Bioremediation of mercury and cadmium using marine Streptomyces sp. K13: biosorption, bioreactor and its toxicity studies

Heavy metal contamination in marine ecosystems remains a critical environmental issue with serious implications for ecological and human health. This study is the first to highlight the bioremediation ability of the newly identified actinobacterial strain K13, which was collected from marine sediments along the northeast coast of Tamil Nadu, India. Among the 18 isolates screened, strain K13 showed heavy metal tolerance to mercury and cadmium at concentrations up to 150 mg/L. Molecular identification through 16S rRNA gene sequencing confirmed the presence of the Streptomyces genus. The strain metal tolerance efficiency was optimized using response surface methodology, achieving a maximum reduction of 73.1% for mercury by the dithizone method at an initial concentration of 150 mg/L within 15 days. In contrast, cadmium tolerance and growth efficiency was evaluated by turbidimetric assays on the basis of the minimal inhibitory concentration, where a gradual decrease was observed at an OD of 0.336 and growth at an OD of 1.752 at a concentration of 1280 mg/mL. Strain K13 also showed strong biofilm induction and exopolysaccharide production under metal concentrations, indicating a defensive biosorption mechanism. Adsorption studies using the Langmuir, Freundlich, and Temkin models, along with kinetic modelling, revealed that the process was chemisorption and more favourable for mercury binding. Scanning electron microscope imaging revealed noticeable surface alterations after metal exposure, and FT-IR analysis revealed that functional groups such as hydroxyl, carbonyl, and sulfoxide groups were key in metal interactions. In a lab-scale bioreactor setup, metal removal reached 76% for mercury and 73% for cadmium on the 20th day with an initial concentration of 150 mg/L. A genotoxicity study using the Allium cepa root tip assay revealed that treatment with strain K13 significantly reduced chromosomal aberrations induced by heavy metals. Overall, strain K13 is a promising candidate for ecofriendly remediation of mercury and cadmium, although the present findings are limited to laboratory-scale conditions; therefore, future studies should focus on pilot and field-scale validation to confirm its applicability in environmental remediation.