Lysobacter may drive the hormetic effects of Pb on soil alkaline phosphatase

ABA-regulated MAPK signaling pathway promotes hormesis in sugar beet under cadmium exposure

Sugar beet (Beta vulgaris L.) shows potential as an energy crop for cadmium (Cd) phytoremediation. To elucidate its in vivo response strategy to Cd exposure, seedlings were treated with 1, 3, and 5 mmol/L CdCl2 (Cd-1, Cd-3, and Cd-5) for 6 h, using 0 mmol/L CdCl2 (Cd-0) as the control. The results showed that Cd-3 promoted a unique "hormesis" effect, leading to superior growth performance, increased levels of chlorophyll, soluble protein, and SOD activity, and reduced MDA content in sugar beet, compared to Cd-1, Cd-5, and even Cd-0. GO and KEGG enrichments and PPI networks of transcriptomic analysis revealed that the differentially expressed genes (DEGs) were primarily involved in lipid metabolism, cellular protein catabolism, and photosynthesis. Notably, the MAPK signaling pathway was significantly enriched only under Cd-3, with the up-regulation of ABA-related core gene BvPYL9 and an increase in ABA content after 6 h of Cd exposure. Furthermore, overexpression of BvPYL9 in Arabidopsis thaliana (OE-1 and OE-2) resulted in enhanced growth (fresh weight, dry weight, and root length), as well as higher ABA and soluble protein contents under different Cd treatments. Cd-induced transcriptional responses of BvPYL9 were also evident in OE-1 and OE-2, especially at 10 µmol/L, indicated by qRT-PCR. These findings suggest that ABA-mediated MAPK signaling pathway is activated in response to Cd toxicity, with BvPYL9 being a key factor in the cascade effects for the Cd-induced hormesis in sugar beet.

The succession of microbial community and distribution resistance gene in response to enrichment cultivation derived from a long-term toxic metal(loid)s polluted soil

Microbial communities as the most important and active component of soil play a crucial role in the geochemical cycling of toxic metal(loid)s in the Pb and Zn smelting site soils. However, the relationships between soil microbial communities and the fractions of toxic metal(loid)s and the succession of soil microbial community and functions after enrichment cultivation have rarely been analyzed. In this study, the diversity and composition of microbial communities in soils before and after enrichment cultivation were investigated by high-throughput sequencing. And the co-occurrence relationships between soil microbial community after enrichment cultivation and MRGs genes were also analyzed through the BacMet database. Results showed that the dominant genus in the soils was Lactobacillus and Stenotrophomonas. The soil microbial community exhibited a notable correlation with Cd, Pb, and As, among which Cd exerted the most profound impact. Alishewanella, Pseudomonas, Massilia and Roseibacillus were significantly correlated with the fraction of Cd. After enrichment cultivation, the number of genera decrease to 96. And the dominant genus changed to Acinetobacter, Bacillus, Comamonas, Lysobacter, and Pseudoxanthomonas. High abundance of metal resistance genes (MRGs) including zntA, fpvA, zipB, cadA, czcA, czcB, czcC, zntA, arsR, pstS and pstB was found in the microbial community after enrichment cultivation. The potential host genus for MRGs was Acinetobacter, Comamonas, Lysinibacillus, Azotobacter, Bacillus, Lysobacter, Cupriavidus, Pseudoxanthomonas, and Thermomonas. Additionally, these microbial community after enrichment cultivation possessing pathways of bacterial chemotaxis and two-component systems was enabled them to adapt to the polluted environment. These observations provided potential guidance for microbe isolation and the development of strategies for the bioremediation of toxic metal(loid)s polluted soils.

©Relationship between endophytic fungal diversity and colonization and soil factors of cultured blueberry roots in Guizhou Province, Southwest China

Dark septate endophytes (DSEs) inhabit plant roots and soil in ecosystems and host plants worldwide. DSE colonization is influenced by cultivars, soil factors, and specific habitat conditions. The regular diversity of DSEs in blueberries in Guizhou, China, is still unclear. In this study, four cultivars (Gardenblue, Powderblue, O'Neal, and Legacy) in three areas (Gaopo, Majiang, and Fenggang) in Guizhou were used to identify DSEs by morphological and molecular biological methods and to clarify the relationship between DSE diversity and DSE colonization and soil factors of cultivated blueberries in Guizhou. The DSEs isolated from cultivated blueberry roots in 3 areas in Guizhou Province were different, belonging to 17 genera, and the dominant genera were Penicillium, Phialocephala, and Thozetella. DSEs isolated from Majiang belonged to 12 genera and 16 species, those from Gaopo belonged to 7 genera and 15 species, and those from Fenggang belonged to 5 genera and 7 species. Among the different blueberry varieties, 11 genera were isolated from O'Neal, 12 genera were isolated from Powderblue, 11 genera were isolated from Legacy and 13 genera were isolated from Gardenblue. Coniochaeta is endemic to O'Neal, Chaetomium and Curvularia are endemic to Powderblue, and Thielavia is endemic to Legacy. Correlation analysis showed that DSE diversity was significantly correlated with DSE colonization and soil factors.

Induced mechanism of phosphatase hormesis by Cd ions and rhizosphere metabolites of Trifolium repens L

Rhizosphere phosphatases can exhibit hormetic effects in response to cadmium (Cd) ion stimulation. However, understanding the mechanisms underlying hormesis effects on soil ecosystems is challenging as studies on hormesis are usually specific to an organism, cell, or organ. To comprehensively investigate the mechanism of phosphatase hormesis, this study utilized in situ zymography and metabolomics to analyze the rhizosphere of Trifolium repens L. (white clover). Zymograms showed that rhizosphere phosphatase displayed a hormetic effect in 10 mg kg-1 Cd contaminated soil, with a hotspot area 1.8 times larger than non-Cd contaminated soil and a slight increase in enzyme activity. Nevertheless, the phosphatase activity was substantially suppressed upon elevating the Cd concentration in the soil to 50 mg kg-1. Differential metabolite identification and KEEG pathway enrichment analysis revealed that both rhizosphere organic acids and amino acid compounds positively affected phosphatase activity, and both were able to stabilize complexation with Cd ions via carboxyl groups. Besides, molecular docking models suggested that Cd ions act as cofactors to induce the formation of hydrogen bonds between amino acids/organic acids and phosphatase residues to form a triplet complex with a more stable structure, thereby improving phosphatase activity. The results indicated that amino acids and organic acids are heavily enriched in the rhizosphere of white clover and form a particular structure with soil Cd ions and phosphatase, which is essential for inducing the phosphatase hormesis as a detoxification mechanism in the rhizosphere micro-ecosystem.

The role of the ABF1 gene in regulation of Cd-induced hormesis in Arabidopsis thaliana

Hormesis is important in plant performance in contaminated environments, but the underlying genetic mechanisms are poorly understood. This study aimed at mining key genes in regulating Cd-induced hormesis in Arabidopsis thaliana and verifying their biological function. Hormesis of fresh weight, dry weight, and root length occurred at concentrations of 0.003-2.4, 0.03-0.6, and 0.03-0.6 µM Cd, respectively. Superoxide dismutase and catalase activities, and chlorophyll content displayed inverted U-shaped curves, indicating that the antioxidant defense system and photosynthesis system played roles in hormesis. Based on KEGG pathway analysis with the trend chart of differentially expressed genes and weighted correlation network analysis, the key gene ABF1 in the metabolic pathway of abscisic acid was identified. Subsequently, genetic experiments with wild, overexpressing, and knockdown lines of A. thaliana were conducted to further verify the biological function of ABF1 involving Cd-induced hormesis in A. thaliana. The results revealed that the resistance capability of the overexpressing type to Cd stress was significantly enhanced and implicated that the ABF1 gene is essential for Cd-induced hormesis in A. thaliana. Mining key genes that regulate Cd-induced hormesis in plants and stimulate them could have a transformative impact on the phytoremediation of metal-contaminated environments.

Bio-organic fertilizer promoted phytoremediation using native plant leymus chinensis in heavy Metal(loid)s contaminated saline soil

Heavy metal(loid)s (HMs) contaminated saline soil appeared around the world, however, remediation regarding these collected from field conditions remains unknown. Native plants cultivation and bio-organic fertilizer (BOF) application were two efficient tools for soil amelioration. Herein, a pot experiment was conducted to examine the feasibility of a native plant (Leymus chinensis) for phytoremediation, and investigate the impacts of lignite based bio-organic fertilizer (LBOF) and manure based bio-organic fertilizer (MBOF) on phytoremediation of the soil contaminated by Pb, Cd, As, Zn, Cu, Ca²⁺, and SO₄^2-. The results demonstrated the effectiveness of L. chinensis and highlighted the positive impacts of BOF according to the improved plant growth, HMs phytostabilization, salt removal, and soil properties. LBOF and MBOF changed soil microbiome to assist phytoremediation in addition to physiological modulation. Having enhanced fungal and bacterial richness respectively, LBOF and MBOF recruited various plant growth promoting rhizobacteria with different functions, and shifted microbial co-occurrence networks and keystone taxa towards these different but beneficial forms. Structural equation models comprehensively reveled the strategy discrepancy of LBOF and MBOF to regulate the plant biomass, HMs uptake, and soil salt. In summary, L. chinensis coupled with BOF, especially LBOF, was a effective strategy to remediate HMs contaminated saline soil.

Selenium and Bacillus proteolyticus SES synergistically enhanced ryegrass to remediate Cu-Cd-Cr contaminated soil

Heavy metal compound contaminated soil is an ecological threat, and soil containing copper (Cu), cadmium (Cd) and chromium (Cr) simultaneously is widely distributed. The application of phytoremediation in heavy metal combined contamination is still limited. In this study, to explore whether and how exogenous selenium (Se) and Bacillus proteolyticus SES enhance the remediation of combined Cu-Cd-Cr contaminated soil by ryegrass, pot experiments were carried out. Se alone or in combination with B. proteolyticus SES treatment increased the removal rates of heavy metals in the rhizosphere soil by 17.38%-157.25% relative to the control, while Se + B. proteolyticus SES treatment played a greater role in improving the heavy metals tolerance of ryegrass and increasing the activity of soil acid phosphatase. Moreover, Se and B. proteolyticus SES favored the preferential recruitment of specific taxa with the capacity of plant growth promotion and heavy metals resistance to the rhizosphere. The rhizosphere soil of Se treatment was specifically enriched with Lysobacter, Rhodanobacter, Micrococcales, Paenarthrobacter, and Adhaeribacter, while from class Bacilli to genus Bacillus enriched extensively and specifically in the rhizosphere of B. proteolyticus SES + Se treatment. Furthermore, five functional beneficial rhizosphere microbes including: Microbacterium sp., Pseudomonas extremaustralis, Bacillus amyloliquefaciens, Priestia megaterium, and Bacillus subtilis were isolated from the two treatments with the best remediation effect and synthetic communities (SynComs) were constructed. SynComs inoculation experiment further demonstrated the role of specific beneficial microbes in regulating the bioavailability of heavy metals. Results revealed that Se supplementation efficiently facilitated the phytoextraction of combined Cu-Cd-Cr contaminated soil, and B. proteolyticus SES inoculation showed the synergistical enhancement effect in the presence of Se.

Effects of different fertilization methods on Lolium multiflorum Lam. growth and bacterial community in waste slag

Waste slag has low nutrient content, so it has insufficient nutrient cycling and transformation in the soil ecosystem. There are few studies on the application of oligotrophic phosphate-solubilizing bacteria and phosphate (P) fertilizer to improve the properties of waste slags. In this study, three oligotrophic bacterial strains with P solubilizing activity, namely, Bacillus subtilis 2C (7.23 μg/mL), Bacillus subtilis 6C (4.07 μg/mL), and Bacillus safensis 2N (5.05 μg/mL), were isolated from waste slags. In the pot experiment, compared with no application of P fertilizer, inoculation of Bacillus subtilis 2C with a 50% recommended dose of P fertilizer significantly increased the available phosphorus (AP), total phosphorus (TP), and total nitrogen (TN) in slag by 33.16%, 76.70%, and 233.33%, respectively. The N, P uptake and fresh weight of Lolium multiflorum Lam. were significantly improved by 114.15%, 139.02%, and 100%, respectively. The analysis of the bacterial community showed that the application of P fertilizer decreased the diversity and richness of the bacterial community, and with the addition of phosphorus fertilizer and Bacillus subtilis 2C, the bacterial community in the slag developed towards eutrophication. Redundancy analysis (RDA) showed that the TP content in the slag was significantly correlated with the bacterial community (P = 0.001, < 0.01), followed by the TN content. This study on different P fertilizer application methods can provide some basic ideas for improving the performance of waste slag.

Exopolysaccharide-producing bacteria enhanced Pb immobilization and influenced the microbiome composition in rhizosphere soil of pakchoi (Brassica chinensis L.)

Lead (Pb) contamination of planting soils is increasingly serious, leading to harmful effects on soil microflora and food safety. Exopolysaccharides (EPSs) are carbohydrate polymers produced and secreted by microorganisms, which are efficient biosorbent materials and has been widely used in wastewater treatment to remove heavy metals. However, the effects and underlying mechanism of EPS-producing marine bacteria on soil metal immobilization, plant growth and health remain unclear. The potential of Pseudoalteromonas agarivorans Hao 2018, a high EPS-producing marine bacterium, to produce EPS in soil filtrate, immobilize Pb, and inhibit its uptake by pakchoi (Brassica chinensis L.) was studied in this work. The effects of strain Hao 2018 on the biomass, quality, and rhizospheric soil bacterial community of pakchoi in Pb-contaminated soil were further investigated. The results showed that Hao 2018 reduced the Pb concentration in soil filtrate (16%–75%), and its EPS production increased in the presence of Pb2+. When compared to the control, Hao 2018 remarkably enhanced pakchoi biomass (10.3%–14.3%), decreased Pb content in edible tissues (14.5%–39.2%) and roots (41.3%–41.9%), and reduced the available Pb content (34.8%–38.1%) in the Pb-contaminated soil. Inoculation with Hao 2018 raised the pH of the soil, the activity of several enzymes (alkaline phosphatase, urease, and dehydrogenase), the nitrogen content (NH4+-N and NO3−-N), and the pakchoi quality (Vc and soluble protein content), while also raising the relative abundance of bacteria that promote plant growth and immobilize metals, such as Streptomyces and Sphingomonas. In conclusion, Hao 2018 reduced the available Pb in soil and pakchoi Pb absorption by increasing the pH and activity of multiple enzymes and regulating microbiome composition in rhizospheric soil.

Hormetic dose responses induced by organic flame retardants in aquatic animals: Occurrence and quantification

The organic flame retardants (OFRs) have attracted global concerns due to their potential toxicity and ubiquitous presence in the aquatic environment. Hormesis refers to a biphasic dose response, characterized by low-dose stimulation and high-dose inhibition. The present study provided substantial evidence for the widespread occurrence of OFRs-induced hormesis in aquatic animals, including 202 hormetic dose response relationships. The maximum stimulatory response (MAX) was commonly lower than 160% of the control response, with a combined value of 134%. Furthermore, the magnitude of MAX varied significantly among multiple factors and their interactions, such as chemical types and taxonomic groups. Moreover, the distance from the dose of MAX to the no-observed-adverse-effect-level (NOAEL) (NOAEL: MAX) was typically below 10-fold (median = 6-fold), while the width of the hormetic zone (from the lowest dose inducing hormesis to the NOAEL) was approximately 20-fold. Collectively, the quantitative features of OFRs-induced hormesis in aquatic animals were in accordance with the broader hormetic literature. In addition, the implications of hormetic dose response model for the risk assessment of OFRs were discussed. This study offered a novel insight for understanding the biological effects of low-to-high doses of OFRs on aquatic animals and assessing the potential risks of OFRs in the aquatic environment.

Measurement and modeling of hormesis in soil bacteria and fungi under single and combined treatments of Cd and Pb

Heavy metals are considered major environmental pollutants. Soil microorganisms represent a predominant component of soils ecosystems, yet there is little information regarding hormetic responses of soil microorganisms to single and combined exposures to heavy metals. In the present study, to explore and predict the hormetic response of soil microorganisms, dose-response relationships of bacterial and fungal populations to single and combined treatments of cadmium (Cd) and lead (Pb) were evaluated. The results revealed hormetic responses of bacterial and fungal populations to both single and combined Cd and Pb treatments. The maximum stimulation (M_max; relative to control treatment with no metals) of bacterial and fungal populations was 40% at 2 mg Cd/kg and 60% at 160 mg Pb/kg. An enhanced M_max occurred in bacterial (50%) and fungal (75%) populations in the presence of the binary mixtures of 0.6 mg Cd/kg + 160 mg Pb/kg and 4.0 mg Cd/kg + 200 mg Pb/kg, suggesting positive additivity. This study showed that the hormetic effects of the mixtures were related to the independent effect of Cd and Pb, but they could not be predicted by the single effect of Cd or Pb. These new findings of the hormetic response of soil microorganisms to single treatments of Cd and Pb and their binary mixtures can facilitate the determination and minimization of ecological risks in heavy metal-polluted soils.

Hormetic responses of soil microbiota to exogenous Cd: A step toward linking community-level hormesis to ecological risk assessment

We investigated hormetic responses of soil microbial communities to exogenous Cd by assessing microbial count, bacterial and fungal abundance, and microbial community diversity. We found that the bacterial count (BC) decreased (3-40%) by 0.2-40 mg Cdkg^-1. Addition of 0.6-2.0 mgkg^-1 significantly increased fungal count (FC) by 7-42%, while addition of 4.0-40 mgkg^-1 Cd decreased FC by 29-51%, indicating a hormetic dose response. We also found that the FC/BC ratio increased by 0.6-2.0 mg Cdkg^-1, with a maximum stimulation of 51%, and decreased (18-27%) by 4.0-40 mg Cdkg^-1. Cd had no adverse effect on the α-diversity of bacterial or fungal communities. For relative abundances (RAs) of bacteria and fungi at phylum level, Bacteroidetes RA exhibited a biphasic dose-response curve, with an 18-24% increase at 0.6-4.0 mgkg^-1 and a 10% decrease at 40 mgkg^-1 compared with control. The results of FC, FC/BC, and Bacteroidetes RAs suggest that hormesis occurred at microbial community level, with positive effects occurring at 0.6-2.0 mgkg^-1. This study can contribute to incorporating microbial community hormesis into the ecological risk assessments in the future.

The role of bacterial communities in shaping Cd-induced hormesis in 'living' soil as a function of land-use change

Bacterial communities and soil physicochemical properties shape soil enzymes activities. However, how environmental factors and bacterial communities affect the relationship between increasing doses of soil pollutants and soil alkaline phosphatase (ALP), an index of soil microbiota activity, remains poorly understood. In this study, we investigated the response of soil ALP to 13 doses of Cd (0 and 0.01-100 mg/kg) under four land uses, viz. grassland (GL), natural forest (NF), plantation forest (PF), and wheat field (WF). We found that Cd commonly induced hormetic-like responses of soil ALP, with a maximum stimulation of 10.7%, 10.1%, 11.6%, and 14.5% in GL, NF, PF, and WF, respectively. The size of the hormetic zone (Hor_zone), an integrated indicator of the stimulation phase and biological plasticity, was in the order GL > WF > PF > NF, and the hormetic zone occurred in the dose range of 5-10, 0.3-10, 0.8-3, and 3-5 mg/kg, respectively. These results indicate highly pleiotropic responses of 'living' soil system to promote resilience to Cd contamination, with soil microbiota potentially contributing to soil ALP's hormetic-like response under different land uses. The hormetic-like response of 'living' soil ALP in different land uses offers a new insight into the identification and minimization of the ecological risks of land-use change in Cd-contaminated lands.

Cd induced biphasic response in soil alkaline phosphatase and changed soil bacterial community composition: The role of background Cd contamination and time as additional factors

Hormesis is an intriguing phenomenon characterized by low-dose stimulation and high-dose inhibition. The hormetic phenomena have been frequently reported in the past decades, but the researches on the biphasic responses of soil enzymes are still limited. The main objective of this study is to explore dose response of alkaline phosphatase (ALP) to Cd (0, 0.003, 0.03, 0.3, 3.0 and 30 mg/kg) in the presence of different levels of background Cd contamination (bulk soil with no added Cd, BS; low background Cd, LB; medium background Cd, MB; and high background Cd, HB). ALP activity at 0.003-0.3 mg Cd/kg was 13-39% higher than that of the control (0 mg Cd/kg) for HB after 7 d. Similarly, the enzyme activities at 0.003-0.03 mg Cd/kg were 2-25% and 14-17% higher than those of the controls for MB and HB after 60 d. After 90 d, ALP activities at 0.3-3.0 mg Cd/kg increased by 11-17% for LB. The dose-response curves had the shape of an inverted U, showing biphasic responses at days 7 (HB), 60 (MB and HB) and 90 (LB). After 60 days of exposure, total operational taxonomic units (OTU) numbers and unique species exposed to Cd stress displayed hormetic-response curve for MB. The relative abundances of Agrobacterium, Salinimicrobiums, Bacilllus, and Oceanobacillus displayed significantly positive correlations with ALP activity. This suggested that bacterial communities potentially contribute to ALP's hormesis. This study further provides new insights into the ecological mechanisms of pollutant-induced hormesis, and substantially contributes to the ecological risk assessment of Cd pollution.

Effect of Pseudomonas moorei KB4 Cells' Immobilisation on Their Degradation Potential and Tolerance towards Paracetamol

Pseudomonas moorei KB4 is capable of degrading paracetamol, but high concentrations of this drug may cause an accumulation of toxic metabolites. It is known that immobilisation can have a protective effect on bacterial cells; therefore, the toxicity and degradation rate of paracetamol by the immobilised strain KB4 were assessed. Strain KB4 was immobilised on a plant sponge. A toxicity assessment was performed by measuring the concentration of ATP using the colony-forming unit (CFU) method. The kinetic parameters of paracetamol degradation were estimated using the Hill equation. Toxicity analysis showed a protective effect of the carrier at low concentrations of paracetamol. Moreover, a pronounced phenomenon of hormesis was observed in the immobilised systems. The obtained kinetic parameters and the course of the kinetic curves clearly indicate a decrease in the degradation activity of cells after their immobilisation. There was a delay in degradation in the systems with free cells without glucose and immobilised cells with glucose. However, it was demonstrated that the immobilised systems can degrade at least ten succeeding cycles of 20 mg/L paracetamol degradation. The obtained results indicate that the immobilised strain may become a useful tool in the process of paracetamol degradation.

Toluene induces hormetic response of soil alkaline phosphatase and the potential enzyme kinetic mechanism

Hormesis of soil enzyme that involved in heavy metal has been attracting much more attention for risk assessment of heavy metal toxicity, but insufficient studies were conducted to define the hormetic responses induced by toluene or other organic pollutions. The objectives of this study were to investigate the hormetic responses of soil enzyme induced by toluene and explore the potential enzyme kinetic mechanism. Soil alkaline phosphatase (ALP) activity was regarded as the endpoint to explore the hormetic responses under different doses of toluene (0.0, 0.1, 0.5, 1.0, 2.0, 3.0, 5.0, 10.0, 50.0 and 100.0 μL g^-1). Subsequently, we conducted the experiments of enzymatic reaction kinetics and pure enzyme to further verify the potential mechanisms of soil ALP's hormesis. Results showed that ALP activities at 0.1-1.0 μL g^-1 toluene were significantly increased in contrast to the control (0 μL g^-1 toluene) (P < 0.05) at the exposure time of 30, 36, 48 and 54 h, with the maximum stimulation magnitudes of 24-43%. ALP activities were almost not affected by toluene (2-100 μL L^-1) in the whole experimental period (6-54 h). Meanwhile, the values of catalytic efficiency (the radio V_max/K_m, V_max: maximum reaction velocity and K_m: Michaelis constant) and V_max significantly increased compared with the control, but the value of K_m decreased from 2.5 to 1.6. Overall, low dose toluene can induce hormesis of soil ALP. The potential reason is that low-dose toluene could enhance the combination of soil ALP and substrates. We believe that this study will provide a new viewpoint for ecological risk assessment of toluene contaminated soils.