Enhanced Biphenyl Degradation by Rhodococcus sp. TG-1 Under Cr(VI) Stress via Modified Biochar Immobilization

Co-contamination of biphenyl and heavy metals is widespread in industrial environments, but systematic studies on the simultaneous treatment of both pollutants using a single microbial strategy remain limited. In this study, we characterized the biphenyl degradation performance, metabolic pathway, transcriptomic response, and Cr(VI) tolerance of Rhodococcus sp. TG-1, and developed an alkali-modified biochar immobilization system to enhance its degradation efficiency for biphenyl under Cr(VI) stress. Degradation experiments were carried out under optimal conditions (30 °C, pH 7.0), and it was found that strain TG-1 degraded 76.84% of 300 mg/L biphenyl within 3 days. Intermediate metabolites were identified by LC-MS, and five key intermediates were detected, confirming that TG-1 metabolizes biphenyl via the classical 2,3-dihydroxybiphenyl dioxygenase pathway, with subsequent entry into the tricarboxylic acid cycle. Transcriptomic analysis was performed to profile gene expression, revealing 845 differentially expressed genes under biphenyl stress, including 672 upregulated genes significantly enriched in aromatic degradation pathways. Seven complete bph gene clusters responsible for biphenyl catabolism were also identified. Strain TG-1 exhibited high tolerance to Cr(VI), with a minimum inhibitory concentration (MIC) of 500 mg/L. However, its biphenyl degradation efficiency dropped to 51.32% in the presence of 200 mg/L Cr(VI). After immobilization using alkali-modified straw biochar (JBC), heavy metal toxicity was alleviated, and the biphenyl removal rate increased to 99.30% under co-contamination conditions. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analyses confirmed that TG-1 was stably loaded onto the biochar surface through hydrogen bonding and electrostatic interactions. Altogether, this study provides a promising bacterial strain and a green immobilization strategy for enhancing biphenyl removal in the presence of Cr(VI), offering a practical approach for the treatment of environments co-contaminated with aromatic compounds and heavy metals.