Abstract

Heavy metal contamination poses a serious threat to both the ecosystem and human and requires expensive cleanup costs. Bioremediation based on microorganisms, plants, or other biological systems offers cost-effective and environment friendly metal clean-up methods. Studies on bacterial diversity in heavy metal contaminated sites have demonstrated a high diversity of microorganisms that are adapted to the new environment. Bacteria that are resistant to and grow on metals play an important role in the biogeochemical cycling of those metal ions. In pursuit to identify bacteria that are tolerant to different heavy metals and can have a potential in bioremediation, surveys, and collection of samples from several presumptive heavy metal-polluted sites of India were carried out. A total of 77 bacterial morphotypes were obtained, and based on minimum inhibitory concentrations (MIC) of different heavy metals, that is Pb⁺², Ni⁺², Cd⁺², Cr⁺³, Hg⁺², Cu⁺², Zn⁺², Co⁺², and As⁺², thirteen potential bacterial isolates were identified possessing very high and multiple heavy metal tolerance like arsenic (50–1100 mg kg⁻¹), lead (100–2000 mg kg⁻¹), chromium (250–500 mg kg⁻¹), cadmium (50–100 mg kg⁻¹), and other heavy metals. All potential bacteria were morphologically characterized, identified based on the 16 s rRNA gene sequences, and studied for plant growth promoting attributes. Bacterial strains were found to be phosphate solubilizers, siderophore and ammonia producers, and nitrate reducers. Bacillus cereus MB1, Bacillus amyloliquefaciens RD4, Bacillus megaterium MF7, and E. cloacae MC4 were evaluated for alleviation of As, Cr, Ni, and Pb toxicity, respectively in spinach. The inoculation of plants with respective heavy metal-tolerant bacteria under study gave higher records of all estimated growth parameters, total chlorophyll content and antioxidant enzyme, superoxide dismutase, activity and differential response in proline biosynthesis when compared to respective uninoculated heavy metal controls. Overall selected plant growth-promoting heavy metal-tolerant bacterial inoculations were found to promote growth and reduce the respective heavy metal toxicity in spinach plant. Since heavy metal contamination in agricultural lands is becoming serious environmental concern, the heavy metal-tolerant plant growth-promoting strains reported in this study can offer suitable economical and eco-friendly base for development of the bioremediation strategies.

摘要

重金属污染对生态系统和人类都构成严重威胁，需要昂贵的清理成本。基于微生物、植物或其他生物系统的生物修复提供了经济高效且环境友好的金属清理方法。对重金属污染场地细菌多样性的研究表明，适应新环境的微生物具有高度多样性。对金属具有抗性并在其上生长的细菌在这些金属离子的生物地球化学循环中起着重要作用。为了确定对不同重金属具有耐受性并具有生物修复潜力的细菌，我们从印度的几个假定的重金属污染地点进行了调查和样本收集。共获得77个细菌形态型，⁺² , Ni ⁺² , Cd ⁺² , Cr ⁺³ , Hg ⁺² , Cu ⁺² , Zn ⁺² , Co ⁺²和 As ⁺² , 鉴定出 13 株潜在的细菌分离株，具有很高的多种重金属耐受性如砷（50-1100 mg kg ^-1）、铅（100-2000 mg kg ^-1）、铬（250-500 mg kg ^-1）、镉（50-100 mg kg ^{-1 ）}）和其他重金属。对所有潜在细菌进行形态学表征，基于 16 s rRNA 基因序列进行鉴定，并研究植物生长促进属性。发现细菌菌株是磷酸盐增溶剂、铁载体和氨生产者以及硝酸盐还原剂。蜡样芽孢杆菌MB1、解淀粉芽孢杆菌RD4、巨大芽孢杆菌MF7 和阴沟肠杆菌MC4 分别用于减轻菠菜中的 As、Cr、Ni 和 Pb 毒性。与各自未接种的重金属对照相比，在研究中用各自的重金属耐受细菌接种植物给出了所有估计的生长参数、总叶绿素含量和抗氧化酶、超氧化物歧化酶、脯氨酸生物合成的活性和差异反应的更高记录。发现总体选择的植物生长促进重金属耐受细菌接种可促进菠菜植物的生长并降低其各自的重金属毒性。由于农田重金属污染正成为严重的环境问题，