Radial Oxygen Loss from the Roots of Mangrove Seedlings Enhances the Removal of Polycyclic Aromatic Hydrocarbons

The presence of polycyclic aromatic hydrocarbons (PAHs) in soil poses a significant global environmental concern, particularly in coastal wetlands. Mangrove ecosystems exhibit enormous potential in environmental purification; however, the underlying mechanisms involved in the degradation of pollutants (e.g., PAHs) remain ambiguous. In the present investigation, a soil pot experiment was conducted with the addition of pyrene to evaluate the effect of radial oxygen loss (ROL) from roots on PAH degradation using three mangrove seedlings (<i>Rhizophora stylosa</i>, <i>Aegiceras corniculatum</i>, and <i>Avicennia marina</i>). The results showed that mangrove plantation can significantly promote the efficiency of pyrene removal. As for the three mangrove species studied, the greatest removal rate (90.75%) was observed in the soils associated with <i>A. marina</i>, followed by <i>A. corniculatum</i> (83.83%) and <i>R. stylosa</i> (77.15%). The higher PAH removal efficiency of <i>A. marina</i> can be partially attributed to its distinctive root anatomical structure, characterized by a thin exodermis and high porosity, which facilitates ROL from the roots. The results from qPCR further demonstrate that ROL is beneficial for promoting the abundance of PAH-ring hydroxylating dioxygenase gene, leading to a higher removal efficiency. Additionally, Rhizobiales, Defferrisomatales, and Ardenticatenales may also play important roles in the process of pyrene degradation. In summary, this study provides evidence for elucidating the mechanism of PAH removal from the perspective of ROL, thereby contributing valuable insights for species selection during mangrove restoration and remediation.