Enzyme-mediated synergistic bioremediation of PAH and heavy metal co-contaminated soil using nocardia species and Helianthus annuus

Heavy metals and polycyclic aromatic hydrocarbons (PAHs) are persistent soil contaminants posing risks to human health, environmental stability, and agricultural productivity via oxidative stress, neurotoxicity, carcinogenicity, and reduced crop yields. This study evaluated the bioremediation potential of native Iranian Nocardia species combined with sunflower (Helianthus annuus) remediation for restoring PAHs and heavy metal-co-contaminated soils. Thirty-six soil samples from Fars Province, Iran, were analyzed to isolate bioremediating Nocardia using turbidometric, chromatographic, biochemical, molecular, and enzymatic assays. Greenhouse experiments employed spiked soils (PAHs: 100-200 mg/kg; heavy metals: As, Hg, Cd, Pb, Cr; 3-400 mg/kg) across low, medium, and high levels, with five treatments (control, phytoremediation(P), bioremediation(B), combined phytoremediation and bioremediation (MB), sterile control) assessed over 90 days for soil properties, contaminants, plant growth, translocation (TF)/bioaccumulation factors (BAF). Thirteen Nocardia isolates (36.11%) were identified belonging to eight species. N. farcinica (85%), N. otitidiscaviarum (82%), and N. cyriacigeorgica (80%) led in PAH degradation; N. wallacei (75%), N. kruczakiae (70%), and N. cyriacigeorgica (65%) excelled in heavy metal removal. This results showed that the first report of identification of N. wallacei and N. veterana as PAHs degrading and heavy metals removal agents. A consortium of these five species in the MB treatment achieved 84.5-92.3% PAH degradation and 69.7-78.9% heavy metal removal, significantly outperforming B (76.8-85.6%, 63.4-72.5%), P (55.2-65.4%, 48.6-58.7%), and controls (p < 0.001). MB enhanced soil fertility (OC: 1.8%, TN: 42 mg/kg), microbial activity, and reduced metal translocation (TF/BAF: 0.20-0.60). In conclusion integrated Nocardia bioremediation and sunflower phytoremediation offers a promising sustainable strategy for co-contaminated soils, with potential for field-scale application in industrial and agricultural contexts following further validation.