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Article

Impacts of Selenium Supplementation on Soil Mercury Speciation, Soil Properties and Mercury-Resistant Microorganisms and Resistant Genes

by
Guangpeng Pei
1,2,
Yuxin Li
2,3 and
Hua Li
2,4,*
1
College of Resources and Environment, Shanxi Agricultural University, Jinzhong 030815, China
2
School of Environment Science and Resources, Shanxi University, Taiyuan 030006, China
3
Research Institute of Pomology, Shanxi Agricultural University, Jinzhong 030815, China
4
Shanxi Yellow River Laboratory, Taiyuan 030006, China
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(9), 1928; https://doi.org/10.3390/agronomy14091928 (registering DOI)
Submission received: 30 July 2024 / Revised: 24 August 2024 / Accepted: 26 August 2024 / Published: 28 August 2024
(This article belongs to the Section Innovative Cropping Systems)
Browse Figures
Versions Notes

Abstract

:
Soil mercury (Hg) contamination is a serious threat to local ecology and public health. Exogenous selenium (Se) supplementation can effectively reduce the toxicity of Hg. However, the mechanisms affecting the changes in soil Hg speciation, soil properties and the microbial Hg-resistant system during the Se–Hg interaction after exogenous Se supplementation are not clear. Therefore, in this study, soil culture experiments were conducted to analyze the effects of different Se additions on the transformation of Hg speciation, soil properties and Hg-resistant microorganisms and resistant genes (mer operon). The results indicated that Se supplementation facilitated the transformation of soil Hg from bioavailable (exchangeable and carbonate-bound) to stable forms (organic material-bound and residual), significantly reducing Hg bioavailability. Se supplementation notably decreased the electrical conductivity of Hg-contaminated soil, but had no significant effect on the soil pH, organic matter content, cation exchange capacity or alkaline phosphatase and catalase activities. The maximum activity levels of soil sucrase and urease were observed when 1 mg kg−1 Se was added. Se significantly inhibited soil peroxidase and ascorbate oxidase activities, thereby alleviating the oxidative stress in the soil system caused by Hg. Additionally, Se significantly activated the Hg-resistant system in soil microorganisms by either decreasing or increasing the regulatory genes merD and merR, and it significantly upregulated the cytoplasmic protein gene merP and the membrane protein genes merC, merF and merT. This further increased the abundance of the organomercury lyase gene merB and the mercuric reductase gene merA, promoting the conversion of Hg species to Hg⁰. Furthermore, the abundance of mer operon-containing microorganisms, such as Thiobacillus ferrooxidants, Pseudomonas, Streptomyces and Cryptococcus, significantly increased with Se addition, explaining the role of soil microorganisms in mitigating soil Hg stress via Se supplementation.

1. Introduction

Mercury (Hg) is a widely recognized environmental contaminant because of its toxicity, contamination capacity, persistence and bioaccumulation qualities [1,2]. The Hg contamination of agricultural soils is very serious in some areas due to human activities and geological processes [3,4]. Especially in Hg mining areas, Hg-containing slag, wastewater and atmospheric deposition lead to the serious Hg contamination of agricultural land [5]. About 1.6% of agricultural land in China is contaminated with Hg [6], mainly due to wastewater irrigation, mining activities, improper fertilizer use, chemical applications containing mercury compounds and atmospheric deposition [7]. Once in the soil, Hg undergoes a variety of biogeochemical processes that have significant impacts on the environment and food safety [8]. Among these, the bioavailability of Hg depends on the soil properties, Hg concentration and its speciation. Therefore, it is essential to take measures to control the biogeochemical behavior of Hg in the environment and prevent its transfer to living organisms.
Selenium (Se) is an essential trace element required by humans. Moreover, when organisms are exposed to heavy metals, Se may act as a natural detoxifier and reduce the toxicity of Se and heavy metals by forming metal–selenoprotein complexes [9]. Since the first discovery of Se-Hg antagonism by Parizek and Ostadalova (1967) [10], the mechanisms have been extensively investigated in aquatic organisms, birds and mammals, including, but not limited to, the formation of Se-Hg compounds, the Se-induced reduction in Hg bioavailability and the Se inhibition of Hg toxicity [11,12,13]. With further studies on Se-Hg antagonism, the use of Se as an efficient remediation technique for Hg-contaminated soils has received an increasing amount of attention from scholars. As an antagonist to Hg, Se can sequester Hg into highly stable HgSe, inhibiting the transformation of Hg to methylmercury (MeHg) [14,15]. Wang et al. [12] found that Hg in soil mainly exists in the form of HgSe or HgSe nanoparticles using TEM-EDX and XANES. However, the mechanisms by which Se supplementation affects soil Hg speciation and bioavailable Hg changes remain unclear. Hg exists in various chemical forms in soil and can bind to multiple matrices [16,17]. Chemical and biological reactions can alter Hg speciation and its binding to different substances, affecting the solubility, toxicity and bioavailability of Hg-bound chemicals [16,18]. The speciation of Hg in soil is usually in equilibrium [19]; however, this equilibrium is disturbed after the addition of Se and the formation of HgSe [15]. Therefore, it is of great importance to further investigate the transformation and impact of soil Hg and Se speciation during Se-Hg interactions.
While Hg is toxic to humans and other large organisms, certain microorganisms can withstand this toxic element [20]. These Hg-resistant microorganisms not only endure high concentrations of Hg, but can also convert toxic forms of Hg into non-toxic or less harmful forms [21]. These capabilities are found in microorganisms such as Pseudomonas, Staphylococcus aureus, Thiobacillus ferrooxidants, Streptomyces, Cryptococcus and Escherichia coli [22,23,24]. Hg-resistant microorganisms exhibit different Hg resistance mechanisms, such as the direct efflux of Hg [25], extracellular/intracellular sequestration [26] and the enzymatic transformation of Hg compounds [27]. This enzymatic transformation can be encoded in the mer operon, which consists of regulatory genes (merR and merD), transporters (merT, merP, merD, merF and merC) and two enzymes, the presence of which differs between microorganisms (merA and merB) [28,29]. Moreover, the main mechanism for Hg detoxification in microbial entities is the mercuric reductase enzyme MerA, which is encoded by the merA gene. This enzyme regulates the reduction in toxic Hg2⁺ to relatively less toxic Hg0, which is then further volatilized [20]. Various studies have reported the widespread presence of Hg-resistant microorganisms and their potential role in detoxifying toxic metals [30,31]. The Se-Hg antagonism process is influenced by both microbial and non-microbial factors in the soil environment, with the mainstream view suggesting that microbial processes are the main drivers of Se-Hg antagonism [32]. Microbial-mediated Se-Hg antagonism is regulated by key factors such as inorganic Hg bioavailability, Hg-resistant microorganisms and the mer operon [22,30,33]. The addition of exogenous Se may influence these key factors, ultimately reducing Hg stress on the soil ecosystem. However, studies focusing on the effects of exogenous Se supplementation on Hg-resistant microorganisms and resistant genes in Hg-contaminated soil are still limited.
In this research, we investigated the interaction between Hg and Se in the soil system under Hg stress by applying different concentrations of Se; analyzed the change in soil Hg and Se speciation, physicochemical properties, enzyme activities, the abundance of Hg-resistant microorganisms and mer operon abundance; and explored the possibility of Se supplementation to reduce the risks posed by Hg in the soil.

2. Materials and Methods

2.1. Se- and Hg-Contaminated Soil

Exogenous Se was provided in the form of sodium selenite (Na2SeO3, 99.0%), purchased from Sigma Aldrich Chemical Co. of St Louis, MO, USA.
The contaminated soils were collected from the farmland around a gold mine in Shanxi Province, China, which suffered intense Hg contamination due to artisanal and small-scale gold mining. Bulk soils from the tillage layer (0~20 cm) were collected during the fallow season and transported to our laboratory. The total Hg (THg) content was 5.38 mg kg−1, which is 1.58-fold higher than the risk screening value for the soil contamination of agricultural land defined by the Chinese government (China GB 15618-2018). The basic physicochemical properties of the soil are listed in Table 1.

2.2. Experimental Design

A laboratory soil incubation experiment was conducted. Different doses of exogenous Se were added to the Hg-contaminated soil in the form of Na2SeO3 solution and fully mixed to make the soil Se contents reach 1.00, 2.00 and 3.00 mg kg−1, respectively, with an untreated control (CK). Each treatment was replicated three times. The treated soil was placed in a flowerpot with a diameter of 15.5 cm and a height of 12.0 cm and deionized water was added to keep the soil moisture at 60.0% of the maximum water-holding capacity in the field. Then, all the samples were cultured at room temperature for 30 d.
After incubation, a part of the fresh soil was stored at −80 °C to determine the Hg-resistant microorganisms and the quantity of mer operon. The remaining soil was air-dried and sieved to determine the physicochemical properties of the soil, enzyme activities, the total and available content and the speciation of Hg and Se.

2.3. Chemical Analysis

2.3.1. Soil Properties

The pH value of the soil was measured using a pH meter (soil–water = 1:2.5) [34]. The electric conductivity (EC) of the soil was measured using the electrode method (China HJ 802-2016). The soil organic matter (OM) content was measured using the external heating potassium dichromate volumetric method [35]. The soil cation exchange capacity (CEC) was measured using the hexamminecobalt trichloride solution–spectrophotometric method (China HJ 889-2017).
For the nutrient conversion enzymes, sucrase, urease and alkaline phosphatase activities were determined using 3,5-dinitrosalicylic acid colorimetry, phenol–sodium hypochlorite and sodium diphenyl phosphate colorimetric methods, respectively [36]. For oxidoreductase enzymes, catalase, peroxidase and ascorbate oxidase activities were determined using the potassium permanganate, iodine and 2,6-dichlorophenol titration methods, respectively [36].

2.3.2. Soil Hg and Se Contents

The content of THg in the soil was determined according to China GB/T 22105.1-2008. Briefly, approximately 0.200 g dry sample was digested with 10.0 mL freshly prepared Aqua regia (HCl:HNO3 = 3:1, v/v) in a water bath at 100 °C for 2 h. An aliquot of the digested solution was measured using liquid chromatography–atomic fluorescence spectrometry (LC-AFS 6500, Beijing Haiguang Instruments Co., Ltd., Beijing, China). The content of total Se (TSe) in the soil was determined according to China NY/T 1104-2006. Briefly, the soil sample was digested with HNO3-HClO4, and the digestion liquid was determined using LC-AFS 6500. The available Hg (AHg) content in the soils was analyzed via DTPA solvent extraction and determined using LC-AFS 6500. The available Se (ASe) content of the soil was extracted with the DTPA and analyzed using LC-AFS 6500. The effectiveness of Se supplementation and its impact on Hg availability were evaluated using Equations (1) and (2).
PAHg = The content of AHg/The content of THg × 100%
PASe = The content of ASe/The content of TSe × 100%
where PAHg represents the proportion of AHg in the THg (%) and PASe denotes the proportion of ASe in TSe (%).

2.3.3. Sequential Extraction of Hg and Se in Soil

The sequential extraction procedure (SEP) was performed according to Tessier et al. [37]. Hg and Se in the soil were divided into five consecutive fractions: F1 (exchangeable), F2 (carbonate-bound), F3 (Fe–Mn oxide-bound), F4 (organic material-bound) and F5 (residual). The order of relative bioavailability and migration is F1 > F2 > F3 > F4 > F5. The proportions of different fractions of Hg and Se in the soil were calculated using Equations (3) and (4).
PFi-Hg = The content of different fractions of Hg (Fi-Hg)/The content of THg × 100% (i = 1~5)
PFi-Se = The content of different fractions of Se (Fi-Se)/The content of Se × 100% (i = 1~5)
where PFi-Hg denotes the proportion of different fractions of Hg in THg (%) and PFi-Se represents the proportion of different fractions of Se in TSe (%).

2.4. Hg-Resistant Microorganisms and Mer Operon Quantification

Soil microbial total DNA was extracted using the TIANGEN Soil DNA Kit. The quantities of Hg-resistant microorganisms and mer operons were quantified using qPCR. The reaction system (10.0 μL) included 5.00 μL 2 × SG Green qPCR Mix, 0.200 μL forward and reverse primers (10.0 μM), 1.00 μL cDNA and 3.60 μL ddH2O, with each gene analyzed in triplicate. The PCR program was conducted as follows: 95 °C for 3 min, followed by 40 cycles of 95 °C for 10 s and 60 °C for 30 s. The primer sequences for the Hg-resistant microorganisms and mer genes are listed in Tables S1 and S2.

2.5. Data Analysis

Duplicates and standard reference materials (GBW07403 (GSS-3) and GBW07413a (ASA-2)) were used for quality assurance and to control the Hg and Se measurements. The results are expressed as the mean ± standard error (SE). A one-way analysis of variance (ANOVA) was used to compare the treatments. Statistical analyses were performed using Statistical Packages for Social Sciences (SPSS, version 25.0) and Origin. Significant differences between the treatment groups are denoted by different lowercase letters.

3. Results

3.1. Effects of Se Supplementation on Soil AHg and ASe

The AHg content of the soil decreased significantly with the increased Se addition (p < 0.05) (Figure 1a). Specifically, when 3.00 mg kg−1 Se was added, the soil AHg content decreased to 0.384 mg kg−1, which was 77.2% lower than that of the CK. Simultaneously, as indicated in Table 2, Se supplementation could significantly reduce PAHg, with the most significant reduction observed when 3.00 mg kg−1 Se was added, which was 78.4% lower compared to that of the CK. These results demonstrate the capability of Se to decrease soil AHg.
The effect of Se supplementation on the ASe content of the soil is shown in Figure 1b. Compared to the CK, the soil ASe content significantly increased with the increased Se addition, which increased by 225%, 603% and 1300%, respectively. Considering the different levels of exogenous Se added to the soil, the PASe values were further calculated (Table 2). Only when 3.00 mg kg−1 Se was added did the PASe significantly increase compared to that of the CK by 33.3%. The other treatments showed no significant effect on PASe.

3.2. Effects of Se Supplementation on Soil Hg and Se Speciation

Using the Tessier fractionation method, the Hg-Se antagonistic effect on soil Hg and Se speciation was further studied. As shown in Figure 2, the contents of the available (F1-Hg and F2-Hg) and intermediate fractions (F3-Hg) of Hg significantly decreased with the increased Se addition, decreasing by 0.159~0.548 mg kg−1, 0.613~0.825 mg kg−1 and 0.061~0.093 mg kg−1, respectively, compared to those of the CK. However, the stable fractions (F4-Hg and F5-Hg) of Hg significantly increased with the increased Se addition by 0~0.693 mg kg−1 and 0.0630~1.18 mg kg−1, respectively, compared to those of the CK. Table 3 shows a consistent trend, with the proportions of available (PF1-Hg and PF2-Hg) and intermediate fractions (PF3-Hg) of Hg significantly decreasing and the stable fractions (PF4-Hg and PF5-Hg) of Hg significantly increasing with the increased Se addition. When 3.00 mg·kg−1 Se was added, PF1-Hg and PF2-Hg were the lowest, decreasing by 85.0%, while PF4-Hg and PF5-Hg were the highest, increasing by 38.5% compared to that of the CK. These results suggest that increasing the Se addition can promote the transformation of Hg from available to stable forms, reducing its availability and mobility in soil.
The changes in Se speciation in soil are shown in Figure 3. Except for intermediate Se (F3-Se), the contents of the other Se fractions significantly increased with the increased Se addition, which is consistent with the trend concerning the ASe content. Compared to the CK, the contents of different Se fractions increased by 0.159~0.670 mg kg−1 (F1-Se), 0.0158~0.459 mg kg−1 (F2-Se), 0.0972~0.531 mg kg−1 (F4-Se) and 0.209~1.08 mg kg−1 (F5-Se) after Se supplementation. The impact of Se supplementation on the proportions of different Se fractions in TSe is shown in Table 3. The proportions of the available speciation of Se (PF1-Se and PF2-Se) were the highest in the treatment with 3.00 mg kg−1 Se, significantly increasing by 32.7% compared to that of the CK. The other Se additions had no significant effect on PF1-Se and PF2-Se. Compared to the CK, Se supplementation significantly reduced the proportion of the intermediate form of Se (PF3-Se) by 56.4%~86.2% while significantly increasing the proportion of the stable speciation of Se (PF4-Se and PF5-Se) by 138%~266%. These results suggest that Se supplementation promotes the transformation of available to stable Se, enhancing its stability and reducing its environmental risk.

3.3. Effects of Se Supplementation on Soil Physicochemical Properties

The effects of Se supplementation on the pH, EC, OM and CEC of soil are shown in Figure 4. Compared to the CK, the increased Se addition significantly reduced the EC value, which was 10.9%~14.3% lower than that of the CK. However, there was no significant change in the EC within the Se addition range from 1.00 to 3.00 mg kg−1. Additionally, Se supplementation had no significant impact on the pH, OM or CEC of the soil. These results indicate that Se supplementation has no significant effects on the physicochemical properties of soil.

3.4. Effects of Se Supplementation on Soil Enzyme Activities

Se supplementation had varying effects on various enzyme activities in the soil (Figure 5). The levels of soil sucrase and urease activity initially increased and then decreased with the increasing Se addition. The highest enzyme activity levels were observed when 1.00 mg·kg−1 Se was added, showing significant increases of 25.2% and 42.2% compared to that of the CK (Figure 5a,b). Furthermore, Se supplementation did not significantly affect the activities of soil alkaline phosphatase and catalase (Figure 5c,d). However, the level of soil peroxidase activity significantly decreased with the increasing Se addition (Figure 5e) by 37.1%, 67.7% and 71.0% compared to that of the CK. Regarding the soil ascorbate oxidase activity (Figure 5f), the influence of Se supplementation was only significant when 3.00 mg kg−1 was added, showing a decrease of 1.81% compared to that of the CK.

3.5. Effects of Se Supplementation on Hg-Resistant Genes

The enzymes encoded by genes containing mer operons can transform the speciation and oxidation state of soil Hg through enzymatic reactions, thereby reducing Hg toxicity. Figure 6a shows that, with the increasing Se addition, the number of merA genes significantly increased by 19.1%~78.12% compared to that of the CK. The trend in the merB gene numbers was similar to that of merA, with a significant increase of 59.7%~216% compared to that of the CK (Figure 6b). The impact of Se supplementation on the Hg transport genes merC, merP and merT followed a consistent pattern: less Se addition increased the number of Hg transport genes, while a greater soil Se addition lowered this (Figure 6c,f,h). Specifically, the numbers of merC and merP reached maximum values when 1.00 mg kg−1 Se was added, increasing by 102% and 36.3% compared to that of the CK, respectively. The number of merT reached its maximum when 2.00 mg kg−1 Se was added, increasing by 62.5% compared to that of the CK. However, when 3.00 mg kg−1 Se was added, the numbers of these three genes significantly decreased from their respective peaks. Another Hg transport gene, merF, significantly increased by 25.7%~31.4% after Se addition, but no significant differences were found among the different soil Se treatments (Figure 6e). merD, which inhibits mer operon expression, reached its lowest number when 2.00 mg kg−1 Se was added, decreasing by 13.3% compared to that of the CK. This result indicated a higher expression level of the mer operon at a soil Se content of 2.00 mg kg−1 than that of the other treatments (Figure 6d). From Figure 6g, it can be seen that, compared to the CK, Se addition did not cause significant differences in the regulatory gene merR in the soil, while significant differences were observed between the treatments with soil Se additions of 1.00 mg kg−1 and 2.00 mg kg−1.
From the figure, it can be seen that, compared to the control, a selenium addition does not cause significant differences in the merR gene expression in the soil, but there is a significant difference between the treatments with soil selenium additions of 1.00 mg/kg and 2.00 mg/kg.

3.6. Effects of Se Supplementation on Hg-Resistant Microorganisms

The soil Hg-resistant microorganisms, as influenced by Se supplementation, are shown in Figure 7. In terms of the Hg-resistant microorganisms in this study, Staphylococcus aureus and Pseudomonas were dominant in the soil, with the quantities both exceeding 109 copies g−1. These are followed by Streptomyces and Thiobacillus ferrooxidans, with quantities greater than 108 copies g−1. Escherichia coli had a content of 107 copies g−1, while the lowest quantity of Hg-resistant microorganisms was Cryptococcus, with only 104 copies g−1. When 1.00 mg kg−1 of Se was added, the quantity of Staphylococcus aureus significantly decreased by 7.88% compared to that of the CK (Figure 7a). Moreover, Se supplementation had no significant effect on the quantity of Escherichia coli (Figure 7c), except for Cryptococcus in the treatment with 2.00 mg kg−1 Se, where its quantity significantly decreased compared to those in the other treatments. The quantities of Thiobacillus ferrooxidants, Pseudomonas, Streptomyces and Cryptococcus all showed an increasing trend after Se addition (Figure 7b,d–f). Specifically, compared to the CK, when 1.00, 2.00 and 3.00 mg kg−1 Se were added, the quantities of Thiobacillus ferrooxidants significantly increased by 12.4%~20.6%, Pseudomonas increased by 3.23%~10.3%, Streptomyces increased by 5.77%~7.75% and Cryptococcus increased by 30.1%~131%. These results indicated that increasing the soil Se addition can increase the quantities of Hg-resistant microorganisms containing mer operon genes in the soil. These microorganisms store Hg in their cytoplasm, thereby reducing its effectiveness and promoting the transformation of inorganic or organic Hg complexes into Hg0, which is volatile, thereby reducing the Hg content of the soil. Through these pathways, soil Se supplementation ultimately reduces Hg stress in the soil system.

4. Discussion

4.1. Se Supplementation Promotes the Transformation of Soil Hg and Se Fractions

The results of the Tessier fractionation analysis have often been used as an assessment tool to establish the mobility and availability of Hg in soil. Based on the different forms and affinities of Hg in soil, soil samples have been continuously extracted using different extractants under different extraction conditions to obtain different fractions of Hg in soil, so as to better understand the distribution pattern, existing forms and migration pathways of Hg in soil. In this research, with an increase in soil Se addition, the content and proportion of available forms of Hg (available, F1, F2, PAHg, PF1-Hg and PF2-Hg) significantly decreased, while the content and proportion of the stable forms of Hg (F3-Hg, F4-Hg, PF3-Hg and PF4-Hg) increased markedly. These results indicate that exogenous Se supplementation can reduce the bioavailability of Hg in soil. Similarly, except for the intermediate speciation of Se (F3-Se), the contents of other speciations of Se and the ASe in soil increased significantly with increased Se addition. This result indicates that Se-Hg antagonism can simultaneously reduce the bioavailability of Hg and Se in soil, effectively immobilizing both Hg and Se. These findings confirmed that soil is an important site for Se-Hg antagonism [1,38]. Wang et al. [39] found that the content of AHg in soil decreased significantly with the increasing Se addition. Yan et al. [40] discovered that Se addition can significantly reduce the bioavailability of Hg, particularly affecting organic-bound and elemental (strongly bound) Hg. Xu et al. [15] demonstrated that after Se addition, soil Hg transforms into strongly complexed forms due to the stronger binding capacity of the Se functional groups, forming stable complexes. Therefore, the decrease in the AHg content and the transformation to a stable form will further effectively reduce Hg transfer and accumulation in relation to plants [41].
In this research, 11.9% of Hg exists in the F1 fraction and 16.4% exists in the F2 fraction, which all show the active state of Hg in soil. With the exogenous addition of Se, under the action of soil reductants and microorganisms, some exogenous SeO32− is reduced to Se0 or Se2− [42], which directly chelate with F1-Hg and F2-Hg to form Se-Hg complexes, reducing the contents of F1-Hg and F2-Hg. This is because Se0 or Se2− can form stable HgSe complexes (with a stability constant lgK = 45) directly with Hg0 or Hg2+ that are free in the soil or have a weak carbonate-binding capacity [43] or replace S in HgS to form insoluble HgSe or isostructural series HgS-HgSe (solubility product constants: HgSe (~10−60) < HgS (~10−52)). The experimental soil in this study has a high OM content (19.4 g kg−1), which contributed to the formation of a higher F4-Hg content [44]. Despite the fact that this study found that Se had no significant effect on the OM content of the soil, increasing the Se additions led to a significant increase in the soil F4-Hg content, indicating that Se can encourage soil AHg to compete for organic matter binding sites, forming stable Hg–organic complexes. HgSe complexes can also bind with soil OM to form large molecular HgSe compounds, further reducing the bioavailability of Hg and Se in the soil [45].

4.2. Changes in Soil Enzyme Activity under Se Supplementation

Soil enzymes respond very promptly to environmental changes and are often used as biological indicators of soil microbial functions, soil health and ecosystem changes [46,47]. In this study, the impact of exogenous Se on soil enzyme activities varies with the amount of Se added and the type of soil enzyme. Additionally, the inhibitory effect of heavy metal Hg on soil enzymes is also an important influencing factor [48]. Exogenous Se supplementation exhibited a trend of initially promoting and subsequently inhibiting the activities of soil sucrase and urease enzymes, which is similar to the findings of Hu et al. [49]. The reason for this is that low concentrations of Na2SeO3 (1.00 mg kg−1) are decomposed and absorbed by microorganisms, promoting their growth and reproduction, thereby stimulating the synthesis and secretion of soil sucrase and urease enzymes [50]. When more Se is added to the soil, Na2SeO3 reacts with Hg to form a large number of Se-Hg compounds or can generate selenides under the action of microorganisms, binding to the soil enzyme active site to form a stable complex, which inhibits the enzyme activity and results in competitive adsorption with the enzyme reaction substrate. The insignificant impact of Se supplementation on the soil alkaline phosphatase activity may be related to the sufficient biologically available phosphorus content in the soil. Soil peroxidase and ascorbate oxidase activities significantly decreased after the Se treatment, indicating that Se can effectively alleviate soil peroxide and ascorbate accumulation, reducing the oxidative stress caused by Hg contamination in the soil.

4.3. The Impact of Se Supplementation on Microbial Hg-Resistant System

In order to adapt to Hg pollution in the environment, some microorganisms have evolved a set of Hg-resistant gene systems. The mechanism of Hg resistance may involve changing the form and valence of Hg via an enzymatic reaction and reducing the toxicity of Hg in the environment [51,52]. Notably, the most valuable molecular mechanism of microbial Hg resistance is mediated by the mer operon [53]. Regulatory components like merD (which downregulates mer operon expression) and/or merR (which upregulates mer operon expression) modulate the expression of the mer operon in response to induction by external Hg complexes [54,55]. Subsequently, Hg2+ binds to the cysteine residues of the cytoplasmic protein encoded by merP, forming a complete membrane protein [56]. This process forms a Hg2+ bridge structure, allowing Hg2+ to transfer from the cysteine residues of the above membrane proteins to cysteine residues on the membrane proteins encoded by merT, merC or merF, facilitating Hg2+ entry into the cytoplasm [57,58,59]. The cleavage enzyme encoded by merB then severs the chemical bonds linking Hg2+ to the cysteine residues [60], freeing Hg2+ to be reduced by the Hg ion reductase encoded by merA into Hg0, which has high vapor pressure and can be volatilized from the cell to protect microorganisms from Hg toxicity [61,62]. Hg0 reduced by microorganisms enters the soil and can further volatilize into the atmosphere through soil pores, reducing the toxic effect of Hg on the soil system.
In contrast to the above process, the number of merD genes in this study showed a decreasing trend after the addition of exogenous Se, while the number of merR genes showed an increasing trend, indicating that Se supplementation stimulation had a significant promoting effect on the number and expression of mer operons [30]. The changes in the number of genes encoding cytoplasmic and membrane proteins follow a consistent trend with the increasing Se addition, initially rising and then stabilizing and declining, which correlates with the initial binding of Hg2+ to cysteine residues [20]. When Hg2+ reached the cytoplasm, the number of merB and merA genes increased significantly after the addition of Se, which could accelerate the transformation of Hg speciation and reduce its toxicity. In addition, Se supplementation significantly promoted the growth and reproduction of Thiobacillus ferrooxidants, Pseudomonas, Streptomyces and Cryptococcus containing the mer operon in the soil (Figure 7). This result was consistent with the change rule of Hg-resistant genes (Figure 6), indicating that the addition of Se has an activating effect on the microbial Hg-resistant system, which can accelerate the transformation of Hg and reduce its toxicity.
Consequently, Se supplementation is an effective method to reduce soil Hg stress and promote soil Hg resistance. However, the application of Se must be carefully managed to balance benefits and risks [63] as excessive Se levels can be toxic, presenting substantial risks to ecosystem health and public safety [64]. As the results of this research show, the soil ASe content significantly increased with the increased Se addition. Zhang et al. [41] also demonstrated that supplementing with 1~3 mg kg−1 of Se can effectively reduce Hg accumulation and increase the Se content of grains. Therefore, in practical applications of selenium, it is necessary to investigate and establish safe Se application thresholds to avoid plant toxicity and environmental contamination.

5. Conclusions

In conclusion, our study indicates that (1) exogenous Se supplementation can reduce the bioavailable Hg content in soil, promoting the transformation of Hg from bioavailable forms to stable forms; (2) exogenous Se can alleviate the oxidative stress caused by soil peroxides and ascorbate due to Hg pollution; and (3) exogenous Se activates the Hg-resistant system of soil microorganisms. With the increasing soil Se addition, the number of mer genes related to Hg resistance significantly increases (except for the merD genes, which decrease in number) and there is a notable increase in the numbers of Thiobacillus ferrooxidants, Pseudomonas, Streptomyces and Cryptococcus with Hg resistance.
Thus, the findings of our study highlight the significant role of Se supplementation in mitigating Hg toxicity and improving soil Hg resistance, providing a strategy for the effective utilization of Hg-contaminated farmland soil.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14091928/s1, Table S1: Primer sequences of Hg-resistant microorganisms; Table S2: Primer sequences of mer operon genes.

Author Contributions

Conceptualization, G.P. and Y.L.; methodology, Y.L.; validation, G.P. and Y.L.; formal analysis, G.P.; investigation, G.P.; resources, H.L.; data curation, G.P. and Y.L.; writing—original draft preparation, G.P.; writing—review and editing, Y.L. and H.L.; visualization, G.P. and Y.L.; supervision, H.L.; project administration, H.L.; funding acquisition, G.P., Y.L. and H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Basic Research Program of Shanxi, China (No. 202303021212103, No. 202103021223124), the Key R&D Program of Shanxi, China (No. 201703D211014, No. 201603D21110-1) and the Scientific Research Starting Project for the Doctor of Shanxi Agricultural University, China (No. 2023BQ14, No. 2021BQ93).

Data Availability Statement

The original contributions presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Effect of Se supplementation on the content of available Hg and Se in soil. (a) The content of available Hg. (b) The content of available Se. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 1. Effect of Se supplementation on the content of available Hg and Se in soil. (a) The content of available Hg. (b) The content of available Se. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 2. Effect of Se supplementation on Hg fractions in soil. F1, exchangeable fraction; F2, carbonate-bound fraction; F3, Fe-Mn oxide-bound fraction; F4, organic material-bound fraction; F5, residual fraction. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 2. Effect of Se supplementation on Hg fractions in soil. F1, exchangeable fraction; F2, carbonate-bound fraction; F3, Fe-Mn oxide-bound fraction; F4, organic material-bound fraction; F5, residual fraction. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 3. Effect of Se supplementation on Se fractions in soil. F1, exchangeable fraction; F2, carbonate-bound fraction; F3, Fe-Mn oxide-bound fraction; F4, organic material-bound fraction; F5, residual fraction. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 3. Effect of Se supplementation on Se fractions in soil. F1, exchangeable fraction; F2, carbonate-bound fraction; F3, Fe-Mn oxide-bound fraction; F4, organic material-bound fraction; F5, residual fraction. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 4. Effect of Se supplementation on soil physicochemical properties. (a) pH. (b) Electric conductivity. (c) Organic matter. (d) Cation exchange capacity. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 4. Effect of Se supplementation on soil physicochemical properties. (a) pH. (b) Electric conductivity. (c) Organic matter. (d) Cation exchange capacity. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 5. Effect of Se supplementation on soil enzyme activities. (a) Sucrase activity. (b) Urease activity. (c) Alkaline phosphatase activity. (d) Catalase activity. (e) Peroxidase activity. (f) Ascorbate oxidase activity. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 5. Effect of Se supplementation on soil enzyme activities. (a) Sucrase activity. (b) Urease activity. (c) Alkaline phosphatase activity. (d) Catalase activity. (e) Peroxidase activity. (f) Ascorbate oxidase activity. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 6. Effect of Se supplementation on the number of Hg-resistant genes. (a) merA. (b) merB. (c) merC. (d) merD. (e) merF. (f) merP. (g) merR. (h) merT. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 6. Effect of Se supplementation on the number of Hg-resistant genes. (a) merA. (b) merB. (c) merC. (d) merD. (e) merF. (f) merP. (g) merR. (h) merT. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Figure 7. Effect of Se supplementation on the quantity of soil Hg-resistant microorganisms. (a) Staphylococcus aureus. (b) Thiobacillus ferrooxidants. (c) Escherichia coli. (d) Pseudomonas. (e) Streptomyces. (f) Cryptococcus. Different lowercase letters indicate significant differences between treatments (p < 0.05).
Figure 7. Effect of Se supplementation on the quantity of soil Hg-resistant microorganisms. (a) Staphylococcus aureus. (b) Thiobacillus ferrooxidants. (c) Escherichia coli. (d) Pseudomonas. (e) Streptomyces. (f) Cryptococcus. Different lowercase letters indicate significant differences between treatments (p < 0.05).
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Table 1. Basic physicochemical properties of the soil in this study.
Table 1. Basic physicochemical properties of the soil in this study.
Soil Physicochemical PropertiesValue
Clay (%)5.14
Silt (%)41.8
Sand (%)53.1
pH7.70
Electric conductivity (μs cm−1)269
Cation exchange capacity (mmol kg−1)15.2
Organic matter (g kg−1)19.4
Hg (mg kg−1)5.38
Se (mg kg−1)0.272
Table 2. Effect of Se supplementation on the proportions of available Hg and Se.
Table 2. Effect of Se supplementation on the proportions of available Hg and Se.
Se Supplementation (mg kg−1)PAHg (%)PASe (%)
CK31.4 ± 0.58 a33.0 ± 3.63 b
1.0015.3 ± 0.44 b32.5 ± 0.63 b
2.008.28 ± 0.30 c32.8 ± 0.36 b
3.006.79 ± 0.39 d44.0 ± 0.59 a
Note: Different letters in the same column indicate significant differences between treatments (p < 0.05). PAHg, the proportion of AHg in the THg. PASe, the proportion of ASe in the TSe.
Table 3. The effect of Se supplementation on the proportions of different fractions of Hg and Se in the soil.
Table 3. The effect of Se supplementation on the proportions of different fractions of Hg and Se in the soil.
ElementSe Supplementation (mg kg−1)PF1 (%)PF2 (%)PF3 (%)PF4 (%)PF5 (%)
HgCK11.9 ± 0.78 a16.4 ± 1.20 a5.57 ± 0.80 a42.6 ± 0.58 c23.5 ± 2.09 c
1.009.22 ± 0.41 b4.74 ± 0.28 b4.56 ± 0.07 ab49.2 ± 0.20 b32.3 ± 0.15 b
2.007.69 ± 0.31 c5.74 ± 0.55 b4.35 ± 0.09 ab56.7 ± 1.25 a25.5 ± 1.18 c
3.002.32 ± 0.06 d1.91 ± 0.49 c4.13 ± 0.11 b44.8 ± 1.29 c46.9 ±0.84 a
SeCK0.00 ± 0.00 d33.1 ± 1.71 a49.0 ± 5.62 a2.73 ± 0.44 c15.2 ± 7.51 c
1.0017.3 ±1.04 b18.6 ± 1.94 b21.4 ± 0.18 b12.0 ±0.12 b30.8 ± 2.63 b
2.0015.0 ± 0.81 c9.00 ± 1.08 c10.4 ± 0.09 c7.32 ± 3.40 bc58.4 ± 3.30 a
3.0022.9 ± 0.42 a21.0 ± 1.02 b6.77 ± 0.14 c18.6 ± 1.00 a30.8 ± 1.82 b
Note: Different letters in the same column indicate significant differences between treatments (p < 0.05). PF1, PF2, PF3, PF4, PF5, the proportion of different fractions of Hg and Se in THg and TSe, respectively.
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Pei, G.; Li, Y.; Li, H. Impacts of Selenium Supplementation on Soil Mercury Speciation, Soil Properties and Mercury-Resistant Microorganisms and Resistant Genes. Agronomy 2024, 14, 1928. https://doi.org/10.3390/agronomy14091928

AMA Style

Pei G, Li Y, Li H. Impacts of Selenium Supplementation on Soil Mercury Speciation, Soil Properties and Mercury-Resistant Microorganisms and Resistant Genes. Agronomy. 2024; 14(9):1928. https://doi.org/10.3390/agronomy14091928

Chicago/Turabian Style

Pei, Guangpeng, Yuxin Li, and Hua Li. 2024. "Impacts of Selenium Supplementation on Soil Mercury Speciation, Soil Properties and Mercury-Resistant Microorganisms and Resistant Genes" Agronomy 14, no. 9: 1928. https://doi.org/10.3390/agronomy14091928

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