Phytoremediation effect of Scirpus triqueter inoculated plant-growth-promoting bacteria (PGPB) on different fractions of pyrene and Ni in co-contaminated soils
Graphical abstract
Few reveal the mechanism of inoculation plants with PGPB to remediate PAH-metal co-contaminated soil by analyzing the chemical speciations of contaminants. This literature investigated the influence of inoculation plants with PGPB on different fractions of pyrene and Ni in rhizospheric and non-rhizospheric soil. FDA activities were studied to determine the activities of soil microorganisms. In addition, plant dry weight was studied to reflect the resistance of the inoculated plants to environmental stress. The addition of PGPB increased the tolerance of plants in Ni and Ni-pyrene contaminated soil, especially in single Ni-contaminated soil, but not increased the plant biomass in single pyrene-contaminated soil. Compared to single pyrene contaminated soil, the presence of Ni significantly promoted the degradation of pyrene in both rhizospheric and non-rhizospheric soil. On the contrary, the presence of pyrene hindered the inoculated plant from accumulating Ni to some extent.
Introduction
Coexistence of polycyclic aromatic hydrocarbons (PAHs) and heavy metals in the environment has caused worldwide concerns [1]. PAHs and heavy metals are often risky to human health, due to their carcinogenicity, cytotoxicity, teratogenicity, and mutagenicity [2], [3]. Moreover, co-contamination of PAHs and heavy metals even had higher toxicity in the environment, which increased the difficulty in remediating the polluted soil [4]. For example, PAHs mineralization was significantly inhibited by high levels of heavy metals in the co-contaminated soil [5]. Phytoremediation is an environmentally-friendly and cost-effective remediation technology widely used for the removal of PAHs and heavy metals [6]. However, many factors such as low tolerance to environmental stress limited the remediation efficiency [7]. Plants associated bacteria could significantly increase the phytoremediation efficiency due to their interaction with plants [8], [9]. Plants have a cooperative relationship with microbes for the removal of pollutants in soil. The presence of plants could change the microbial community structure and make it more conductive to remove the contaminations in soil [10]. Plant-associated bacteria protected plants against damage from plant pathogens as a consequence of promoting the growth of plants [11].
Plant growth promoting bacteria (PGPB), a kind of beneficial bacteria isolated from the rhizospheric soil, were utilized to combine plants to remove contaminants from soil [12]. Inoculation plants with PGPB enhanced the tolerance of plants to environmental stresses by the synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase [13]. PGPB had the ability to solubilize phosphate and fix nitrogen, which provided plants with more nutrients [14]. In addition, indole-3-acetic (IAA) and siderophores produced by PGPB directly and indirectly increased plant biomass [15]. PGPB were widely used to remediate heavy metal contaminated soil [8], [12], [16]. Weyens et al. (2009) also investigated the feasibility of inoculating plants with PGPB to remediate organics [17]. Moreover, few reveal the mechanisms of inoculating plants with PGPB to remediate the PAH-metal co-contaminated soil by analyzing the chemical speciations of contaminants.
In case of remediating heavy metals, the key factor in phytoremediation was the bioavailability of heavy metal, which determined the remediation efficiency [18]. Root exudates such as organic acids efficiently increased the bioavailability of heavy metals [19]. Biosurfactants produced by some PGPB significantly promoted the mobilization of contaminants [20]. The presence of heavy metals stimulated PGPB to produce siderophores, which in turn increased the bioavailability of heavy metals [21], [22]. The addition of PGPB could significantly increase the accumulation of heavy metals in plants [8]. As far as the remediation of PAHs was concerned, bioavailability and appropriated soil microorganisms both play important roles in the degradation of PAHs. Bioavailable fraction of PAHs was the most easily available for bioremediation [23]. So increasing bioavailability is the first step for the efficient degradation of PAHs. Organic acids in root exudates and biosurfactants produced by some PGPB could increase the bioavailability of PAHs [19], [20]. At the presence of PAHs, plants changed microbial communities and stimulated the more efficient microbes [2]. Highly efficient microbes may be the key for the degradation of PAHs. Plant-promoted microbial degradation was the major way for the removal of PAHs in soil [24].
Fluorescein diacetate (FDA) assay was an easy and convincing method to determine soil microorganism activities [25], [26]. FDA activity was used to reflect the conditions of soil microorganisms and assess the soil quality [27]. In this study, FDA activity not only expressed the changes of soil microorganism activity directly, but also indicated the activity of PGPB in different types of contaminated soil indirectly. In addition, FDA activity also reflected high tolerance of PGPB to pyrene and Ni in this study.
Some researches have reported that the addition of PGPB helped to promote the remediation of single contamination caused by PAH or heavy metal, however few study focuses on the influence of inoculation plants with PGPB on the chemical speciations of PAH and heavy metal in the PAH-metal co-contaminated soil. Notably, the selected PGPB in this experiment was also a strain of pyrene-degrading bacteria. The aims of this study were as follows: (1) to estimate the effect of inoculation with PGPB on the efficiency of phytoremediation of pyrene-Ni co-contaminated soil; (2) to investigate the shifts of chemical speciations of pyrene and Ni in rhizospheric and non-rhizospheric soil after inoculation Scirpus triqueter with PGPB; (3) to ascertain the response of soil microorganism activity and the growth conditions of Scirpus triqueter to pyrene and Ni after PGPB inoculation.
Section snippets
Chemicals
Pyrene with a purity of 98% was purchased from Aladdin Reagent (Shanghai, China). The rest reagents, at least analytical grade, were purchased from Sinopharm (Shanghai, China).
Soil
The experiment soil (air-dried, 2 mm sieved), never exposed to any PAHs or heavy metals contaminants, was collected from the topsoil (0–20 cm) at Shanghai University, China. The test soil consisted of 60.4 ± 2.7% silt, 32.2 ± 1.6% sand and 7.4 ± 0.5% clay. The pH of test soil was 8.31 ± 0.06. In addition, the nutrient elements were
The characters of the isolated PGPB
The results of biochemical tests of the isolated PGPB were showed in Table 3. The isolated PGPB were able to utilize ACC as sole nitrogen source and synthesize ACC deaminase, which lowered the levels of ethylene [28]. IAA as a kind of auxins had direct influence on the growth of plants [37]. Secretion of siderophores could efficiently inhibit phytopathogens by competing iron in soil [15]. In addition, the isolated strains also had the ability to solubilize phosphate. Remarkably, the isolated
Conclusions
The addition of PGPB increased the tolerance of Scirpus triqueter in Ni and Ni-pyrene contaminated soil, especially in single Ni-contaminated soil, but not increased the plant biomass in single pyrene-contaminated soil. Co-contamination of pyrene and Ni had higher toxicity to soil microorganisms than single pyrene or Ni contamination. The addition of PGPB significantly increased the FDA activity in pyrene-Ni co-contaminated soil. The addition of PGPB increased the efficiency of phytoremediation
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgements
The work was funded by the National Natural Science Foundation of China (Nos.41373097, 21677093), Program for Innovative Research Team in University (No.IRT13078).
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2022, ChemosphereCitation Excerpt :This study also emphasized the impact of pyrene on the phytoremediation of Ni. Some important indicators of plant resistance to Ni can determine the efficiency of Ni uptake by plants (Chen et al., 2017). When bioavailable heavy metals enter plants, they accumulate in different tissues which show the varying degrees of toxicity to plants (He et al., 2020).