Aerobic degradation of hexachlorobenzene and pentachloronitrobenzene by Cupriavidus nantongensis HB4B5: Dechlorination mechanisms and bioremediation potential

Organochlorine pesticides (OCPs), including hexachlorobenzene (HCB) and pentachloronitrobenzene (PCNB), are highly toxic and persistent pollutants that pose significant ecological and human health risks. Their chemical stability makes them particularly resistant to biodegradation. In this study, we isolated and characterized Cupriavidus nantongensis HB4B5, a novel aerobic bacterium capable of efficiently degrading HCB and PCNB, without the accumulation of toxic intermediates. The strain demonstrated high tolerance to multiple OCPs (HCB, PCNB, hexachlorocyclohexane, and chlordane), utilizing them as carbon sources. Within 7 days, strain HB4B5 achieved degradation rates of 91.74 % for HCB and 79.75 % for PCNB. Whole genome sequencing revealed that HB4B5 not only degrades polysubstituted chlorobenzene through aerobic dechlorination but also possesses genetic markers for degrading other aromatic pollutants, establishing its potential as a bioremediation candidate for contaminated sites. Notably, we functionally characterized two flavoprotein monooxygenases (C-hcbA and C-pcpB) as key catalysts in chlorobenzene dechlorination. Catalytic assays confirmed their involvement in HCB degradation and revealed a novel meta-position degradation pathway that prevents toxic intermediate accumulation. Optimal degradation conditions were established at 35 °C/pH 7 for HCB and 30 °C/pH 7 for PCNB. This study provides the first evidence of HCB degradation by a Cupriavidus species and highlights the potential of strain HB4B5 as a bioremediation agent for sites contaminated with complex chlorinated aromatic hydrocarbons.