Microbial Degradation and Debromination of Polybrominated Diphenyl Ethers (PBDEs) in Aquatic Sediments
Student thesis: Doctoral Thesis
Related Research Unit(s)
Polybrominated diphenyl ethers (PBDEs), a group of newly emerged toxic and persistent organic pollutants, are likely to accumulate in sediments, especially in aquatic ecosystems. However, research on PBDE degradation and functional microbial communities in aquatic sediments is rare. Therefore, this study aimed to compare the temporal changes in microbial community and the intrinsic potential of four different types of aquatic sediments to anaerobically remove BDE-153, a hexa-BDE congener commonly found in aquatic environment. The study also investigated the intrinsic potential of one aquatic sediment at different depths to anaerobically remove BDE-47, a typical and prevalent tetra-BDE congener in aquatic sediments, and related the removal to the abundance of dehalogenating and other functional bacteria. The degradation pathways of BDE-47 in this aquatic sediment under alternating anaerobic-aerobic conditions were further explored.
The removal potential of BDE-153 varied significantly among four sediment types collected from Hong Kong Special Administrative Region. The removal rates in mangrove, mudflat, marine, and freshwater sediments were 0.013, 0.013, 0.011, and 0.009 day-1, respectively, indicating that sediment from intertidal wetland was more efficient in removing BDE-153. The relative abundance of phylum Proteobacteria, especially the five genera belonging to this phylum, namely, Acinetobacter, Sulfurimonas, Pseudomonas, Psychromonas, and Pelobacter, in sediment at the beginning of the experiment (background level) determined the removal rate. Mangrove and mudflat sediments harbored higher proportions of microorganisms from these five genera than marine and freshwater sediments, and these higher proportions could explain their higher BDE-153 removal rates. The species richness, Shannon, and Simpson indices were comparable among the four types of sediments and did not change significantly during the 150-day experiment. However, the microbial community composition in mangrove, mudflat, and marine sediments was altered by the contamination of BDE-153 at the end of the experiment but not in the freshwater sediment. Distance-based multivariate multiple regression analysis revealed that salinity, total organic carbon (TOC), and pH were the main contributing factors in control sediments without BDE-153. Conversely, salinity, TOC, and BDE-52, the major debromination product of BDE-153, were revealed to be the three main factors explaining the variations in microbial community composition in BDE-treated sediments.
Mangrove sediment collected at different depths, namely, 0–5, 5–10, 10–15, 15–20, 20–25, and 25–30 cm, showed different potentials to remove BDE-47 anaerobically. Surface sediment (0–5 cm) had a higher BDE-47 removal rate (0.008 day-1) than deep sediment (5–30 cm), which was 0.006 day-1 on average. Surface sediment also had higher abundance of 16S rRNA gene copies of total bacteria, Dehalococcoides spp., Dehalogenimonas spp., Desulfitobacterium spp., and Geobacter spp., with an average of 1.68 × 1014, 1.45 × 108, 8.05 × 107, 0.81 × 106, and 1.23 × 108 copies g-1 dry weight (dw), respectively, than deep sediment. The higher bacterial abundance could be attributed to the higher content of total organic matter (15.45%), total Kjeldahl nitrogen (2.72 mg N g-1 dw), and electron acceptors (1709 µg g-1 dw for sulfate and 4520 µg g-1 dw for iron III) in surface sediment. Significant positive linear relationships were found between anaerobic removal rate of BDE-47 and 16S rRNA gene copy number of total bacteria, Dehalococcoides spp., and Dehalogenimonas spp. in sediment, and the corresponding R2 values were 0.636, 0.64, and 0.468 (all having P < 0.01). Stepwise multiple regression analyses further revealed that Dehalococcoides spp. were the most important dehalogenating bacteria affecting the anaerobic removal of BDE-47 in mangrove sediment (R2 = 0.617, p < 0.001).
Mangrove surface sediment (0–5 cm) was subjected to different alternating anaerobic-aerobic conditions created by three tidal flushing regimes: 1-day high tide and 1-day low tide (1:1d), 1-week high tide and 1-week low tide (1:1w), and 2-week high tide and 2-week low tide (2:2w). The redox potential was maintained at around −200 mV for the 1:1d flushing regime but decreased to −50 mV and −100 mV for the 1:1w and 2:2w flushing regimes, respectively, during the flooding period (high tide), and the corresponding values increased to 310 mV and 320 mV during the draining period (low tide). At the end of the 40-week experiment, more reduction of BDE-47 was detected in the sediment under the 1:1d flushing regime (75.2%) than that under the 1:1w (58.8%) or 2:2w (51.4%) flushing regime. Less brominated congeners, including BDE-28, -17, -8 and -7, were generated as debromination products, and their total concentrations varied among sediments under different tidal flushing regimes. The sediment subjected to a longer duration of aerobic period (1:1w and 2:2w) had significantly lower concentrations of debromination products, especially BDE-17, which is the most dominant debromination product. The concentration of BDE-17 was 150.2, 143.8, and 198.4 ng g−1 dw in sediments under the 2:2w, 1:1w, and 1:1d flushing regimes, respectively. The concentration of 4′-OH-BDE17, which is the most dominant hydroxylation product, was 17.9, 20.17, and 6.34 ng g−1 dw in sediments under the 2:2w, 1:1w, and 1:1d flushing regimes, respectively. Microbial analyses based on ester-linked fatty acids further supported that a longer duration of aerobic period (2:2w and 1:1w) favored the growth of aerobic microorganisms, thus enhancing the aerobic degradation of PBDEs. A higher abundance of bphC gene was also observed in the 2:2w (1.2 × 107 copies g-1 dw) and 1:1w flushing regimes than in the 1:1d flushing regime (0.7 × 107 copies g-1 dw). On the contrary, a longer duration of anaerobic period (1:1d) promoted the growth of anaerobic bacteria, such as Dehalococcoides spp. and Dehalogenimonas spp., the abundance of which (6.3 × 107 and 5.9 × 107 copies g-1 dw, respectively) was much higher under the 1:1d flushing regime than under the 1:1w or 2:2w flushing regime.
In summary, the present study revealed that mangrove sediment harbored more functional bacteria and exhibited better potential to remove BDE-153 anaerobically than other types of aquatic sediments. Surface mangrove sediment (0–5 cm) had higher BDE-47 removal rate than deep sediment under an anaerobic condition, and the removal rate was significantly related to the abundance of functional bacteria involved in the reductive debromination process. In the same mangrove surface sediment, BDE-47 could go through both anaerobic debromination and aerobic degradation mechanisms under alternating anaerobic-aerobic conditions created by different flushing regimes. The 1:1w and 2:2w flushing regimes with a longer duration of aerobic period had a higher abundance of bphC gene and were more efficient in the hydroxylation of lower brominated congeners. The present findings demonstrated that alternating anaerobic-aerobic conditions could be a promising in situ natural attenuation strategy to achieve the complete removal of PBDEs without generating secondary contamination. Further investigation should be conducted to optimize this strategy for the effective bioremediation of PBDE-contaminated sediments.