Bioremediation of polycyclic aromatic hydrocarbons (PAHs) by microorganisms in mangrove sediments
Student thesis: Doctoral Thesis
Contamination of Polycyclic Aromatic Hydrocarbons (PAHs) in mangrove sediments is a serious environmental problem around the world due to its toxicity, carcinogenicity and mutagenicity. In recent decades, bioremediation, the use of microorganisms to degrade contaminants into less toxic or non-toxic compounds, becomes an alternate cost-effective and environmental acceptable remediation technique. However, the presence of PAH-degrading microorganisms in mangrove swamps and their intrinsic biodegradation ability have seldom been explored. The present study therefore aims to investigate the biodegradation potential of indigenous microorganisms in mangrove sediments and their effectiveness to remedy sediments contaminated by a mixture of PAHs, namely, fluorene (Fl), phenanthrene (Phe) and pyrene (Pyr). The first two, 3-ring PAHs, are considered as low molecular weight PAHs and the last one with 4-ring represents a high molecular weight PAH. A PAH-degrading bacterial consortium was successfully enriched from surface sediments collected from Ho Chung, a typical mangrove swamp in Hong Kong SAR, using mixture of fluorene, phenanthrene and pyrene. Two PAH-degraders, Rhodococcus sp. and Acinetobacter sp., and one PAH-resistant bacteria, Pseudomonas sp. were isolated from the consortium. The effectiveness of the consortium, the individual bacterial isolate, and all possible combinations of the three isolates to degrade mixture of fluorene, phenanthrene and pyrene in liquid culture were compared. The degradation percentages of the 3-ring PAHs were higher than that of pyrene (4-ring) in most cases, and the consortium had significantly higher degradation ability than the isolates. At the end of 4-week degradation, over 99% of the 3-ring PAHs (Fl and Phe) and 90% pyrene were degraded by the consortium, which were significantly higher than the respective values by Acinetobacter, and the percentages by Rhodococcus were even lower. Positive interactions happened between two PAH-degraders as well as between PAH-resistant bacteria and both PAH-degraders. These results indicate that the consortium is a better candidate than individual isolates in degrading PAHs in liquid culture. The ability of the enriched consortium for remediation of sediment slurries contaminated with Fl, Phe and Pyr, the bioaugumentation technique, was compared with natural attenuation (intrinsic degradation by indigenous microorganisms without addition of any nutrient or microbial inoculum) and biostimulation (addition of nutrient). At the end of the 4-week degradation, natural attenuation gave more than 99% degradation of Fl and Phe but only around 30% of Pyr were degraded. Bioaugmentation had similar degradation performance with natural attenuation, however, some inhibitory effect was observed in week 1. Biostimulation degraded over 97% of all three PAHs after 4 weeks, showing that nutrient amendment could enhance Pyr degradation. The in-situ degradation ability of the mixture of PAHs in mangrove microcosm vegetated with Kandelia obovata under different bioremediation strategies were examined. The Fl and Phe degradation percentage of natural attenuation (intrinsic degradation by indigenous microorganisms with plant intact) was comparable with non sterile control microcosm (intrinsic degradation by indigenous microorganisms without plant intact), but the natural attenuation enhanced Pyr degradation at the end of 3-month degradation. Addition of nutrients and inoculum in the vegetated microcosms (i.e. biostimulation and bioaugmentation) did not further enhance the Fl and Phe degradation. However, the pyrene degradation percentage of bioaugmentation is higher that that of non sterile control microcosm, indicating that combination of plant and inoculum had a positive effect on pyrene degradation. The PAH metabolites produced during the degradation of fluorene, phenanthrene and pyrene by the enriched bacterial consortium were analyzed using the on-fiber silylation solid-phase microextraction (SPME) combining with Gas Chromatography – Mass Spectrometry (GC-MS). Seventeen metabolites were identified by the full scan mass spectra and selected ion mass (SIM) scans in different PAH degradation cultures. Five novel metabolites, including 1 and 3-hydroxyfluorene in fluorene degradation cultures, trihydroxy phenanthrene in phenanthrene degradation cultures as well as lactone and 4-hydroxyphenanthrene in pyrene degradation cultures were identified, indicating the new pathways in fluorene, phenanthrene and pyrene metabolism by bacteria. The present study reveals that indigenous bacteria in Ho Chung mangrove sediments have high potential to remedy the PAH-contaminated sediments. This study confirms positive interaction among isolates can enhance PAH degradation. This study also demonstrated addition of nutrients and bacterial inoculum can increase the degradation efficiency of PAH-contaminated sediments.
- Bioremediation, Polycyclic aromatic hydrocarbons, Mangrove soils, Microbiology