Degradation of phenanthrene in mangroves at different rhizosphere distances and changes in microbial community structure

【Objective】 The purpose was to study the degradation of phenanthrene（Phe）in different rhizosphere distances and the structural changes of microbial community in the rhizosphere region，and to obtain the rhizosphere distances and degrading microbial species with the best degradation effect，in order to provide reference for elucidating the enhancement of mangrove rhizosphere effects on the degradation of polycyclic aromatic hydrocarbons（PAHs）in the coastal zone and its mechanism.【Method】 In this study，four consecutive compartments with rhizosphere distances of 0-2 mm ，2-4 mm，4-6 mm（rhizosphere）and >6 mm（non-rhizosphere）were separated using rhizosphere boxes，and the Phe residue concentration，sediment physicochemical properties，microbial biomass and bacterial community structure were determined at different rhizosphere distances at 0，10，20，30，40，and 50 d. Redundancy analysis （RDA）was used to investigate the correlation between Phe residue concentration and environmental factors and bacterial community structure. 【Result】The results of the study showed that the Phe degradation efficiency（74.73%）was the highest in the sediments close to the rhizosphere surface（0-2 mm compartment） . After 50 d of incubation， the physicochemical properties of different compartments were compared. Compared with 0 d，the most great changes were observed in the 0-2 mm compartment，with a decrease of 3.84% in pH and a 16.14% increase in organic matter content；humus content was elevated by 12.56% in the >6 mm compartment. During the degradation process of Phe，the pH and humus content of the sediment increased with the increase of the rhizosphere distance of the mangrove forests，and the content of organic matter decreased with the increase of the rhizosphere distance. After 50 d of incubation ，microbial biomass carbon（MBC），microbial biomass nitrogen（MBN）and microbial respiratory strength（SMR）were the highest in the 0-2 mm compartment in general， which at 216.45 mg/kg，68.20 mg/kg and 147.35 mg/ （kg·h）respectively，and all of them decreased with the increasing of rhizosphere distance. At the end of incubation，in the dominant functional genera of 0-2 mm，2-4 mm and 4-6 mm compartments，relative abundance of Brevibacterium and Lactobacillus continued to decrease with the increasing of rhizosphere distance，and relative abundance of Alicyclobacillus continued to increase with increasing of rhizosphere distance. RDA showed that Phe residue concentration was mainly affected by degradation function microorganisms and environmental factors，and was negatively correlated with Alicyclobacillus，Halothiobacillus，Thiomonas，Brevibacterium，and Lactobacillus，as well as organic matter.【Conclusion】Mangrove rhizosphere effects can enhance the degradation of Phe by affecting the physicochemical properties of sediments in the rhizosphere zone，thereby changing the structure of bacterial community and microbial activity，and have a positive impact on the elimination of PAHs pollution in the coastal zone ecosystem.