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ORIGINAL RESEARCH article

Front. Plant Sci., 21 September 2022
Sec. Plant Symbiotic Interactions
This article is part of the Research Topic Rhizospheric Microbiota-Plant Interactions: A Bioremediation Strategy for Inorganic Pollutants View all 5 articles

Combined effects of Bacillus sp. M6 strain and Sedum alfredii on rhizosphere community and bioremediation of cadmium polluted soils

\r\nAbbas Ali AbidAbbas Ali Abid1Gengmiao ZhangGengmiao Zhang2Dan HeDan He1Huanhe WangHuanhe Wang3Itrat BatoolItrat Batool4Hongjie DiHongjie Di1Qichun Zhang*Qichun Zhang1*
  • 1Zhejiang Provincial Key Laboratory of Agricultural Resources and Environment, Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, Zhejiang University, Hangzhou, China
  • 2Zhuji Agricultural Technology Extension Center, Zhuji, China
  • 3Zhuji Economic Speciality Station, Zhuji, China
  • 4Institute of Food Science, Chinese Academy of Agricultural Sciences, Beijing, China

Concerns regarding inevitable soil translocation and bioaccumulation of cadmium (Cd) in plants have been escalating in concomitance with the posed phytotoxicity and threat to human health. Exhibiting a Cd tolerance, Bacillus sp. M6 strain has been reported as a soil amendment owing to its capability of reducing metal bioavailability in soils. The present study investigated the rhizospheric bacterial community of the Cd hyperaccumulator Sedum alfredii using 16S rRNA gene sequencing. Additionally, the Cd removal efficiency of strain Bacillus sp. M6 was enhanced by supplementing with biochar (C), glutamic acid (G), and rhamnolipid (R) to promote the phytoremediation effect of hyperaccumulator S. alfredii. To the best of our knowledge, this is the first time the amendments such as C, G, and R together with the plant-microbe system S. alfredii-Bacillus sp. M6 has been used for Cd bioremediation. The results showed that soil CaCl2 and DTPA (Diethylenetriamine penta-acetic acid) extractable Cd increased by 52.77 and 95.08%, respectively, in all M6 treatments compared to unamended control (CK). Sedum alfredii with Bacillus sp. M6 supplemented with biochar and rhamnolipid displayed a higher phytoremediation effect, and the removal capability of soil Cd (II) reached up to 16.47%. Moreover, remediation of Cd polluted soil by Bacillus sp. M6 also had an impact on the soil microbiome, including ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and cadmium transporting ATPase (cadA) genes. Quantitative PCR analysis confirmed the Bacillus sp. M6 strain increased the abundance of AOB and cadA in both low Cd (LC) and high Cd (HC) soils compared to AOA gene abundance. Besides, the abundance of Proteobacteria and Actinobacteria was found to be highest in both soils representing high tolerance capacity against Cd. While Firmicutes ranked third, indicating that the additionof strain could not make it the most dominant species. The results suggested the presence of the hyperaccumulator S. alfredii and Cd tolerant strain Bacillus sp. M6 supplemented with biochar, and rhamnolipid, play a unique and essential role in the remediation process and reducing the bioavailability of Cd.

GRAPHICAL ABSTRACT
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Graphical Abstract. LC, low cadmium; HC, high cadmium; C, biochar; R, rhamnolipid; G: glutamic acid.

Introduction

As the intensification of anthropogenic activities such as fertilization, mining, sewage sludge, pesticide abuse, and metallurgy continues to evolve in the social economy it has consequently contributed to soil pollution in the form of heavy metals (Zhou et al., 2016). Cadmium (Cd) contamination of soil is becoming a ubiquitous environmental problem due to uncontrolled industrialization, unsustainable urbanization, and intensive agricultural practices. Cadmium is a toxic element, and severely threatens food safety, human health, and soil quality. As a matter of fact, the soil is exploited as an ultimate source of waste disposal and utilization. The activities in different natural and anthropogenic sources release Cd in the soil, which bio-accumulates in food crops. In China, at least 2 × 105 km2 area of farmland (one-fifth of the farmland) has been polluted by Cd (Fan et al., 2013). The mean soil Cd concentrations were found at 1.698 mg kg–1 in Zhangshi, an irrigated area of Shenyang (Sun et al., 2006), 0.35 mg kg–1 in Shanghai (Hu et al., 2004), 0.8 mg kg–1 in Hangzhou (Zhang and Ke, 2004), 0.04 mg kg–1 in Guangdong Province (Zhang et al., 2011), and 1.152 mg kg–1 in Tianjin (Wu and Cao, 2010). The presence of Cd is quite alarming for the cumulative soil environment because Cd is toxic to plants even at very low concentrations (0.5 μg Cd g–1 soil).

Reportedly, Cd influences nitrogen cycling microbial biomass in the soil (Liu et al., 2012a,b). Afzal et al. (2019) reported that nitrifying amoA gene abundance in paddy soils was reduced by adding Cd to 5 and 10 mg kg–1. Smolders et al. (2001) found that microbially mediated nitrification is suppressed up to 80% by adding Cd in the form of CdCl2 (200 mg kg–1). Similarly, Khan et al. (2010) confirmed a decrease in microbial enzyme urea amidohydrolase and acid phosphatase activities by up to 33.0% after 2 weeks of soil incubation with 5 mg Cd kg–1. Another study by Liu et al. (2018) showed that Cd decreases amoA gene abundance in the soils, reducing ammonium oxidation. Like nitrifiers, the denitrifying bacterial community was also sensitive to Cd, significantly impacting the denitrifier community structure (Afzal et al., 2019).

A plethora of heavy metal remediation techniques, such as phytoremediation, stabilization, solidification, electroremediation, and excavation, have been investigated under both controlled and field conditions (Mulligan et al., 2001). Among these techniques, the method of phytoremediation is frequently applied because it exhibits great potential for cost-effective performance and is environment-friendly. However, phytoremediation is mainly affected by the time for phytoremediation and plants’ physiological damage under high-stress conditions. Organic acids can chelate free heavy metal ions, reduce their bioavailability in the soil and eventually reduce their toxic effect (Yao et al., 2022). Besides, some organic acids can also mobilize heavy metals around the roots, making them easier to absorb by plants and achieve a cumulative effect. Biosurfactants, such as rhamnolipids, produced by microorganisms, i.e., Pseudomonas aeruginosa, play an essential role in reducing heavy metal toxicity in soil (Juwarkar et al., 2007). Rhamnolipid-mediated bioremediation is one of the frequently proposed potential strategies in the case of indigenous soil bacteria (Mulligan et al., 2001). Glutamic acid is another non-toxic amino acid produced by Bacillus species, which has been widely used in heavy metals adsorption (Lei et al., 2016; Luo et al., 2016). It contains several functional groups (-COOH), which can bind metal ions. Alternatively, biochar has a specific structure, and a high surface area can sorb contaminants and heavy metals (Tang et al., 2013).

Sedum alfredii has attracted increasing attention from researchers in China due to its distinct Cd co-hyperaccumulator tendencies (Yang et al., 2004a,b). The prevailing paradigm showed that metal-tolerant bacteria isolated from S. alfredii rhizosphere significantly enhanced Cd extraction (Pan et al., 2017). However, the basic ecology of indigenous soil microbial communities associated with the phytoextraction process in polluted soils by plant species needs further appraisal. In this study, a strain of Cd tolerant Bacillus sp. M6, isolated previously by Zhou et al. (2017), was used to mobilize heavy metals (mobilization capacity > 30%) by different carrier loading to increase the bioavailability of Cd in S. alfredii soil. For the first time, Bacillus sp. M6 was supplemented with biochar, glutamic acid, and rhamnolipid amendments to enhance the activity for Cd bioremediation. The effects of all these biochar, glutamic acid, and rhamnolipid amendments combined with Bacillus sp. M6 strain apropos of Cd remediation necessitates further exploration. Hence, the objectives of the present study are: (1) to explore the feasibility of Cd tolerant strain Bacillus sp. M6 as an inoculum to detoxify Cd; (2) to observe if soil inoculation with Bacillus sp. M6 and some amendments increase the efficiency of Cd removal from soil; (3) to analyze the composition and functionality of bacterial rhizobium upon inoculation; and (4) to evaluate the accumulation potential of Cd in S. alfredii.

Materials and methods

Soil sampling

To explore the phytoremediation strategies of Cd contaminated soil, low Cd contaminated (LC) and high Cd contaminated (HC) agricultural soils were sampled. The soils with Cd contents < 0.70 mg kg–1 (considered low-contaminated soils) were collected from Zhuji County of Shaoxing, Zhejiang Province, China. The study area belongs to a subtropical humid monsoon climate with an average annual temperature of 16.3?, having four distinct seasons, more rain, and sufficient light. The mean annual precipitation is about 1,373.6 mm, and the mean annual sunshine is about 1,887.6 h. The soils with Cd contents of 0.70 mg kg–1 or more were considered highly polluted soils, and HC was collected from farmland in the Fuyang District of Hangzhou City, Zhejiang Province. The sampling site of the Fuyang District has a subtropical monsoon climate, with an average annual temperature of 16.10?, annual average sunshine of 1,927.7 h, and a frost-free period of 231 days, and an annual rainfall of 1,441.9 mm. Random soil samples were collected from 0 to 20 cm of soil surface. The soil cores were packed in sterile plastic bags, sealed, and transported to the laboratory. Each sample was divided, and one subsample was incubated for the remediation experiment, while another sample was sieved through a 2.0 mm mesh for subsequent physicochemical analysis (Table 1).

TABLE 1
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Table 1. Basic physical and chemical properties of soil.

Microbial inoculum preparation

The isolated strain Bacillus sp. M6 with minimum inhibitory concentration (MIC) 100 mg L–1 CdCl2 was cultured in lysogeny broth (LB medium) at 28°C. The preliminary experiment indicated the strain has the increasing capability of Cd (II) biosorption and bioaccumulation (Zhou et al., 2017). The strain was then deposited in the NCBI gene bank1 with accession number KM349307. The cells were harvested after culturing in LB medium for 48 h at 28°C, at the mid-log phase of the growth curve, showing the optical density (microbial concentration/absorbance) of 0.1–1.2 at 600 nm to ensure cfu ≥ 1 × 108 cells/ml. After harvesting, the cells were centrifuged (5,000 rpm) at 4°C for 10 min, washed five times with deionized water, and re-suspended in a 0.9% NaCl solution. The strain was supplemented by (1) 1 g sterilized pine bamboo biochar (prepared in an oxygen-free closed container at 600? for 2 h), and 35 ml re-suspended strain were mixed and shaken at 160 rpm at 25°C for 6 h; (2) 1 g glutamic acid and 35 ml re-suspended strain were mixed and shaken at 160 rpm at 25°C for 6 h; (3) 1 mmol rhamnolipid (Ruijie Biology) and 35 ml re-suspended strain were mixed and shaken at 160 rpm at 25°C for 6 h.

Morphology by scanning electron microscopy

The effects of amendments on the structural morphology of the M6 strain were studied using the scanning electron microscopic (SEM) method. The supernatant was removed from the bacterial suspension by centrifugation. A small amount of the sample was wrapped with filter paper and placed overnight in a 2.5% glutaraldehyde solution at 4°C. After that; the samples were rinsed three times for 15 min with 0.1 M phosphoric acid buffer solution having pH 7. The samples were treated with 1% osmic acid solution for 2 h and rinsed again with phosphoric acid buffer solution. The samples were dehydrated with gradient concentration ethanol (including 30, 50, 70, 80, 90, and 95%) for 15 min and then dehydrated by 100% ethanol for 20 min and repeated two times. Finally, the samples were treated with a mixture of ethanol and isoamyl acetate (v/v = 1:1) for 30 min and then treated with pure isoamyl acetate. After keeping it overnight, the samples were dried at a critical limit. Finally, the treated samples were observed using a Hitachi TM-1,000 scanning electron microscope.

Pot experimental design/layout

A pot experiment was conducted to evaluate the effect of Cd on the soil microbial community. A PVC plastic pot with a diameter of 20 cm × 15 cm was used in the experiment. About 2.5 kg of air-dried soil was weighed and placed into the pots. The soil water contents were adjusted to 60% field capacity (FC), and five S. alfredii plants were transferred to each pot. Afterward, different bacterial suspensions were incorporated. Five sets of each treatment, including (1) Bacillus sp. M6 (M6); (2) Bacillus sp. M6 plus biochar (C); (3) Bacillus sp. M6 plus glutamic acid (G); (4) Bacillus sp. M6 plus rhamnolipid (R) were prepared, in addition, to control having ddH2O (CK). Each treatment had three further replicates. The different bacterial suspensions were poured into the roots of S. alfredii plant growing in the corresponding pot at a rate of 150 ml per pot after every 10 days. The S. alfredii plants were harvested after applying five doses of the bacterial suspension. After harvesting the plant, fresh soil samples were collected from each pot. Each replicated soil sample was then divided, and one subsample was stored at –80°C for subsequent microbial analysis. The other soil sample was sieved through a 2.0 mm mesh for subsequent analysis of chemical properties.

Plant sample analysis and soil characterization

Soil pH was measured with a pH meter (1:25) and soil available Cd contents were determined using graphite furnace atomic absorption spectrophotometer (PE AA800) with CaCl2 extraction. Soil ammonium and nitrate content were determined using the KCL extraction method using a continuous flow analyzer (SAN + + Skalar. Holland). The stems and leaves of S. alfredii were separated, washed with tap water followed by deionized water, and placed in the oven for 30 min at a temperature of 105°C to deactivate the enzymes. After obtaining the constant weight at 70°C, the plants were ground with a ball mill (JXFSTPRP-II, Shanghai Jingxin Industrial Development Co., Ltd.), passed through a 100 mm sieve then stored in the plastic zipper bag. Total Cd content in plants was determined by graphite furnace atomic absorption spectrophotometer (PE AA800) with microwave digestion method.

Extraction of soil DNA and quantitative polymerase chain reaction of functional genes

The Fast DNA Spin Kit was used to extract soil DNA from 0.5 g fresh soil samples according to the manufacturer’s instructions (MP Biomedicals, LLC, Solon, OH, United States). The size and integrity of DNA were examined by 0.7% agarose gel electrophoresis, and the quality and purity of DNA were analyzed by NanoDrop ND-2000 UV-vis (NanoDrop Technologies, Wilmington, DE, United States). The extracted DNA was stored at –20°C for further microbial analyses.

The ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and cadmium transporting ATPase (cadA) genes were quantified by quantitative PCR (qPCR) according to Muema et al. (2015). The qPCR was performed using Light Cycler 480 (Roche, Germany). A total of 20 μl reaction mixture was used for the amplification, including 10 μl of SYBR Premix Ex Taq (Takara, Dalian, China), 1 μl of DNA template (1–10 ng), 1 μl of each primer (10 μM), and added sterilized ddH2O 7 μl (Supplementary Table 1). The primers of each target gene and thermal cycling conditions are given in Supplementary Table 1. The standards of the targeted genes AOB, AOA, and cadA genes were prepared using the positive clones isolated from the plasmid. Plasmid DNA concentration was determined by NanoDrop 2,000 UV-Vis Spectrophotometer (NanoDrop Technologies, Wilmington, DE, United States) by calculating the AOB, AOA, and cadA genes copy numbers. Ten-fold serial dilutions were performed to generate the standard curve of the plasmid DNA. Clones with efficiency and correlation coefficients above 95% and 0.98, respectively, were employed to make the standard curve.

Bacterial 16S amplification and high throughput sequencing

The extracted DNA of soil samples was sequenced using the V4 region of 16S rRNA by Novogene, Shanghai, China.2 The primers 515F (5’-GTGCCAGCMGCCGCGGTAA-3’) and 806R (5’-G GACTACHVGGGTWTCTAAT-3’) were used for bacterial community analysis. High-Fidelity polymerase chain reaction (PCR) Master Mix with GC Buffer was purchased from New England Biolabs’ Phusion. High-efficiency and high-fidelity enzymes were used for PCR to ensure the efficiency and accuracy of amplification. The PCR mixture contained a 30 μl reaction volume, including 15 μl Phusion Master Mix (2 ×), 3 μl primer (2 μM), 15 μl gDNA (1 ng/μl), and 0 μl of sterilized ddH2O. The amplification was done for 1 min at 98°C, followed by 30 cycles of 10 s at 98°C, 30 s at 50°C, and 30 s at 72°C. The PCR products were then purified by 2% agarose gel electrophoresis. The band size of 400–450 bp as the target bands was recovered and purified using the GeneJET gel recovery kit (Thermo Scientific). A gene library was constructed using the New England Biolab’s NEB Next Ultra DNA Library Prep Kit. After Qubit quantification and library detection, the library was sequenced by HiSeq 2,500 PE250. QIIME 1.9.1 was used to filter, optimize, and control the sequence information. Finally, 97% of sequences with similar levels were classified into one OTUs. In addition, cluster analysis was performed based on calculated Bray-Curtis dissimilarities between the samples. The specific bacterial taxa were recognized separately by LEfSe from phylum to genus level.

Statistical data analysis

The standard errors and means of the data were computed using Excel (Microsoft, United States). The significance of the difference between the treatments was performed by one-way analysis of variance (ANOVA) using SPSS 20.0 (SPSS Inc., United States). The results were considered significant at p < 0.05. Graphs were prepared by using Origin Pro version 2021 (Origin lab corporation, Wellesley Hills, Wellesley, MA, United States).

Results and discussion

The hyperaccumulator S. alfredii not only tolerates high levels of Cd but also efficiently takes up and accumulates large amounts of Cd in its aboveground parts (Ge et al., 2021), and thereby could prove to be useful in the phytoremediation of Cd polluted soils (Yang et al., 2004a). Besides, rhamnolipids are bio-surfactants produced by microorganisms, i.e., Pseudomonas aeruginosa, with the potential to mobilize heavy metals in the soil (Mulligan et al., 2001; Juwarkar et al., 2007). A Cd tolerant Bacillus sp. M6 strain was used, with a tolerance capacity of 14.1% toward Cd (Zhou et al., 2017). The inoculation of Bacillus can improve the mobility and bioavailability of Cd in soil and increases plants’ extraction rate by two times. In this study, we found that the addition of Bacillus sp. M6 could effectively increase the exchangeable state of Cd in the rhizosphere soil of S. alfredii (Jeong et al., 2012; Mo et al., 2022). Since this remains challenging how the supplemented Cd tolerant Bacillus sp. M6 strain influences the rhizosphere community composition and activity in Cd contaminated soils. Therefore, a 16 S high-throughput sequencing approach was applied to gain insights into the rhizosphere microbial community composition that may influence metal accumulation in the hyperaccumulator S. alfredii.

Morphology of the supplemented strain

The surface membrane of bacteria under Bacillus sp. M6 strain supplemented with rhamnolipid was rough (Figures 1b,d) in contrast to the bacteria without rhamnolipid (Figures 1a,c), the surface membrane was smooth, indicating rhamnolipid impact on the permeability of the bacterial surface membrane. The same results were found by Ragheb et al. (2000) demonstrated rhamnolipid as a biosurfactant, mobilizes heavy metals in soil by metal desorption from rhizosphere through complexation, ion exchange, or accumulation on the surface by reducing interfacial tension (Lal et al., 2018; Mishra et al., 2021) and changes the membrane permeability of bacteria that could help bacteria to release organic acids and enhance the activation effect. The strain Bacillus sp. M6 adsorbed significantly in the gaps and on the surface of biochar, respectively (Figures 1e,f). A porous microstructure and lacunose surfaces of biochar provide habitats for bacteria. Moreover, the soluble organic carbon and other nutrients adsorbed by biochar can provide substrates for the survival of microorganisms, thus improving the abundance and activity of microbes in the soil (Atkinson et al., 2010; Beesley and Marmiroli, 2011).

FIGURE 1
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Figure 1. The bacterial structure of electron microscopy by using Bacillus sp. M6. And amendments (a) M6 strain 2 μm scale; (b) M6 + rhamnolipid 2 μm scale; (c) M6 strain 500 nm scale; (d) M6 + rhamnolipid 500 nm scale; (e,f) M6 + biochar.

Effects of the supplemented strain on soil pH and available Cd content

In order to precisely predict metal availability in soils, we use CaCl2 and DTPA (diethylenetriaminepentaacetic acid) extractions measurements to assess metal availability in soils (Hseu and Lai, 2017). The availability of the metals in soils cannot be precisely predicted unless we measure CaCl2 and DTPA extractions (Hseu and Lai, 2017). Moreover, the metals adsorbed on organic matter are potentially available and can be extracted by DTPA. Hence, it is important to extract DTPA-Cd to make our results more precise.

The CaCl2 and DTPA extractable Cd were significantly affected by the supplemented strain. In HC soils, CaCl2 extractable Cd increased by 45.91∼52.77% in all supplemented treatments compared to CK. However, no significant difference among the supplemented treatments was found (Figure 2A). Besides, DTPA extractable Cd content was enhanced by 23.31∼93.84% compared to CK, and the increase in treatments M6 and R was recorded as significantly higher than that of C and G treatments in both high and low Cd soils (Figures 2A,B). In LC soils, CaCl2 extractable Cd increased significantly by 21.14∼48.98% in all supplemented treatments compared to CK. While comparing supplemented treatments, M6 promoted the most CaCl2 extractable Cd (Figure 2B). Besides, DTPA extractable Cd content in LC soils increased by 28.52∼95.08%, and the highest increase was observed in treatments M6 and R. The same results were found in HC soils, indicating that rhamnolipid promoted extractable Cd because of its biosurfactant, which has less toxicity than an active chemical agent and can degrade itself in soil due to its activation effect (Zhao et al., 2018). In addition, it can enhance the migration ability of heavy metals by reducing the interfacial tension between heavy metals and soil (Wang and Mulligan, 2004).

FIGURE 2
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Figure 2. Changes in pH and available cadmium in LC and HC soils. (A) Changes in extractable Cd of DTPA and CaCl2 in HC soils; (B) changes in extractable Cd of DTPA and CaCl2 in LC soils; (C) changes in soil pH; (D) correlation between soil pH and CaCl2-extracted Cd. The arrow bar above the line shows standard error. The different letters show significant differences while same letters represent no significant differences among the treatments. CK, control; M6, M6 strain; C, M6 + Biochar; G, M6 + Glutamic acid; R, M6 + rhamnolipid; FY, Fuyang; ZJ, Zhuji; LC, low cadmium; HC, high cadmium; DTPA, Diethyene triaminepenta acetic acid.

It has been found that Bacillus sp. M6 strain can reduce soil pH to enhance soil Cd availability (Zhou et al., 2017). Compared to CK, the pH of all treatments decreased significantly in both soils (Figure 2C). The soil pH influences the solubility of heavy metals and the ionization state of numerous functional groups on a large surface area (Argun et al., 2009). At low pH, large amounts of H + ion cause strong electrostatic repulsion, which decreases the binding of Cd (II) by carboxyl groups and other acidic functional groups. A significant negative correlation between CaCl2 extractable Cd and pH in LC and HC soils, respectively, was found (p < 0.05, p < 0.001) (Figure 2D), which is in accordance with the previous studies (McBride, 2002; Römkens et al., 2011). After amendment application, the pH of the soil was reduced, and the amount of extractable Cd was improved, favoring the accumulation of Cd by hyper-accumulators.

Effect of the different supplemented strains on soil functional gene abundance

The introduction of Bacillus sp. M6 strain increased the abundance of AOB in all supplemented treatments, while the AOA gene abundance was decreased in all supplemented treatments compared to CK (Figures 3A,B). In HC soils, the AOA gene abundance ranged from 0.99 × 1010-1.74 × 1010 copies/g in dry soils. The AOA gene abundance in M6, C, and R treatments was significantly decreased, and the lowest abundance was observed in R treatment (Figure 3A). Although, rhamnolipid can mobilize soil heavy metals by itself. It also changes the membrane permeability of some bacteria, which may negatively impact the growth and survival of some bacteria. In LC soils, the AOA gene abundance was 3.78 × 108 – 5.13 × 108 copies/g in dry soil, and the decrease was not evident compared to CK, except in the biochar treatment (Figure 3B). Interestingly, the AOB gene abundance increased significantly in all supplemented treatments than CK, indicating the abundance of nitrifying bacteria being increased with the addition of exogenous bacteria. This AOB increase might be due to the introduction of exogenous microbial agents, increasing soil mineralization. The active carbon and nitrogen improved by mineralization can promote amoA AOB growth (Liu et al., 2018), whereas amoA AOA plays a role in the soil environment with low nitrogen content. Also, more bacterial activity was found in low contaminated soil than with high contamination because of the growth restrictions by high Cd concentration manifesting as the inhibition of cell growth and cell detoxification mechanism (i.e., intracellular efflux of Cd+2).

FIGURE 3
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Figure 3. Abundance of soil functional genes amoA AOA and amoA AOB in (A) HC (B) LC soils collected from two different soils. (C) The abundance of cadA genes in two soils. Data points represent means the standard deviations (n = 3). The various lowercase letters indicate significant differences among all treatments at p < 0.05. CK, control; M6, M6 strain; C, M6 + Biochar; G, M6 + Glutamic acid; R, M6 + rhamnolipid; FY, Fuyang County; ZJ, Zhuji County; LC, low cadmium; HC, high cadmium.

The cadA functional gene can encode P-type ATPase in the cad-ABC system of Gram-positive bacteria. The strain can pump Cd out of the bacteria through the enzyme encoded by the cadA gene to achieve detoxification (Tsai et al., 1992; Sebastian et al., 2019). The abundance of cadA gene in HC and LC soils ranged from 8.81 × 107-1.22 × 108 copies/g dry soil and 1.06 × 108-1.56 × 108 copies/g dry soil, respectively (Figure 3C). It was found that the cadA gene abundance in all supplemented treatments was increased compared to CK, which was beneficial in enhancing the resistance of soil microorganisms to Cd. The cadA abundance of G treatment was significantly increased by 38.48 and 47.17% in Fuyang and Zhuji, respectively, than that of the CK treatment, indicating that glutamic acid was more conducive to bacterial growth.

Effect of the supplemented strain on soil microbial community

The soil microbial community is the leading player in heavy metal remediation in the soil. Also, plant growth and the availability of heavy metals can be stimulated by microbial communities. Hence, understanding the basic ecology of soil communities associated with plant remediation is very important to optimize the plant remediation process (Wood et al., 2016). The results showed the soil microbial community was affected to a certain extent after applying different treatments. The abundance of Proteobacteria and Actinobacteria was found highest in both soils (Supplementary Figures 1a,b), showing high tolerance capacity against Cd (Liu et al., 2020a,b). While Firmicutes ranked third, indicating the addition of strain could not make it the most dominant species. Most of the heavy metal-resistant genes are associated with phyla Firmicutes, and that is why phyla Firmicutes can promote their adaptation to heavy metal polluted environments (Guo et al., 2019). Contradictory to that, the phyla Firmicutes growth in our study was not higher than Proteobacteria and Actinobacteria. The reason might be due to increasing bacterial suspension doses (five doses). Hence, the strain was no more competitive in the rhizosphere, since the Firmicutes population is not the most relevant taxa upon repeated inoculation. In HC soils, the relative abundance of Firmicutes ranged from 5.91 to 14.16% (Supplementary Figure 1A). Moreover, the abundance of Firmicutes in R treatment was reduced relatively. In contrast, the highest abundance was found in the C treatment, indicating that biochar had promoted the survival efficiency of Firmicutes while glycolipids reduced the survival efficiency of Firmicutes. A similar trend of Firmicutes abundance was found In LC soils (Supplementary Figure 1B). The number of OTUs from low to high was represented by blue to red (Supplementary Figure 1A), and it can be observed clearly that the abundance of Firmicutes in the C and M6 treatment was high. The rationale for the high abundance of Firmicutes in C treatment was due to adding biochar as a sole carbon source. The phospholipid bilayer structure on the cell surface of the strain was more compatible with rhamnolipid. After long-term Cd addition, the rhamnolipid may damage the membrane structure and affect its uptake and assimilation mechanism (Kuyukina et al., 2005). Subsequently, the porous structure of biochar facilitates the growth of bacteria and changes microbial community structure in soil (Wang et al., 2018). A previous study has also shown that biochar can affect soil microbial community composition by regulating the redox process of heavy metal ions (Chen et al., 2016).

The cluster analysis of relative abundance by Bray Curtis distance showed the bacterial community relative abundance was different in C and M6 treatments from that of the un-amended control (Figure 4). Linear discriminant analysis (LEfSe) was further used to study the effects of different treatments on soil microbial abundance at different classification levels (Figure 5). The LDA > 3.0 was chosen as a significant difference in abundance. In HC soil, the abundance of 24 differentiated branches increased in C treatment from the phylum level to the species level, which was the highest among all treatments. The same results were found in LC soil with a total of 14 branches. Besides, the biochar with strain Bacillus sp. M6 exhibited a significant response in HC soil from order to genus level, suggesting that biochar treatment positively affected the relative activity of the target strain, more evidently in HC soils. The biochar has an advantage over other amendments because it has large porosity, specific surface area, and abundant oxygen-containing functional groups on the surface. Also, because of the strong electrostatic field and adsorption reaction, it can bind small and charged particles, having a size < 0.25 mm, to form larger aggregates (Wang et al., 2019). Besides, the effect of biochar on soil microbial and enzyme activities might enhance the soil organic matter storage significantly and improve soil aggregate stability (Tang et al., 2022).

FIGURE 4
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Figure 4. The cluster dendrogram of Bray-Curtis dissimilarity of samples. CK, control; M6, M6 strain; C, M6 + Biochar; G, M6 + Glutamic acid; R, M6 + rhamnolipid; F, Fuyang County; Z, Zhuji County.

FIGURE 5
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Figure 5. LEfSe cladogram indicating the phylogenetic distribution of bacteria lineages under different treatments in HC (A) and LC (B) soils. In the figure, red, green, dark blue, purple, and light blue circles, respectively represent the flora of C, CK, G, M6, and R treatments which are significantly higher than those of the other four treatments. LDA > 3 is taken to represent the significant difference, and the circles from inside to outside, respectively represent the classification level of domain, phylum, class, order, family, genus, and species. CK, control; M6, M6 strain; C, M6 + Biochar; G, M6 + Glutamic acid; R, M6 + rhamnolipid; FY, Fuyang County; ZJ, Zhuji County; LC, low cadmium; HC, high cadmium; LEfSe, Linear discriminant analysis effect size; LDA, Linear discriminant analysis.

Cadmium contents of Sedum alfredii under different treatments

It was found that S. alfredii has the ability to absorb more Cd, which in turn led to a decrease of Cd concentration in the rhizosphere, in line with the results of Tang et al. (2020), who revealed in a field trial that S. alfredii uptake Cd from the rhizosphere. Hou et al. (2018) confirmed after 150 days of the experiment that S. alfredii exhibited a significant difference in metal tolerance and accumulation in Cd contaminated soils (Hou et al., 2018). After the addition of treatments, Cd level increased in the S. alfredii leaves compared to CK, especially under C and R treatments, increased by 54.05 and 47.11%, respectively (Figure 6A), showing that C and R treatments promoted phytoremediation. However, Cd content in leaf and stem was significantly reduced in G treatment compared to CK. The inoculation of Cd-resistant bacteria, which is immobilized by biochar, significantly increased the accumulation of Cd in the stem and roots of the plant (Chuaphasuk and Prapagdee, 2019). Li et al. (2007) found that the combined effects of bacteria and S. alfredii increased the accumulation of heavy metals in plants, which was inconsistent with our results. In stem, significantly higher Cd accumulation was found in R and C treatments compared to CK. The total plant Cd content in R was 3.58 mg kg–1, which was higher than CK, while total soil Cd content in C and R treatments was found to be lowest (Figure 6C). Among C and R, comparatively lower Cd contents in the soils was found in R treatment (0.43 mg kg–1), significantly lower than that of CK by 16.47% (Figure 6C).

FIGURE 6
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Figure 6. Cd content in stems and leaves of Sedum alfredii (A) and total Cd and available Cd content in LC soil (B), Cd content of Sedum alfredii and total soil Cd content in HC (C). The data is the means and standard deviations of three replicates (n = 3). The significant differences among all treatments (p < 0.05) are presented by lowercase letters. LC, low cadmium; HC, high cadmium.

Under the influence of S. alfredii root absorption, the soil available Cd changed significantly (Figure 6B). Soil available Cd in C treatments decreased significantly compared to CK, while a 14.33% increase was observed in M6 treatment compared to CK (Figure 6B). However, a significant increase was observed in Cd content of S. alfredii plants in R treatment, and the increase was 10.59% in comparison with CK in HC soils (Figure 6C). The results also reveal that the total soil Cd content decreased in all treatments, especially in C and R treatment, which reduced by 24.60∼26.70% compared with CK. The main reason might be the growth inhibition of strain to some extent, with the increase of Cd pollution, resulting in a reduced activation effect.

Conclusion

The specific function of strain Bacillus sp. M6 was supplemented by bamboo biochar, glutamic and rhamnolipid. A hyperaccumulating plant and a bacterial strain were combined to remediate low and high Cd contaminated soils. The results indicated that soil pH decreased after being treated with supplemented strain and S. alfredii. In C treatment, S. alfredii showed a good phytoremediation effect, and the total Cd content of the soil was significantly reduced, especially in low Cd contaminated soil. Besides, the microbial abundance of soil nitrogen cycling function was also affected by adding microbial agents. Compared to the control, the Cd accumulation in stems and leaves of S. alfredii under R and C treatment was increased. It has been concluded that R and C amendments combined with M6 strain are the best options. Further studies are needed to evaluate the mechanism of Cd accumulation and remediation using the M6 strain with different R and C levels.

Data availability statement

The data can be extracted from the NCBI gene bank with accession number: KM349307.

Author contributions

AA: conceptualization, methodology, software, data curation, and writing–original draft preparation. GZ and HD: writing—review and editing. DH: methodology. IB and HW: methodology, writing, and software. QZ: conceptualization, writing—review and editing, validation, funding acquisition, and resources. All authors contributed to the article and approved the submitted version.

Funding

This work was supported by the National Key Research and Development Program of China (2021YFD1700803), the National Natural Science Foundation of China (41877044), and the Natural Science Foundation of Zhejiang Province (LZ21C030002 and LGN21C200017).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2022.913787/full#supplementary-material

Footnotes

  1. ^ https://www.ncbi.nlm.nih.gov
  2. ^ www.novogene.com

References

Afzal, M., Yu, M., Tang, C., Zhang, L., Muhammad, N., Zhao, H., et al. (2019). The negative impact of cadmium on nitrogen transformation processes in a paddy soil is greater under non-flooding than flooding conditions. Environ. Int. 129, 451–460. doi: 10.1016/j.envint.2019.05.058

PubMed Abstract | CrossRef Full Text | Google Scholar

Argun, M. E., Dursun, S., and Karatas, M. (2009). Removal of Cd (II) and Pb (II), Cu (II) and Ni (II) from water using modified pine bark. Desalination 24, 519–527. doi: 10.1016/j.desal.2009.01.020

CrossRef Full Text | Google Scholar

Atkinson, C. J., Fitzgerald, J. D., and Hipps, N. A. (2010). Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review. Plant Soil 337, 1–18. doi: 10.1007/s11104-010-0464-5

CrossRef Full Text | Google Scholar

Beesley, L., and Marmiroli, M. (2011). The immobilisation and retention of soluble arsenic, cadmium and zinc by biochar. Environ. Pollut. 159, 474–480. doi: 10.1016/j.envpol.2010.10.016

PubMed Abstract | CrossRef Full Text | Google Scholar

Chen, Z., Wang, Y., and Xia, D. (2016). Enhanced bioreduction of iron and arsenic in sediment by biochar amendment influencing microbial community composition and dissolved organic matter content and composition. J. Hazard. Mat. 311, 20–29. doi: 10.1016/j.jhazmat.2016.02.069

PubMed Abstract | CrossRef Full Text | Google Scholar

Chuaphasuk, C., and Prapagdee, B. (2019). Effects of biochar-immobilized bacteria on phytoremediation of cadmium-polluted soil. Environ. Sci. Pollut. Res. 26, 23679–23688. doi: 10.1007/s11356-019-05661-6

PubMed Abstract | CrossRef Full Text | Google Scholar

Fan, T., Ye, W. L., Chen, H. Y., Lu, H. J., Zhang, Y. H., and Li, D. X. (2013). Review on contamination and remediation technology of heavy metal in agricultural soil. Ecol. Environ. Sci. 22, 1727–1736.

Google Scholar

Ge, J., Tian, S., Yu, H., Zhao, J., Chen, J., Pan, L., et al. (2021). Exogenous application of Mn significantly increased Cd accumulation in the Cd/Zn hyperaccumulator Sedum alfredii. Environ. Pollut. 278:116837. doi: 10.1016/j.envpol.2021.116837

PubMed Abstract | CrossRef Full Text | Google Scholar

Guo, D., Fan, Z., Lu, S., Ma, Y., Nie, X., Tong, F., et al. (2019). Changes in rhizosphere bacterial communities during remediation of heavy metal-accumulating plants around the Xikuangshan mine in southern China. Sci. Rep. 9:1947. doi: 10.1038/s41598-018-38360-2

PubMed Abstract | CrossRef Full Text | Google Scholar

Hou, D., Lin, Z., Wang, R., Ge, J., Wei, S., Xie, R., et al. (2018). Cadmium exposure-Sedum alfredii planting interactions shape the bacterial community in the hyperaccumulator plant rhizosphere. Appl. Environ. Microbiol. 84:e02797-17. doi: 10.1128/AEM.02797-17

PubMed Abstract | CrossRef Full Text | Google Scholar

Hseu, Z. Y., and Lai, Y. J. (2017). Nickel accumulation in paddy rice on serpentine soils containing high geogenic nickel contents in Taiwan. Environ. Geochem. Health 39, 1325–1334. doi: 10.1007/s10653-017-9925-6

PubMed Abstract | CrossRef Full Text | Google Scholar

Hu, X. F., Wu, H. X., Hu, X., Fang, S. Q., and Wu, C. J. (2004). Impact of urbanization on Shanghai’s soil environmental quality. Pedosphere 14, 151–158.

Google Scholar

Jeong, S., Moon, H. S., Nam, K., Kim, J. Y., and Kim, T. S. (2012). Application of phosphate- solubilizing bacteria for enhancing bioavailability and phytoextraction of cadmium (Cd) from polluted soil. Chemosphere 88, 204–210. doi: 10.1016/j.chemosphere.2012.03.013

PubMed Abstract | CrossRef Full Text | Google Scholar

Juwarkar, A., Anupa, N., Dubey, K. V., Singh, S. K., and Sukumar, D. (2007). Biosurfactant technology for remediation of cadmium and lead contaminated soils. Chemosphere 68, 1996–2002. doi: 10.1016/j.chemosphere.2007.02.027

PubMed Abstract | CrossRef Full Text | Google Scholar

Khan, S., El-Latif Hesham, A., Qiao, M., Rehman, S., and He, J. Z. (2010). Effects of cd and Pb on soil microbial community structure and activities. Environ. Sci. Pollut. Res. 17, 288–296. doi: 10.1007/s11356-009-0134-4

PubMed Abstract | CrossRef Full Text | Google Scholar

Kuyukina, M. S., Ivshina, I. B., and Makarov, S. O. (2005). Effect of biosurfactants on crude oil desorption and mobilization in a soil system. Environ. Int. 31, 155–161. doi: 10.1016/j.envint.2004.09.009

PubMed Abstract | CrossRef Full Text | Google Scholar

Lal, S., Ratna, S., Said, O. B., and Kumar, R. (2018). Biosurfactant and exopolysaccharideassisted rhizobacterial technique for the remediation of heavy metal contaminated soil: an advancement in metal phytoremediation technology. Environ. Technol. Innov. 10, 243–263. doi: 10.1016/j.eti.2018.02.011

CrossRef Full Text | Google Scholar

Lei, P., Xu, Z., Liang, J., Luo, X., Zhang, Y., Feng, X., et al. (2016). Poly (γ-glutamic acid) enhanced tolerance to salt stress by promoting proline accumulation in Brassica napus L. Plant Growth Regul. 78, 233–241. doi: 10.1007/s10725-015-0088-0

CrossRef Full Text | Google Scholar

Li, W., Ye, Z., and Wong, M. (2007). Effects of bacteria on enhanced metal uptake of the Cd/Zn-hyperaccumulating plant, Sedum alfredii. J. Exp. Bot. 58, 4173–4182. doi: 10.1093/jxb/erm274

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, J., Cao, W., Jiang, H., Cui, J., Shi, C., Qiao, X., et al. (2018). Impact of heavy metal pollution on ammonia oxidizers in soils in the vicinity of a Tailings Dam, Baotou, China. Bull. Environ. Contam. Toxicol. 101, 110–116. doi: 10.1007/s00128-018-2345-1

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, S., Li, F., Wei, X., and Nong, M. (2012a). Effects of alternate partial root-zone irrigation on maize water use and soil microbial biomass carbon. Trans. Chin. Soc. Agric. Eng. 28, 71–77.

Google Scholar

Liu, Y., Zhou, T., Crowley, D., Li, L., Liu, D., Zheng, J., et al. (2012b). Decline in topsoil microbial quotient, fungal abundance and C utilization efficiency of rice paddies under heavy metal pollution across South China. PLoS One 7:e38858. doi: 10.1371/journal.pone.0038858

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, S., Yang, B., and Liang, Y. (2020a). Prospect of phytoremediation combined with other approaches for remediation of heavy metal-polluted soils. Environ. Sci. Pollut. Res. 27, 16069–16085. doi: 10.1007/s11356-020-08282-6

PubMed Abstract | CrossRef Full Text | Google Scholar

Liu, W., Li, Y., Feng, Y., Qiao, J., Zhao, H., Xie, J., et al. (2020b). The effectiveness of nanobiochar for reducing phytotoxicity and improving soil remediation in cadmium-contaminated soil. Sci. Rep. 10:858. doi: 10.1038/s41598-020-57954-3

PubMed Abstract | CrossRef Full Text | Google Scholar

Luo, Z., Guo, Y., Liu, J., Qiu, H., Zhao, M., Zou, W., et al. (2016). Microbial synthesis of poly-γ-glutamic acid: current progress, challenges, and future perspectives. Biotechnol. Biofuels 9:134. doi: 10.1186/s13068-016-0537-7

PubMed Abstract | CrossRef Full Text | Google Scholar

McBride, M. B. (2002). Cadmium uptake by crops estimated from soil total Cd and pH. Soil Sci. 167, 62–67. doi: 10.1097/00010694-200201000-00006

CrossRef Full Text | Google Scholar

Mishra, S., Lin, Z., Pang, S., Zhang, Y., Bhatt, P., and Chen, S. (2021). Biosurfactant is a powerful tool for the bioremediation of heavy metals from contaminated soils. J. Hazard. Mat. 418:126253. doi: 10.1016/j.jhazmat.2021.126253

PubMed Abstract | CrossRef Full Text | Google Scholar

Mo, T., Jiang, D., Shi, D., Xu, S., Huang, X., and Huang, Z. (2022). Remediation mechanism of “double-resistant” bacteria—Sedum alfredii Hance on Pb and Cd-contaminated soil. Ecol. Process. 11, 1–15. doi: 10.1186/s13717-021-00347-9

CrossRef Full Text | Google Scholar

Muema, E. K., Cadisch, G., Roehl, C., Vanlauwe, B., and Rasche, F. (2015). Response of ammonia-oxidizing bacteria and archaea to biochemical quality of organic inputs combined with mineral nitrogen fertilizer in an arable soil. Appl. Soil Ecol. 95, 128–139. doi: 10.1016/j.apsoil.2015.06.019

CrossRef Full Text | Google Scholar

Mulligan, C. N., Yong, R. N., and Gibbs, B. F. (2001). Heavy metal removal from sediments by biosurfactants. J. Hazard Mat. 85, 111–125. doi: 10.1016/S0304-3894(01)00224-2

CrossRef Full Text | Google Scholar

Pan, F., Meng, Q., Luo, S., Shen, J., Chen, B., Khan, K. Y., et al. (2017). Enhanced Cd extraction of oilseed rape (Brassica napus) by plant growth-promoting bacteria isolated from Cd hyperaccumulator Sedum alfredii Hance. Int. J. Phytoremediat. 19, 281–289. doi: 10.1080/15226514.2016.1225280

PubMed Abstract | CrossRef Full Text | Google Scholar

Ragheb, A., Al-Tahhan, Todd, R. S., Adria, A. B., and Raina, M. M. (2000). Rhamnolipid-induced removal of lipopolysaccharide from Pseudomonas aeruginosa: effect on cell surface properties and interaction with hydrophobic substrates. Appl. Environ. Microbiol. 66, 3262–3268. doi: 10.1128/AEM.66.8.3262-3268.2000

PubMed Abstract | CrossRef Full Text | Google Scholar

Römkens, P., Brus, D. J., and Guo, H. Y. (2011). Impact of model uncertainty on soil quality standards for cadmium in rice paddy fields. Sci. Total Environ. 409, 3098–3105. doi: 10.1016/j.scitotenv.2011.04.045

PubMed Abstract | CrossRef Full Text | Google Scholar

Sebastian, A., Nangia, A., and Prasad, M. (2019). Cadmium Toxicity and Tolerance in Micro- and Phytobiomes. Cadmium Toxicity and Tolerance in Plants. Cambridge, MA: Academic Press, 19–46. doi: 10.1016/B978-0-12-814864-8.00002-4

CrossRef Full Text | Google Scholar

Smolders, E., Brans, K., Coppens, F., and Merckx, R. (2001). Potential nitrification rate as a tool for screening toxicity in metal-contaminated soils. Environ. Toxicol. Chem. 20, 2469–2474. doi: 10.1002/etc.5620201111

CrossRef Full Text | Google Scholar

Sun, L. N., Zhang, Y. H., Sun, T. H., Gong, Z. Q., Lin, X., and Li, H. B. (2006). Temporal-spatial distribution and variability of cadmium contamination in soils in Shenyang Zhangshi irrigation area, China. J. Environ. Sci. 18, 1241–1246. doi: 10.1016/S1001-0742(06)60069-7

CrossRef Full Text | Google Scholar

Tang, B., Xu, H., Song, F., Ge, H., Chen, L., Yue, S., et al. (2022). Effect of biochar on immobilization remediation of Cd?contaminated soil and environmental quality. Environ. Res. 204:111840. doi: 10.1016/j.envres.2021.111840

PubMed Abstract | CrossRef Full Text | Google Scholar

Tang, J., Zhu, W., Kookana, R., and Katayama, A. (2013). Characteristics of biochar and its application in remediation of contaminated soil. J. Biosci. Bioeng. 116, 653–659. doi: 10.1016/j.jbiosc.2013.05.035

PubMed Abstract | CrossRef Full Text | Google Scholar

Tang, L., Hamid, Y., Zehra, A., Sahito, Z. A., He, Z. L., Beri, W. T., et al. (2020). Fava bean intercropping with Sedum alfredii inoculated with endophytes enhances phytoremediation of cadmium and lead co-contaminated field. Environ. Pollut. 265:114861. doi: 10.1016/j.envpol.2020.114861

PubMed Abstract | CrossRef Full Text | Google Scholar

Tsai, K. J., Yoon, K. P., and Lynn, A. R. (1992). ATP-dependent cadmium transport by the cadA cadmium resistance determinant in everted membrane vesicles of Bacillus subtilis. J. Bacteriol. 174, 116–121. doi: 10.1128/jb.174.1.116-121.1992

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, M., Zhu, Y., and Cheng, L. (2018). Review on utilization of biochar for metal-contaminated soil and sediment remediation. J. Environ. Sci. 63, 156–173. doi: 10.1016/j.jes.2017.08.004

PubMed Abstract | CrossRef Full Text | Google Scholar

Wang, S., and Mulligan, C. N. (2004). Rhamnolipid foam enhanced remediation of cadmium and nickel contaminated soil. Water Air Soil Pollut. 157, 315–330. doi: 10.1023/B:WATE.0000038904.91977.f0

CrossRef Full Text | Google Scholar

Wang, Y. Q., Niu, W. Q., Li, X. K., Wang, J., Guan, Y. H., and Dong, J. H. (2019). Effects of biochar on soil aggregate structure in greenhouse. Bull. Soil Water Conserv. 39, 190–195.

Google Scholar

Wood, J. L., Zhang, C., and Mathews, E. R. (2016). Microbial community dynamics in the rhizosphere of a cadmium hyper-accumulator. Sci. Rep. 6:36067. doi: 10.1038/srep36067

PubMed Abstract | CrossRef Full Text | Google Scholar

Wu, G. H., and Cao, S. S. (2010). Mercury and cadmium contamination of irrigation water, sediment, soil and shallow groundwater in a wastewater-irrigated field in Tianjin, China. Bull. Environ. Contam. Toxicol. 84, 336–341. doi: 10.1007/s00128-010-9939-6

PubMed Abstract | CrossRef Full Text | Google Scholar

Yang, X. E., Long, X. X., Ye, H. B., He, Z. L., Calvert, D. V., and Stoffella, P. J. (2004a). Cadmium tolerance and hyperaccumulation in a new Zn-hyperaccumulating plant species (Sedum alfredii Hance). Plant Soil 259, 181–189. doi: 10.1023/B:PLSO.0000020956.24027.f2

CrossRef Full Text | Google Scholar

Yang, X. E., Ye, H. B., Long, X. X., He, B., He, Z. L., and Stoffella, P. J. (2004b). Uptake and accumulation of cadmium and zinc by Sedum alfredii Hance at different Cd/Zn supply levels. J. Plant Nutr. 27, 1963–1977. doi: 10.1081/PLN-200030082

CrossRef Full Text | Google Scholar

Yao, W., Huang, L., Yang, Z., and Zhao, F. (2022). Effects of organic acids on heavy metal release or immobilization in contaminated soil. Trans. Nonferrous Metals Soc. China 32, 1277–1289. doi: 10.1016/S1003-6326(22)65873-4

CrossRef Full Text | Google Scholar

Zhang, H. H., Chen, J. J., Zhu, L., Li, F. B., Wu, Z. F., and Yu, W. M. (2011). Spatial patterns and variation of soil cadmium in Guangdong Province, China. J. Geochem. Explor. 109, 86–91. doi: 10.1016/j.gexplo.2010.10.014

CrossRef Full Text | Google Scholar

Zhang, M. K., and Ke, Z. X. (2004). Heavy metals, phosphorus and some other elements in urban soils of Hangzhou City, China. Pedosphere 14, 177–185.

Google Scholar

Zhao, F., Shi, R., Ma, F., Han, S., and Zhang, Y. (2018). Oxygen effects on rhamnolipids production by Pseudomonas aeruginosa. Microb. Cell Fact. 17:39. doi: 10.1186/s12934-018-0888-9

PubMed Abstract | CrossRef Full Text | Google Scholar

Zhou, H., Wang, J., Sun, J., Li, D., and Zhang, Q. (2017). Remediation of cadmium- tolerant bacteria combined with plant absorption on soil heavy metal cadmium pollution. J. Zhejiang Univ. 43, 341–349.

Google Scholar

Zhou, Q., Liu, Z. D., Liu, Y., Jiang, J., and Xu, R. K. (2016). Relative abundance of chemical forms of Cu(II) and Cd(II) on soybean roots as influenced by pH, cations and organic acids. Sci. Rep. 6:36373. doi: 10.1038/srep36373

PubMed Abstract | CrossRef Full Text | Google Scholar

Keywords: Sedum alfredii, Bacillus sp. M6, cadmium, soil microbial community, biochar (BC), bioremediation, rhamnolipid

Citation: Abid AA, Zhang G, He D, Wang H, Batool I, Di H and Zhang Q (2022) Combined effects of Bacillus sp. M6 strain and Sedum alfredii on rhizosphere community and bioremediation of cadmium polluted soils. Front. Plant Sci. 13:913787. doi: 10.3389/fpls.2022.913787

Received: 06 April 2022; Accepted: 01 August 2022;
Published: 21 September 2022.

Edited by:

Ying Ma, University of Coimbra, Portugal

Reviewed by:

Adnane Bargaz, Mohammed VI Polytechnic University, Morocco
Eloisa Pajuelo, Seville University, Spain

Copyright © 2022 Abid, Zhang, He, Wang, Batool, Di and Zhang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Qichun Zhang, qczhang@zju.edu.cn

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.