrates the role of Acidovorax sp. strain PSJ13 isolated from dewatered sludge in efficiently degrading PAM. To be specific, the strain PSJ13 can degrade 51.67% of PAM in 96 h (2.39 mg/(L h)) at 35 °C, pH 7.5 and 5% inoculation amount. Besides, scanning electron microscope, X-ray photoelectron spectroscopy, liquid chromatography-mass spectrometry and high-performance liquid chromatography were employed to analyze samples, and the nitrogen present in the degradation products was investigated. The results showed that the degradation of PAM by PSJ13 started from the side chain and then mainly the -C-C- main chain, which produced no acrylamide monomers. As the first study to report the role of Acidovorax in efficiently degrading PAM, this work may provide a solution for industries that require PAM management.

Characterization and metabolic pathway of Pseudomonas fluorescens 2P24 for highly efficient ammonium and nitrate removal.

The ammonium and nitrate removal performance and metabolic pathways of a biocontrol strain, Pseudomonas fluorescens 2P24, were investigated. Strain 2P24 could completely remove 100 mg/L ammonium and nitrate, with removal rates of 8.27 mg/L/h and 4.29 mg/L/h, respectively. During these processes, most of the ammonium and nitrate were converted to biological nitrogen via assimilation, and only small amounts of nitrous oxide escaped. The inhibitor allylthiourea had no impact on ammonium transformation, and diethyl dithiocarbamate and sodium tungstate did not inhibit nitrate removal. Intracellular nitrate and ammonium were detectable during the nitrate and ammonium transformation process, respectively. Moreover, the nitrogen metabolism functional genes (glnK, nasA, narG, nirBD, nxrAB, nirS, nirK, and norB) were identified in the strain. All results highlighted that P. fluorescens 2P24 is capable of assimilatory and dissimilatory nitrate reduction, ammonium assimilation and oxidation, and denitrification.

Dissimilatory nitrate reduction to ammonium in four Pseudomonas spp. under aerobic conditions.

Dissimilatory nitrate reduction to ammonium (DNRA) has an important role in soil nitrogen retention and is considered to be constrained to anaerobic conditions. However, a recent study found that Pseudomonas putida Y-9 is capable of DNRA under aerobic conditions. In this study, four species of Pseudomonas spp. were found to produce ammonium during the nitrite reduction process under aerobic conditions, similar to the Y-9 strain. The detectable ammonium in the culture supernatant during the nitrite reduction process for each of the four strains originated intracellularly. A subsequent 15N isotope experiment showed that these four strains were able to transform 15NO2 - to 15NH4 + in 3 h under aerobic conditions. The NirBD sequence in each of the four strains showed high similarity with that in the Y-9 strain (approximately 94.61%). Moreover, the nirBD sequences in the four strains and the Y-9 strain were all similar to those of other Pseudomonas spp., while they were relatively distant in terms of their phylogenetic relationship from those of other genera. Overall, these results suggest that these four strains of Pseudomonas spp. are capable of DNRA under aerobic conditions, which might be attributed to the existence of nirBD.

[The toxicity of ZnO and CuO nanoparticles on biological wastewater treatment and its detoxification: a review].

The wide use of ZnO and CuO nanoparticles in research, medicine, industry, and other fields has raised concerns about their biosafety. It is therefore unavoidable to be discharged into the sewage treatment system. Due to the unique physical and chemical properties of ZnO NPs and CuO NPs, it may be toxic to the members of the microbial community and their growth and metabolism, which in turn affects the stable operation of sewage nitrogen removal. This study summarizes the toxicity mechanism of two typical metal oxide nanoparticles (ZnO NPs and CuO NPs) to nitrogen removal microorganisms in sewage treatment systems. Furthermore, the factors affecting the cytotoxicity of metal oxide nanoparticles (MONPs) are summarized. This review aims to provide a theoretical basis and support for the future mitigating and emergent treatment of the adverse effects of nanoparticles on sewage treatment systems.

ß-Alanine Metabolism Leads to Increased Extracellular pH during the Heterotrophic Ammonia Oxidation of Pseudomonas putida Y-9.

The mechanisms underlying the increase in external pH caused by heterotrophic nitrification and aerobic denitrification microorganisms during ammonia oxidation were unclear. This work demonstrated that after culturing Pseudomonas putida Y-9 for 60 h in a medium with ammonium nitrogen as the sole nitrogen source at an initial pH of 7.20, the pH value increased to 9.21. GC-TOF-MS analysis was used to compare the significantly regulated metabolites and related metabolic pathways between different time points. The results showed that the consumption of H+ in the conversion of malonic acid to 3-hydroxypropionic acid in the ß-alanine metabolic pathway was the main reason for the increase in pH. RT-qPCR confirmed that the functional gene ydfG dominated the consumption of H+. This study provides new research ideas for the change of external pH caused by bacterial metabolism and further expands the understanding of the interaction between bacteria and the environment.

Optimistic contributions of plant growth-promoting bacteria for sustainable agriculture and climate stress alleviation.

Global climate change is the major cause of abiotic and biotic stresses that have adverse effects on agricultural productivity to an irreversible level, thus threatening to limit gains in production and imperil sustainable agriculture. These climate change-induced abiotic stresses, especially saline, drought, extreme temperature, and so on affect plant morphological, physiological, biochemical, and metabolic characteristics through various pathways and mechanisms, ultimately hindering plant growth, development, and productivity. However, overuse and other inappropriate uses of agrochemicals are not conducive to the protection of natural resources and the environment, thus hampering sustainable agricultural development. With the vigorous development of modern agriculture, the application of plant growth-promoting bacteria (PGPB) can better ensure sustainable agriculture, due to their ability to improve soil properties and confer stress tolerance in plants. This review deciphered the underlying mechanisms of PGPB involved in enhancing plant stress tolerance and performance under various abiotic and biotic stresses. Moreover, the recent advancements in PGPB inoculation techniques, the commercialization of PGPB-based technology and the current applications of PGPB in sustainable agriculture were extensively discussed. Finally, an outlook on the future directions of microbe-aided agriculture was pointed out. Providing insights into plant-PGPB interactions under biotic and abiotic stresses and offering evidence and strategies for PGPB better commercialization and implementation can inspire the development of innovative solutions exploiting PGPB under climatological conditions.

The nitrite reductase encoded by nirBDs in Pseudomonas putida Y-9 influences ammonium transformation.

It is unknown whether nirBDs, which conventionally encode an NADH nitrite reductase, play other novel roles in nitrogen cycling. In this study, we explored the role of nirBDs in the nitrogen cycling of Pseudomonas putida Y-9. nirBDs had no effect on organic nitrogen transformation by strain Y-9. The △nirBD strain exhibited higher ammonium removal efficiency (90.7%) than the wild-type strain (76.1%; P < 0.05) and lower end gaseous nitrogen (N2O) production. Moreover, the expression of glnA (control of the ammonium assimilation) in the △nirBD strain was higher than that in the wild-type strain (P < 0.05) after being cultured in ammonium-containing medium. Furthermore, nitrite noticeably inhibited the ammonium elimination of the wild-type strain, with a corresponding removal rate decreasing to 44.8%. However, no similar impact on ammonium transformation was observed for the △nirBD strain, with removal efficiency reaching 97.5%. In conclusion, nirBDs in strain Y-9 decreased the ammonium assimilation and increased the ammonium oxidation to nitrous oxide.

[Mechanism and Application of Plant Growth-Promoting Bacteria in Heavy Metal Bioremediation].

Heavy metal contamination is one of the main factors causing ecological and environmental degradation. Soil contamination by heavy metals decreases soil quality, reduces agricultural productivity and quality, and even threatens human health. Therefore, optimizing remediation strategies for soils polluted with heavy metals is of great significance for high-yield, good-quality, and sustainable agriculture. Numerous domestic and foreign scholars have carried out a large number of studies on the phytoremediation of heavy metal contaminated soils. However, the remediation efficiency may be restricted by soil and climatic/environmental conditions. The synergistic remediation of microorganisms and plants is considered an effective means to improve metal remediation efficiency under environmental stresses. Metal-resistant plant growth-promoting bacteria (PGPB) not only promote plant growth and its resistance to biotic (e.g., phytopathogens, etc.) and abiotic (e.g., drought, salinity, extreme temperatures, heavy metals, etc.) stresses but also alter meal bioavailability in soils and metal toxicity in plants, thereby improving phytoremediation efficiency. In this paper, the mechanisms involved in promoting plant growth and its stress tolerance, and affecting metal bioavailability by metal-resistant PGPB, were systematically summarized. Furthermore, research progress on the application of PGPB in ecological restoration in recent years was extensively reviewed.

Plant growth-promoting bacteria in metal-contaminated soil: Current perspectives on remediation mechanisms.

Heavy metal contamination in soils endangers humans and the biosphere by reducing agricultural yield and negatively impacting ecosystem health. In recent decades, this issue has been addressed and partially remedied through the use of "green technology," which employs metal-tolerant plants to clean up polluted soils. Furthermore, the global climate change enhances the negative effects of climatic stressors (particularly drought, salinity, and extreme temperatures), thus reducing the growth and metal accumulation capacity of remediating plants. Plant growth-promoting bacteria (PGPB) have been widely introduced into plants to improve agricultural productivity or the efficiency of phytoremediation of metal-contaminated soils via various mechanisms, including nitrogen fixation, phosphate solubilization, phytohormone production, and biological control. The use of metal-tolerant plants, as well as PGPB inoculants, should hasten the process of moving this technology from the laboratory to the field. Hence, it is critical to understand how PGPB ameliorate environmental stress and metal toxicity while also inducing plant tolerance, as well as the mechanisms involved in such actions. This review attempts to compile the scientific evidence on this topic, with a special emphasis on the mechanism of PGPB involved in the metal bioremediation process [plant growth promotion and metal detoxification/(im)mobilization/bioaccumulation/transformation/translocation] and deciphering combined stress (metal and climatic stresses) tolerance.

Certain Environmental Conditions Maximize Ammonium Accumulation and Minimize Nitrogen Loss During Nitrate Reduction Process by Pseudomonas putida Y-9.

Realizing the smallest nitrogen loss is a challenge in the nitrate reduction process. Dissimilatory nitrate reduction to ammonium (DNRA) and nitrate assimilation play crucial roles in nitrogen retention. In this study, the effects of the carbon source, C/N ratio, pH, and dissolved oxygen on the multiple nitrate reduction pathways conducted by Pseudomonas putida Y-9 are explored. Strain Y-9 efficiently removed nitrate (up to 89.79%) with glucose as the sole carbon source, and the nitrogen loss in this system was 15.43%. The total nitrogen decrease and ammonium accumulation at a C/N ratio of 9 were lower than that at 12 and higher than that at 15, respectively (P < 0.05). Besides, neutral and alkaline conditions (pH 7-9) favored nitrate reduction. Largest nitrate removal (81.78%) and minimum nitrogen loss (10.63%) were observed at pH 7. The nitrate removal and ammonium production efficiencies of strain Y-9 increased due to an increased shaking speed. The expression patterns of nirBD (the gene that controls nitrate assimilation and DNRA) in strain Y-9 were similar to ammonium patterns of the tested incubation conditions. In summary, the following conditions facilitated nitrate assimilation and DNRA by strain Y-9, while reducing the denitrification: glucose as the carbon source, a C/N ratio of 9, a pH of 7, and a shaking speed of 150 rpm. Under these conditions, nitrate removal was substantial, and nitrogen loss from the system was minimal.

Fe2+ Alleviated the Toxicity of ZnO Nanoparticles to Pseudomonas tolaasii Y-11 by Changing Nanoparticles Behavior in Solution.

The negative effect of ZnO nanoparticles (ZnO-NPs) on the biological removal of nitrate (NO3-) has received extensive attention, but the underlying mechanism is controversial. Additionally, there is no research on Fe2+ used to alleviate the cytotoxicity of NPs. In this paper, the effects of different doses of ZnO-NPs on the growth and NO3- removal of Pseudomonas tolaasii Y-11 were studied with or without Fe2+. The results showed that ZnO-NPs had a dose-dependent inhibition on the growth and NO3- removal of Pseudomonas tolaasii Y-11 and achieved cytotoxic effects through both the NPs themselves and the released Zn2+. The addition of Fe2+ changed the behavior of ZnO-NPs in an aqueous solution (inhibiting the release of toxic Zn2+ and promoting the aggregation of ZnO-NPs), thereby alleviating the poisonous effect of ZnO-NPs on the growth and nitrogen removal of P. tolaasii Y-11. This study provides a theoretical method for exploring the mitigation of the acute toxicity of ZnO-NPs to denitrifying microorganisms.

Kitchen waste hydrolysate enhances sewage treatment efficiency with different biological process compared with glucose.

Insufficient carbon source is the primary factor that limits biological nitrogen and phosphorus removal during sewage treatment. This study investigates the feasibility and biological process of kitchen waste hydrolysate (KWH) replacing glucose to improve pollutant removal efficiency. It was found that using KWH as carbon source achieved better removal effect than glucose during sewage treatment. And more than 96% of total nitrogen (TN), total phosphorus (TP), and the chemical oxygen demand were removed after 48 h of acclimation. Nitrogen and phosphorus introduced by adding KHW had no negative effect on the effluent quality. Compared with glucose, KWH decreased the diversity of bacteria and significantly promoted the accumulation of acid-producing bacteria (Propionibacterium) and denitrifying bacteria (Rhodobacteraceae). Moreover, KWH significantly improved the relative abundance of the amo A, nap A, and nos Z genes. This result further indicated that KWH was beneficial for denitrification and was a favorable external carbon source.

Low C/N Ratios Promote Dissimilatory Nitrite Reduction to Ammonium in Pseudomonas putida Y-9 under Aerobic Conditions.

The biogeochemical consequences of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) have a significant influence on nitrogen (N) cycling in the ecosystem. Many researchers have explored these two pathways in soil and sediment ecosystems under anaerobic conditions. However, limited information is available regarding the influence of external environmental conditions on these two pathways in a well-defined experimental system under aerobic conditions. In this study, the impacts of the external environmental factors (carbon source, C/N ratio, pH, and dissolved oxygen) on nitrite reduction through the denitrification and DNRA routes in Pseudomonas putida Y-9 were studied. Results found that sodium citrate and sodium acetate favored denitrification and DNRA, respectively. Furthermore, neutral pH and aerobic conditions both facilitated DNRA and denitrification. Especially, low C/N ratios motivated the DNRA while high C/N ratios stimulated the denitrification, which was opposite to the observed phenomena under anaerobic conditions.

3D Printed Porous Methacrylate/Silica Hybrid Scaffold for Bone Substitution.

Inorganic-organic hybrid biomaterials made with star polymer poly(methyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate) and silica, which show promising mechanical properties, are 3D printed as bone substitutes for the first time, by direct ink writing of the sol. Three different inorganic:organic ratios of poly(methyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate)-star-SiO2 hybrid inks are printed with pore channels in the range of 100-200 µm. Mechanical properties of the 3D printed scaffolds fall within the range of trabecular bone, and MC3T3 pre-osteoblast cells are able to adhere to the scaffolds in vitro, regardless of their compositions. Osteogenic and angiogenic properties of the hybrid scaffolds are shown using a rat calvarial defect model. Hybrid scaffolds with 40:60 inorganic:organic composition are able to instigate new vascularized bone formation within its pore channels and polarize macrophages toward M2 phenotype. 3D printing inorganic-organic hybrids with sophisticated polymer structure opens up possibilities to produce novel bone graft materials.

[Co-fermentation of kitchen waste and excess sludge for organic acid production: a review].

Resource utilization is an effective way to cope with the rapid increase of kitchen waste and excess sludge, and volatile fatty acids produced by anaerobic fermentation is an important way of recycling organic waste. However, the single substrate limits the efficient production of volatile fatty acids. In recent years, volatile fatty acids produced by anaerobic co-fermentation using different substrates has been widely studied and applied. In this paper, we analyze the characteristics of fermentation to produce acid using kitchen waste and excess sludge alone or mixture. Influences of environmental factors and microbial community structure on the type and yield of volatile fatty acids in the anaerobic fermentation system are discussed in detail. Moreover, we propose future research directions, to provide a reference for recycling kitchen waste and excess sludge.

Exogenous Fe2+ alleviated the toxicity of CuO nanoparticles on Pseudomonas tolaasii Y-11 under different nitrogen sources.

Extensive use of CuO nanoparticles (CuO-NPs ) inevitably leads to their accumulation in wastewater and toxicity to microorganisms that effectively treat nitrogen pollution. Due to the effects of different mediums, the sources of CuO-NPs-induced toxicity to microorganisms and methods to mitigating the toxicity are still unclear. In this study, CuO-NPs were found to impact the nitrate reduction of Pseudomonas tolaasii Y-11 mainly through the action of NPs themselves while inhibiting the ammonium transformation of strain Y-11 through releasing Cu2+. As the content of CuO-NPs increased from 0 to 20 mg/L, the removal efficiency of NO3 - and NH4 + decreased from 42.29% and 29.83% to 2.05% and 2.33%, respectively. Exogenous Fe2+ significantly promoted the aggregation of CuO-NPs, reduced the possibility of contact with bacteria, and slowed down the damage of CuO-NPs to strain Y-11. When 0.01 mol/L Fe2+ was added to 0, 1, 5, 10 and 20 mg/L CuO-NPs treatment, the removal efficiencies of NO3 - were 69.77%, 88.93%, 80.51%, 36.17% and 2.47%, respectively; the removal efficiencies of NH4 + were 55.95%, 96.71%, 38.11%, 20.71% and 7.43%, respectively. This study provides a method for mitigating the toxicity of CuO-NPs on functional microorganisms.