Adaptive resistance to and intracellular bioaccumulation of potentially toxic metals in a plant growth-promoting Rhizobium tropici-like isolate: implications for phytoremediation.

The accumulation of potentially toxic metals (PTMs) poses a threat to the cellular integrity of soil microorganisms as well as ecological stability, driving the selection of resistant microorganisms with biotechnological potential. This study aimed to evaluate the behavior of the Rhizobium sp. LBMP-C04 isolate under PTM stress, focusing on adaptive resistance mechanisms and intracellular bioaccumulation. The bacterium, which was isolated from Arachis pintoi root nodules and had previously undergone genomic characterization, was subjected to antibiotic-resistance assays, growth-curve analysis in the presence of Cd²⁺, Cu²⁺, Cr³⁺, Cr⁶⁺, Zn²⁺, and Ni²⁺, metal-removal analyses, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis. The results revealed resistance to multiple antibiotics and differential tolerance to metals, with Cr³⁺ notably allowing sustained bacterial growth. Correlation analyses indicated associations between population dynamics and metal-concentration changes, indicating active involvement in bioaccumulation processes. TEM images showed intracellular metal accumulation without severe structural damage, while FTIR analyses identified functional groups potentially involved in metal ion interactions. Thermogravimetric analysis confirmed changes in biomass composition associated with metal exposure. Overall, the findings demonstrate that the isolate shows efficient adaptive and intracellular sequestration mechanisms for PTMs, although removal efficiency from the medium was limited. Nevertheless, the plant growth-promoting traits and metal resistance of the isolate highlight its potential for application in phytoremediation strategies in contaminated environments.