Heavy metal bioaccumulation potential of indigenous rhizospheric soil fungi with comparative assessment of lead and cadmium resistance

Abstract

Fungi are highly suitable for use in bioremediation processes because of their high degree of metal tolerance. The present investigation focused on comparing the tolerance levels of the two most toxic metals, such as lead (Pb) and cadmium (Cd), and the bioaccumulation capacities of certain unexplored fungi isolated from rhizospheric agricultural soils. Twenty-one (21) fungal strains were isolated, observed for their morphological features and then identified via molecular approach using ITS rDNA regions for Sanger sequencing (ITS1 and ITS4). The tolerance of the fungal isolates to Cd and Pb was assessed by examining their growth response to increasing concentrations of Cd and Pb in the form of cadmium chloride (CdCl₂·H₂O) and lead nitrate [Pb (NO₃)₂], respectively. The majority of the isolates showed tolerance to both Pb and Cd, as indicated by visual observation of growth on solid media. Species-specific differences in metallic tolerance were determined by minimum inhibitory concentration (MIC) and tolerance index (TI) assays, with Penicillium atramentosum (IA47) having the highest resistance to Cd, with MIC of 500 ppm. Against Pb, both Aspergillus calidoustus (IA40) and Penicillium atramentosum (IA47) , showed highest MIC of 400 ppm. The isolates demonstrating the best resistance and growth rates were further investigated for their ability to uptake metals. For that purpose, the fungal biomass was asceptically spiked with 50 and 100 ppm cadmium chloride (CdCl₂·H₂O) and lead nitrate [Pb (NO₃)₂]. The concentration of accumulated metals in the digested biomass was quantified through atomic absorption spectroscopy (AAS). It was found that Phoma herbarum (IA52) had the greatest ability to bioaccumulate Cd (3.693 mg/g) at a 100 ppm concentration. The isolates showed a greater ability to uptake Cd than Pb because Pb tends to be less soluble than Cd and is therefore less readily available for fungal cells. These findings provide valuable insights into the selection of native fungal strains for eco-friendly and sustainable bioremediation of heavy metal contaminated environments and support their potential application in future environmental remediation strategies.