The mechanism of benzene degradation in groundwater by indigenous microbial degradation from the perspectives of isotopes and microorganisms in cold regions of China

Groundwater benzene contamination is widespread, threatening ecosystems and human health. However, the biodegradation mechanisms of benzene under microaerobic conditions in cold regions remain poorly understood. This study selected benzene from groundwater in a chemical industrial park in the northeast China, conducting microcosm dynamic experiments to monitor microbial growth, benzene concentration, and carbon isotope changes, using an isotope fractionation model to elucidate microbial degradation patterns. High-throughput sequencing was also employed to explore microbial community dynamics and degradation pathways. The results indicated that indigenous microorganisms exhibited strong tolerance to benzene concentrations of 10, 20, 30 and 50 mg/L, with degradation efficiencies of 63.66 %, 68.26 %, 69.59 % and 67.23 %. The stable carbon isotopes of benzene shifted towards more positive values, increasing by approximately 3 ‰. The enrichment factor (εc) under microaerobic conditions ranged from −4.5 ‰ to −1.1 ‰. Proteobacteria was the dominant phylum (89.84 %), with Pseudomonas, Acinetobacter, Hydrogenophaga, and Variovorax as key degrading genera. Their abundance first increased and then decreased. Compared to the uncontaminated samples, the abundance increased by 3.1–5.7 times. The key functional genes for benzene degradation include M00548 (dmpK, dmpL, dmpM, dmpN, dmpO, dmpP) and M00547 (todC1, todC2, todB, todA, todD). With the increase in benzene concentration, the functional genes M00548 and M00547 exhibited increases in abundance by factors of 4.407–7.109 and 1.277–6.823, respectively. This elucidates the underlying mechanism behind the changes in benzene degradation efficiency and rate as a function of concentration. The findings provide foundational information to promote the development of more effective bioremediation strategies.