Environment-dependent roles of green TiO₂ nanoparticles in biosurfactant-assisted pyrene biodegradation by Bacillus...

Pyrene is a persistent polycyclic aromatic hydrocarbon (PAH) with limited bioavailability in contaminated environments due to its hydrophobicity and strong sorption to soil organic matter. The present study investigated the role of green-synthesized TiO₂ nanoparticles (TiO₂NPs) and biosurfactant in enhancing pyrene degradation by Bacillus thuringiensis SSL1 in both liquid and soil systems. Biosurfactant-amended liquid cultures exhibited the highest pyrene degradation (77.1 ± 4.2%), whereas nanoparticle-amended treatments showed enhanced intracellular enzyme activities, including catechol 1,2-dioxygenase (2006.4 ml^-1), catechol 2,3-dioxygenase (1925.6 ml^-1), and GST (205 nmol/ml). In soil mesocosms, combined nanoparticle-biosurfactant systems achieved the highest pyrene degradation (89.6 ± 5.5%) and stimulated elevated catA (71.5 ± 9.2) and pcaG (72.5 ± 5.1) expression, coupled with increased dehydrogenase activity and aromatic-ring mineralization. Correlation analysis further demonstrated strong associations between pyrene degradation, catabolic gene expression, and microbial metabolic activity. Metabolite profiling confirmed sequential dioxygenation, hydroxylation, and downstream catechol/protocatechuate-mediated ring cleavage pathways. Collectively, these findings demonstrate that the effects of biosurfactant and TiO₂NPs on pyrene biodegradation are strongly matrix-dependent, with biosurfactant-mediated mobilization contributing substantially to contaminant accessibility and TiO₂NPs influencing pyrene partitioning and microbial-contaminant interactions. This study provides insights into the application of green nanomaterials for designing effective remediation strategies for heterogeneous environmental systems.