Selection and characterization of rhizobacteria from cocoa soils in Ecuador with potential for cadmium mitigation

Bioprospecting for Cd-tolerant rhizobacteria that naturally inhabit cocoa (Theobroma cacao L.) soils contaminated with heavy metals contributes to the development of effective bioremediation strategies.The objective of this study was to select rhizobacteria from cocoa soils in Ecuador with the potential for Cd mitigation.Soil samples were collected from the provinces of Cañar, Esmeraldas, and Manabí.Morphological and biochemical characteristics, Cd tolerance, and molecular identification of the 16S rRNA gene were evaluated.Twelve rhizobacteria morphotypes were selected for their capacity to tolerate 50 mg Cd L -1 for 24 h while producing protease, catalase, and urease enzymes, biofilm production, and solubilizing phosphate and K. Autochthonous strains T-CADM-1, T-CADM-4, T-CADM-5, and T-CADM-6 were incubated at four Cd concentrations (0, 6, 12 and 24 mg L -1 ) for 7 d, maintaining significant cell persistence (p ≤ 0.05) of 2.06  10 9 , 2.02  10 9 , 2.08  10 9 and 1.96  10 9 CFU mL -1 , respectively.The strain T-CADM-5 was the most effective, precipitating bicarbonate (139%) and achieving an in vitro Cd removal rate of 82.4%.GenBank data identified the tolerant strains as Enterobacter sp.(86%), Serratia marcescens (98%), E. asburiae (92%) and E. hormaechei (98%).It was concluded that the strain T-CADM-5 retained its cell viability for 7 d under Cd stress, produced urease and phosphatase, and increased bicarbonate and sulfate anions.These factors positively correlated with Cd immobilization, indicating that these rhizobacteria actively modify their environment to reduce the bioavailability of the heavy metal.