Effect of Exposure Time, Contamination Level, and Type of PAH Compound on Biodegradation Capacity of Mangrove Sediment

Mangrove sediment had high natural attenuation potential with more than 50% of total PAHs being removed within 15 days. The efficiency in degrading PAHs varied with the declining order of phenanthrene (Phe), fluoranthene (Fla), and pyrene (Pyr). The Most Probable Number (MPN) of PAH-degrading bacteria in the PAH-contaminated slurries was 2 to 4 orders of magnitude higher than that in the non-contaminated mangrove slurries. The biodegradation ability of the indigenous microbial community in mangrove sediment slurry was significantly increased after exposure to polycyclic aromatic hydrocarbons. Such enhancement effect was dependent on the level and time of exposure, as well as the types of PAH compounds. The lowest contamination level of 3 mg kg^-1 was effective in promoting the degradation of Phe and Fla after seven days, but the enhancement effect for Pyr degradation was only found in the slurries exposed to contamination levels of 9 mg kg^-1 for 30 days, suggesting a threshold concentration of PAHs to stimulate growth and activity of pyrene-degrading bacteria. The contamination level higher than the threshold concentration did not lead to more degradation. The present study provides insights into the natural attenuation of PAH-contaminated mangrove sediments.