A novel Bacillaceae bacterium enhances phytoprotection and mercury detoxification in contaminated soils

Environmental pollution due to heavy metals has become a global issue over the latest decades, affecting environmental, animal, and human health. This research conducted a phenotypic, genomic, and functional characterization of Methalobacillus radiculiexplorans C204, a bacterial strain isolated from highly mercury-contaminated soils in the Almadén mining district (Ciudad Real, Spain). The main objective was to evaluate its potential as a bioremediation agent and plant growth-promoting bacterium (PGPB) for application in the recovery of heavy metal-degraded soils. Morphological and biochemical analyses revealed typical adaptations of bacteria resistant to abiotic stress. Genomic analysis identified a broad repertoire of genes associated with PGPB functions, as well as key operons conferring resistance to mercury (merA, merB), copper (copA, copB), and zinc/cadmium (czc), along with multiple antibiotic resistance mechanisms. Additionally, species-level genomic divergence from the closest validated relatives (ANI = 83%, dDDH = 28.8%), together with AAI-based differentiation, phylogenomic placement and the partial absence of conserved signature indels characteristic of Neobacillus, support the recognition of this strain as a distinct genus-level lineage within the Bacillaceae family. In plant assays using Lupinus albus grown in soils with varying Hg concentrations, the bacterial inoculation significantly enhanced plant growth, stabilized nutritional profiles, improved physiological indicators, and reduced mercury accumulation in plant tissues by up to 93%. Positive effects were also observed on the soil microbiome, with the strain contributing to the recovery of microbial metabolic activity and the maintenance of functional diversity under toxic conditions. Overall, the results position M. radiculiexplorans C204 as a native, multifunctional bacterial strain of high biotechnological interest and a promising candidate for assisted phytoremediation strategies aimed at restoration of mercury-contaminated soils.