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Title: Biodegradation ability and community structure of bacteria in mangrove sediments contaminated by polycyclic aromatic hydrocarbons (PAHs)
Other Titles: Duo huan fang jing wu ran de hong shu lin di ni zhong xi jun de jiang jie neng li yu jun qun jie gou
Authors: Guo, Chuling (郭楚玲)
Department: Dept. of Biology and Chemistry
Degree: Doctor of Philosophy
Issue Date: 2006
Publisher: City University of Hong Kong
Subjects: Bacteria -- Ecology
Mangrove soils -- Microbiology
Polycyclic aromatic hydrocarbons -- Biodegradation
Notes: CityU Call Number: QD341.H9 G86 2006
Includes bibliographical references (leaves 157-182)
Thesis (Ph.D.)--City University of Hong Kong, 2006
xx, 192 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: Elevated concentrations of polycyclic aromatic hydrocarbons (PAHs) have been found in mangrove sediments. Microbial degradation has been proposed as a major means to remove PAHs in contaminated sediments, but its success depends on the diversity and activity of the PAH-degrading microorganisms in the sediments, which can be affected by the degree of contamination. The present research aims (i) to investigate the PAH-degradation ability of the indigenous microorganisms in surface sediments of different mangrove wetlands in Hong Kong; (ii) to explore the changes of bacterial community composition and degradation ability when the sediment was exposed to different degrees of PAHs contamination; and (iii) to test the biodegradation ability of isolated PAH-degrading bacterial strains from mangrove sediments. The total concentrations of 16 US EPA priority PAHs in surface sediments of seven mangrove wetlands in Hong Kong varied from the lowest in Kei Ling Ha Lo Wai (169 ng g-1dw) to the highest value of 1,058 ng g-1dw in Ma Wan. The bacterial consortia enriched from each of these sediments using phenanthrene (Phe, a 3-ring PAH) as the sole carbon source had different degradation abilities. The consortia enriched from Sai Keng and Ho Chung sediments had significantly higher degradation ability (>80%) than those from the other five mangrove sites. The biodegradation abilities of the enriched consortia did not seem to have any direct relationship with the concentrations of PAHs in the sediments. The effect of PAH contamination levels on biodegradation ability of indigenous microorganisms in Sai Keng mangrove sediments was further explored. A mixture of Phe, fluoranthene (Fla) and pyrene (Pyr) was spiked to the sediment slurry at different amounts: (I) T0 – without PAHs; (II) T1 – 3; (III) T2 – 9; (IV) T3 – 15; (V) T4 – 30; (VI) T5 – 300; and (VII) T6 – 1200 μg g-1 dw sediment. Results showed that mangrove sediment slurries had very high intrinsic degradation ability but the efficiency varied among PAH compounds with Phe > Fla > Pyr. The numbers of PAH-degrading bacteria in all PAH-contaminated slurries were comparable and were 100 fold higher than that of the control (T0), suggesting that the presence of PAHs was essential to stimulate the growth of PAH-degraders. When the slurry was used as the inoculum for biodegradation studies, the slurry pre-exposed to PAHs, even at the lowest contamination level (T1), had significantly higher degradation percentages of Phe and Fla than the control (without any PAHs). For pyrene degradation, the enhancement effect was only found in the slurries exposed to contamination level of 9 μg g-1 (T2) or higher for 30 days. In the three contaminated slurries (T4 to T6), the dominant groups of PAH-degraders changed with the exposure time. According to the traditional cultivation method, Pseudomonas was the dominant species at day 0, and changed to Sphingomonad and Mycobacterium after 15 and 30-day exposure, respectively. The culture-independent molecular biological method, PCR-DGGE (denaturing gradient gel electrophoresis) with the specific primers for Mycobacterium, also showed that the diversity and abundance of Mycobacterium in the most contaminated slurry (T6) after 30-day exposure were higher than that after 15-day exposure, and were more than that in T4 and T5 slurries. Both contamination levels of PAHs and exposure time significantly changed the bacterial community structure and Mycobacterium played an important role in PAHs degradation. Nonetheless, DGGE with universal primers for total bacterial community and the specific primers for Sphingomonas did not show evident shifts in the bacterial composition of the slurries. A total of 21 PAH-degrading bacteria were isolated from the present study. Most of them belonged to Sphingomonad and Mycobacterium, and the latter genus had higher ability to degrade a mixture of Phe, Fla and Pyr. The co-culture of Sphingomonad and Mycobacterium strains also showed better degradation performance than that of a single strain. The present research demonstrated that mangrove sediments had high intrinsic ability to degrade PAHs and such ability could be enhanced by pre-exposing the sediments to some PAH contamination. The biodegradation ability in mangrove sediments was site specific and different sediments harbored different groups of PAH-degrading microorganisms. Nevertheless, Sphingomonad and Mycobacterium strains were the main degraders in the PAH-contaminated mangrove sediments.
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