Augmentation of metal-tolerant bacteria elevates growth and reduces metal toxicity in spinach

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.