Enhanced Anaerobic Biodegradation of PAHs by Rhamnolipid and Earthworm Casts in Contaminated Soil

Rhamnolipids and earthworm casts, as efficient and environmentally friendly biostimulants, influence the biodegradation of organic pollutants. However, it remains unclear how rhamnolipids and earthworm casts affect the anaerobic biodegradation of polycyclic aromatic hydrocarbons (PAHs). This work aimed to investigate the efficacy and mechanism of biostimulants on the anaerobic biodegradation of PAHs through PAH degradation, functional gene abundance, and bacterial community structure. The results revealed that both stimulants promoted the anaerobic degradation of typical PAHs, such as phenanthrene, pyrene, and benzo(a)pyrene. Rhamnolipids and earthworm casts promoted the degradation of phenanthrene and pyrene more significantly, with the degradation rate increasing by 13.75% and 16.92%, respectively, and the degradation rate of benzo(a)pyrene increased by 10.26% and 11.7%, respectively. The addition of rhamnolipids and earthworm casts significantly stimulated the abundance of functional genes ( UbiD , UbiE ) in bacterial communities, and this study indicated a strong association between the abundance of functional genes and PAH degradation efficiency. Furthermore, biostimulants altered the microbial community structure and affected microbial diversity and function. Earthworm casts significantly promoted the Azospirillum (0.02–20.17%) and Acinetobacter (0.01–15.70%) genera, which played an important role in the degradation process of PAHs. Therefore, these findings suggested that the enhancement of anaerobic biodegradation of PAHs by rhamnolipids and earthworm casts is probably due to an increase in abundance of both PAH-degraders and their degrading genes ( UbiD , UbiE ). This study could provide valuable insights for advancing the sustainable remediation of PAH-contaminated soils.