Abstract
Microbial immobilization is considered as a novel and environmentally friendly technology that uses microbes to reduce heavy metals accumulation in plants. To explore microbial resources which are useful in these applications, three water spinach rhizosphere soils polluted by different levels of heavy metals (heavy pollution (CQ), medium pollution (JZ), and relative clean (NF)) were collected. The community composition of heavy metal-immobilizing bacteria in rhizosphere soils and its effects on reducing the Cd2+ and Pb2+ concentrations in water spinach were evaluated. Four hundred strains were isolated from the CQ (belonging to 3 phyla and 14 genera), JZ (belonging to 4 phyla and 25 genera) and NF (belonged to 6 phyla and 34 genera) samples, respectively. In the CQ sample, 137 strains showed a strong ability to immobilize Cd2+ and Pb2+, giving Cd2+ and Pb2+ removal rates of greater than 80% in solution; Brevundimonas, Serratia, and Pseudoarthrobacter were the main genera. In total, 62 strains showed a strong ability to immobilize Cd2+ and Pb2+ in the JZ sample and Bacillus and Serratia were the main genera. A total of 22 strains showed a strong ability to immobilize Cd2+ and Pb2+ in the NF sample, and Bacillus was the main genus. Compared to the control, Enterobacter bugandensis CQ-7, Bacillus thuringensis CQ-33, and Klebsiella michiganensis CQ-169 significantly increased the dry weight (17.16–148%) of water spinach and reduced the contents of Cd2+ (59.78–72.41%) and Pb2+ (43.36–74.21%) in water spinach. Moreover, the soluble protein and Vc contents in the shoots of water spinach were also significantly increased (72.1–193%) in the presence of strains CQ-7, CQ-33 and CQ-169 compared to the control. In addition, the contents of Cd and Pb in the shoots of water spinach meet the standard for limit of Cd2+ and Pb2+ in vegetables in the presence of strains CQ-7, CQ-33 and CQ-169. Thus, the results provide strains as resources and a theoretical basis for the remediation of Cd- and Pb-contaminated farmlands for the safe production of vegetables.
Highlights
With the rapid development of urbanization, environmental problems such as heavy metal accumulation are increasing in farmland areas because of the large amounts of pollutants being discharged and increased use of agricultural inputs, such as fertilizer [1,2,3]
The number of bacteria in NF (1.28 × 106 CFU g−1 ) was higher than those in JZ (1.65 × 104 CFU g−1 ) and CQ (6.4 × 103 CFU g−1 ). These results demonstrated that a higher concentration of heavy metals was associated with a smaller number of culturable bacteria in the rhizosphere of water spinach
Strains CQ-7, CQ-33, and CQ-169 significantly increased the dry weight of water spinach and reduced the contents of Cd2+ and Pb2+ in water spinach
Summary
With the rapid development of urbanization, environmental problems such as heavy metal accumulation are increasing in farmland areas because of the large amounts of pollutants being discharged and increased use of agricultural inputs, such as fertilizer [1,2,3]. In situ passivation is a commonly used technology for the farmland remediation of medium and light heavy metal pollution, and mainly includes chemical passivation remediation and microbial immobilization remediation [11,12,13]. The use of biochar and other chemical passivators immobilizes heavy metals in soil and reduces the absorption of heavy metals by plants [14,15]. Microbial-immobilized remediation technology refers to using soil microorganisms to immobilize heavy metals, causing heavy metals to precipitate or be adsorbed and fixed in the soil, reducing their absorption by plants [17]. Heavy metal-immobilizing bacteria have been widely studied and applied as excellent heavy metal passivators
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