Polycyclic aromatic hydrocarbons (PAHs) released into all environmental
compartments through natural or anthropogenic activities are toxic, carcinogenic and
mutagenic. Mangrove wetlands, located along the coastline of tropical and subtropical
regions and close to human activities, are susceptible to PAH contamination. The
anaerobic properties of mangrove sediments, especially their vertical changes, will
directly affect the PAH contamination and biodegradation; however, studies in this area
are limited. Previous related research was mainly focused on monitoring the temporal
and spatial changes of PAH concentrations. The effects of anaerobic biodegradation of
PAHs in sediment and some important biologically related factors, such as bacterial
population and activity, electron acceptors utilization, and anaerobic gas production, on
PAH degradation have seldom reported. The present research therefore aims (1) to study
the vertical profiles of PAHs and anaerobic properties of the subsurface sediments in Ma
Wan, one of the most contaminated mangrove swamps in Hong Kong; (2) to obtain the
PAH-degrading anaerobic bacterial consortia from subsurface mangrove sediments
under low oxygen (2±0.3% O2) and non-oxygen (0% O2) conditions; (3) to evaluate the
anaerobic biodegradability of PAHs with the amendment of different electron acceptors,
including nitrate, manganese Mn(IV), iron Fe(III), sulfate and carbon dioxide (produced
by NaHCO3).
The vertical distribution of PAHs at different sediment depths, namely 0-2 cm, 2-4
cm, 4-6 cm, 6-10 cm, 10-15 cm and 15-20 cm, in Ma Wan mangrove swamp was
investigated. Results showed that the concentrations of total PAHs (summation of 16 US EPA priority PAHs) increased with sediment depth. The lowest concentration
(about 1300 ng g-1 freeze dried sediment) were found in the surface layer (0-2 cm) while
the highest value (around 5000 ng g-1 freeze dried sediment) was in the deeper layer
(10-15 cm). The percentage of high molecular weight (HMW) PAHs (4 to 6 rings) to
total PAHs was more than 89% at all sediment depths. The ratio of phenanthrene (Phe)
to anthracene (Ant) was less than 10 while fluoranthene (Flua) to pyrene (Pyr) was
around 1. Negative redox potentials (Eh) were recorded in all sediment samples, ranging
from -170 to -200 mv, with a sharp decrease at a depth of 6 cm then declined slowly to
20 cm. These findings suggested that PAHs, particularly HMW ones, in Ma Wan
sediment were originated from diesel-powered fishing vessels and were mainly
accumulated in deep anaerobic layers. Among the electron acceptors commonly used by
anaerobic bacteria, sulfate was the most dominant one, followed by Fe(III), nitrate, and
Mn(IV) was the least. Their concentrations also decreased with sediment depth. The
population size of total anaerobic heterotrophic bacteria increased with sediment depth,
reaching the peak number in the middle layer (4-6 cm). In contrast, the aerobic
heterotrophic bacterial count decreased with sediment depth. The vertical drop of the
electron transport system (ETS) activity under PAH stress suggested that the indigenous
bacteria were still active in the anaerobic sediment layer contaminated with PAHs.
Four PAHs, namely Fluorene (Fl), Phe, Flua and Pyr were selected as the target
compounds in the enrichment and biodegradation experiments because of their
relatively higher concentrations in Ma Wan mangrove sediment. Fresh sediment sample
was mixed with mineral salt medium at a salinity of 25 parts per thousand at a ratio of 1:10 (w/v). The sediment slurry was enriched with the mixed PAHs under low oxygen
and non-oxygen conditions, and the respective enrichment periods were 30 and 45 days.
After three consecutive enrichment periods under each oxygen condition, two enriched
consortia showing satisfactory PAH-degrading ability were obtained. A total of six
strains of anaerobic PAH-degrading bacteria were isolated, the three from the
low-oxygen condition were Microbacterium, Rhodococcus and Sphingomonas, while
only one out of three isolates from the non-oxygen condition was identified and it was a
Sphingomonas species.
A series of biodegradation studies with the inoculation of the enriched consortium
and the amendment of different electron acceptors was performed. Results showed that
nitrate and sulfate significantly enhanced PAH-degrading ability, while Fe(III) and
NaHCO3 did not have any significant effect; Mn(IV) had significantly adverse effect on
the biodegradation of PAHs. Under both low-oxygen and non-oxygen conditions, the
treatment groups with the inoculation of the enriched PAH-degrading consortium with
the addition of electron acceptor had the highest PAHs biodegradation ability, while the
control group (just contained the indigenous bacteria) had the lowest biodegradation
ability. The 3-ring PAHs showed higher biodegradation percentages than the 4-ring
PAHs, irrespective to the oxygen condition and whether the enriched consortia were
inoculated or not. The present study was also proved that the bacteria from mangrove
sediments could utilize different electron acceptors in the sediment simultaneously.
In summary, the present research revealed that Ma Wan mangrove swamp was
contaminated by anthropogenic PAHs, mainly from nearby fishing vessels operated with diesel. PAHs, especially those with high molecular weights, were accumulated in
deep acidic and anaerobic sediments (>10 cm), with pH levels less than 6.0 and Eh at
around -200 mv. Two types of enrichment consortia with PAH-degrading ability were
obtained from its subsurface sediment, and the anaerobic PAH-degrading bacteria were
first time reported. A modified ETS method was successfully applied to evaluate the
bacterial activities under PAH stress. The present study demonstrated that the anaerobic
biodegradation of PAHs could be improved by the amendment of different electron
acceptors, particularly nitrate and sulfate. The biodegradation course was better
understood than previous researches as both biological and chemical parameters were
monitored.
| Date of Award | 15 Jul 2010 |
|---|
| Original language | English |
|---|
| Awarding Institution | - City University of Hong Kong
|
|---|
| Supervisor | Fung Yee Nora TAM (Supervisor) |
|---|