Importance of chemical structure on the development of
hydrocarbon catabolism in soil
Jacqueline L. Stroud1, Graeme I. Paton2,3 & Kirk T. Semple1
1
Faculty of Science and Technology, Department of Environmental Science, Lancaster University, Lancaster, UK; 2School of Biological Sciences, University
of Aberdeen, Aberdeen, UK; and 3Remedios Ltd, Aberdeen Science and Technology Park, Aberdeen, UK
Correspondence: Kirk T. Semple, Faculty of
Science and Technology, Department of
Environmental Science, Lancaster University,
Lancaster, LA1 4YQ, UK. Tel.: 144 1524
594534; fax: 144 1524 593985; e-mail:
[email protected]
Received 27 November 2006; revised 9 March
2007; accepted 28 March 2007.
First published online May 2007.
DOI:10.1111/j.1574-6968.2007.00750.x
Editor: Hans-Peter Kohler
Keywords
hexadecane; PAHs; indigenous catabolic
activity; ageing.
Abstract
A soil was amended with 14C-analogues of naphthalene, phenanthrene, pyrene,
B[a]P or hexadecane at 50 mg kg 1 and the development of catabolic activity
was assessed by determining the rate and extent of 14CO2 evolution at time
points over 180 days. The catabolic potential of the soil was hexadecane 4
naphthalene 4 phenanthrene 4 pyrene 4 B[a]P, determined by the decrease in
lag time (as defined by the time taken for 5% 14CO2 to be evolved from the
minerialization of the 14C-labeled hydrocarbons). The results clearly showed the
difference between constitutive and inducible biodegradation systems. The 0 day
time point showed that hexadecane minerialization was rapid and immediate, with
a 45.4 0.6% mineralization extent, compared with pyrene minerialization at
1.0 0.1%. However, catabolism for pyrene developed over time and after a 95
days soil-pyrene contact time, mineralization extent was found to be 63.1 7.8%.
Strong regression was found (r2 4 0.99) between the maximum rates of mineralization and the partioning coefficient between the mineralized hydrocarbons,
which may indicate linearity in the system.
Introduction
Hydrocarbons are widespread in the environment due to
natural and anthropogenic processes; however, high concentrations of hydrocarbons in soils are often associated
with anthropogenic activities. It is estimated that oil contamination accounts for over 25% of all chemical contamination incidents in the UK (Environment Agency, 2003).
Oils are mixtures of both aliphatic and aromatic hydrocarbons, and the level of hydrocarbon contaminants in soils is a
cause for concern, as these contaminants may persist in soils
and can exert toxic and carcinogenic affects on biological
receptors (Cerniglia, 1992).
Biodegradation is central to hydrocarbon removal from
soils and the physico-chemical characteristics of the hydrocarbon directly impacts biodegradation. For example, microorganisms readily attack the n-alkane open chain
structure via constitutive b-oxidation. In comparison, PAHs
are composed of fused benzene rings, which are relatively
resistant to microbial attack, often requiring the induction
of suitable enzymes for catabolism, and are degraded by
either ortho or meta cleavage of the ring structure (van der
Meer et al., 1992). Larger PAHs, such as benzo[a]pyrene
(B[a]P), are often unsuitable sole carbon sources and a
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c
cometabolism mechanism may be the dominant process for
biodegradation (Kanaly et al., 1997).
Biodegradation of contaminants requires adaptation responses within the microbial community, and may involve
either the enrichment in the numbers of degrading microorganisms or the induction of catabolic enzymes leading to
the degradation of the target contaminant(s) (Leahy &
Colwell, 1990). A range of microorganisms are able to
degrade hydrocarbons, and the ubiquitous nature of hydrocarbon contamination means that competent hydrocarbon
degraders are common (Macleod & Semple, 2002, 2006; Lee
et al., 2003). Frequency of contamination and the length of
exposure to contaminants have been shown to enhance
catabolism for hydrocarbons in soils (Macleod & Semple,
2002, 2006). However, catabolism is complicated by the
decreasing bioaaccessability of contaminants over time due
to associations with soil components (Hatzinger & Alexander, 1995). Macleod & Semple (2002) showed that catabolism for pyrene was limited by pyrene sorption to soil
organic matter in a woodland soil. The extent to which a
hydrocarbon associates with organic matter is described by
the Koc, but this is described by the Kow to a large extent. The
hydrocarbons used in this study had a range of Kow values
from 3.36 (naphthalene) to 9.1 (hexadecane).
FEMS Microbiol Lett 272 (2007) 120–126
121
Biodegradation of hydrocarbons in soil
The development of catabolism by soil microbial communities can be measured using 14C-labelled substrate
respirometry (Reid et al., 2001; Macleod & Semple, 2002,
2006; Lee et al., 2003). The production of 14CO2 is a result of
microbial degradation, making it an ideal technique to
detect microbial adaptation (Spain et al., 1980). The aims
of this study were (1) to quantify the catabolic activity of soil
microbial communities to hydrocarbons with different
physico-chemical properties and (2) to compare the differences between constitutive and inducible biodegradation
systems of ageing hydrocarbons. To address theses aims,
naphthalene, phenanthrene, pyrene, B[a]P and hexadecane
were selected for investigation in this study.
Materials and methods
Materials
All the nonlabelled and labelled [1-14C] naphthalene (specific activity = 2–10 mCi mmol 1, radiochemical purity 4
95%); [9-14C] phenanthrene (specific activity = 40–
60 mCi mmol 1, radiochemical purity 4 95%); [14C-4,5,
9,10] pyrene (specific activity = 10–30 mCi mmol 1, radiochemical purity 4 95%); [7-14C] B[a]P (specific activity =
1–30 mCi mmol 1, radiochemical purity 4 95%) and
[1-14C] n-hexadecane (specific activity = 1–15 mCi mmol 1,
radiochemical purity 4 95%) were obtained from Sigma
Aldrich (UK). The liquid scintillation cocktail, Goldstar and
7 mL and 20 mL glass scintillation vials were obtained from
Meridian (UK). Bibby (UK) supplied the Erlenmeyer flasks
with TeflonTM-lined screw caps. RS (UK) supplied metal
fittings used to make the respirometers. Sodium hydroxide
was obtained from VWR International (UK). Packard Canberra (UK) supplied the sample oxidizer scintillants, Carbosorb and Permafluor as well as combustion cones and caps.
Soil preparation and microcosm set-up
A river alluvium of the Enborne series was collected from
the top 7 to 25 cm of a pasture field at Myescough,
Lancashire. This soil was characterized as a clay loam, pH
6.5, organic matter content 2.7 0.04%. The field-sampled
moisture content of 28 2% (w/w) was maintained for the
duration of the study. Before spiking, the soil was passed
through a 2 mm sieve to remove stones, roots and facilitate
mixing. The soil was then rehydrated to field conditions
using de-ionized water and stored in the dark at 4 1C for
4 days. Soil samples (500 g) were amended with either
naphthalene, phenanthrene, pyrene, B[a]P or hexadecane
in a carrier solvent of acetone to give a concentration of
50 mg kg 1 and c. 100 Bq g 1 using a glass bowl, stainless
steel spoon and hand-mixing method. This method has
been shown to give the most homogenous distribution of
FEMS Microbiol Lett 272 (2007) 120–126
hydrocarbons throughout the soil, with minimal disruption
to the microbial community in comparison with other
methods (Doick et al., 2003). The acetone was allowed to
volatilize over 24 h, and the contaminated soils were placed
in duplicate amber glass microcosms (500 g capacity). One
set of soils was sterilized using g-irradiation (35.1 Kgy;
Isotron, Bradford, UK) shortly (o 24 h) after spiking. The
sterility of each microcosm was ensured by aseptic handling
techniques and checked at each timepoint using standard
microbiological techniques. All of the microcosms were set
up in triplicate and stored in the dark at room temperature
during the ageing period.
Determination of total 14C-hydrocarbon activity
in soil
The total 14C-hydrocarbon activity in the soil was assessed at
each timepoint (0, 30, 60, 90 and 180 days) by combusting
1 g of soil (n = 3) from each microcosm using a sample
oxidizer (Model 307, Packard). The initial recovery (0 day)
of the 14C-hydrocarbon-associated activity in the soil was
normalized to 100%. At each subsequent timepoint, the
14
C- hydrocarbon recovery is compared with the initial
amendment (%), to monitor 14C-losses from each of the
microcosms.
Minerialization of aged 14C-hydrocarbonassociated activity in soil
At 0, 30, 60, 90 and 180 days, subsamples (n = 3) from each
of the microcosms were taken and 14C-respirometry was
used following the procedure developed by Reid et al.
(2001). Briefly, 10 0.2 g soil and 30 mL sterile mineral salts
basal medium (MBS) was added to a 250 mL Erlenmeyer
flask to form a slurry (ratio 1 : 3 w/v). This was sealed with a
modified screw lid incorporating a stainless steel crocodile
clip, which enabled a CO2 trap of 1 mL NaOH (1 M) in a
7 mL glass scintillation vial, to be securely attached inside
the vessel. The sealed flasks were placed in an incubator
shaker at 21 2 1C and shaken at 100 r.p.m. Each CO2 trap
was replaced every 24 h and 5 mL of liquid scintillation fluid
added to the sampled vial. Following overnight storage, the
level of 14C-activity was determined by liquid scintillation
counting and blank corrected (Canberra Packard Tri-Carb
2250Ca). This method enabled the comparison between the
aged 14C-hydrocarbon-associated activity in soil to the
measured evolution of 14CO2 from the 14C-labelled hydrocarbons at each timepoint, thus detecting the development
of catabolism.
Statistical analysis
To investigate the development of catabolism in the soil, the
lag phases (14CO2 evolution 45%), maximum rates of
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122
Results
(a)
Catabolism of 14C-phenanthrene in soil
Loss of 14C-phenanthrene was measured in the soil at each
of the sampling points. There was no removal of the PAH
after 60 days incubation in the nonsterile microcosm;
however, there was a decrease of c. 11% after 90 days. At
180 days incubation, 49% of the initial 14C-activity added to
the soil was recovered (Fig. 1a). In comparison, the sterile
microcosm showed no loss of 14C-activity in the soil, with
99.4 0.5% after 180 days (Fig. 1b).
Despite no appreciable removal of 14C-phenanthrene
from the soil for the first 60 days, degradation of 14C2007 Federation of European Microbiological Societies
Published by Blackwell Publishing Ltd. All rights reserved
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60
40
20
Catabolism of 14C-naphthalene in soil
0
0
20
40
60
80
100
120
140
160
180
200
140
160
180
200
Time (days)
(b) 100
80
remaining (%)
14C-Hydrocarbon associated activity
Loss of 14C-naphthalene was measured in the soil at each of
the sampling points. There was no removal of the PAH from
the nonsterile microcosm after 30 days incubation with
99.6 0.4%; however, there was a decrease of c. 10% of the
initial amendment after 60 days contact time. After 180 days
incubation, c. 70% of the initial 14C-activity remained in the
soil (Fig. 1a). In comparison, the sterile microcosm showed
smaller losses from 60 days ageing, c. 4% of the initial
amendment. However, after 180 days ageing, a decrease of
c. 31% of the initial amendment was found (Fig. 1b).
Despite no appreciable removal of 14C-naphthalene from
the soil incubations at time 0, 14C-naphthalene was significantly (P 0.05) degraded by the indigenous microbial
communities after a short lag time of 2 days compared with
the other hydrocarbons, with a plateau at 58.3 1.4% at the
end of the 14 days incubation, and had the highest rate of
mineralization of 1.4% 14CO2 h 1 (Table 1; Fig. 2a). The
cumulative mineralization extent at the end of the 14 days
assay remained significantly high at both the 30 and 60 days
soil-contact time, reaching a maximum extent of mineralization of 60.7 3.2% at 30 days. Similarly, the maximum
rate of mineralization at 30 days was 1.3% 14CO2 h 1 and the
lag time was 2 days (Table 1, Fig. 2a). However, after ageing
for 95 days, the extent of mineralization significantly
(P 0.05) decreased to just 13.8 1.8%. Similarly, the
maximum rate of mineralization significantly (P 0.05)
decreased by 86–0.2% 14CO2 h 1 when compared with the
other time points (Table 1, Fig. 2a).
100
80
remaining (%)
14C-Hydrocarbon associated activity
minerialization (calculated from the kinetic response observed) and the total cumulative extents of minerialization
were compared at each time point and statistically tested
using ANOVA after normality testing (SIGMA STAT, Version 2.03,
SPSS Inc., Tukey test, P 0.05) following blank correction,
catabolic profiles have been presented using Sigma Plot
(Version 6.10, SPSS).
J.L. Stroud et al.
60
40
20
0
0
20
40
60
80
100
120
Time (days)
Fig. 1. 14C-Recovery compared with the 14C-initial amendment of ()
14
C-napthalene, (&)14C-phenanthrene, (m)14C-pyrene, (,)14C-B[a]P,
(^)14C-hexadecane (50 mg kg 1) from soil over 180 days incubation in
(a) nonsterile and (b) sterile microcosms. SE of the mean where visible
(n = 3).
phenanthrene was measured in the soil at 0, 30 and 60 days.
For 14C-phenanthrene, the initial mineralization profile
was similar to naphthalene (PZ0.05), with decreasing lag
time and a consistently extensive mineralization over time
(Table 1, Fig. 2b). The extent of phenanthrene mineralization reached 60.1 1.6% and a lag time of 4 days in soil
sampled at 0 day incubation. The maximum rate of mineralization for this contaminant was found at this initial time
point at a significant (P 0.05) 1.2% 14CO2 h 1 (Table 1,
Fig. 2b). The lag time decreased by 75% during the 180 days
incubation period to just 1 day. There was no significant
(PZ0.05) difference in the extent of phenanthrene mineralization over the 180 days incubation, with a mean of
59.7% and reaching a maximum of 62.6 1.8% at 95 days
(Table 1, Fig. 2b).
Catabolism of 14C-pyrene in soil
Loss of 14C-pyrene was measured in the soil at each of the
sampling points. There was virtually no loss of pyrene in the
FEMS Microbiol Lett 272 (2007) 120–126
123
45.4 0.6
46.0 0.3
17.4 0.5
17.7 1.3
23.9 5.0
0.4
0.4
0.1
0.1
0.1
1
3
3
3
2
1.0 0.1
34.5 0.1
45.4 1.5
63.1 7.8
58.1 4.4
60.1 1.6
57.1 2.6
60.0 3.5
62.6 1.8
58.5 12.4
1.2
0.9
0.6
1.0
0.6
4 14
12
11
5
1
o 0.1
0.33
0.42
0.58
1.09
o1
o1
o1
o1
o1
Max rate
(%14CO2 h 1)
Lag
(days)
Extent
(%)
Max rate
(% 14CO2 h 1)
Lag
(days)
Extent
(%)
Max rate
(% 14CO2 h 1)
nonsterile soil after 95 days with 99.4 0.3% recovered.
Over the 180 days incubation period, 84.2 0.7% of the
initial 14C-activity remained in the soil (Fig. 1a). No loss of
14
C-pyrene was detected over 180 days in the sterile microcosm, with an activity of 100.4 0.3% recovered at 180 days
(Fig. 1b).
The extent of mineralization of 14C-pyrene was low at
0 day, at just 1.0 0.1% and a lag time greater than the 14
days incubation (Table 1, Fig. 2c). However, after the 30 days
incubation period, a lag time of 12 days and a statistically
significant (P 0.05) pyrene mineralization of 34.5 0.1%
was measured. The maximum rate of mineralization was
estimated at 0.3% 14CO2 h 1 (Table 1, Fig. 2c). The mineralization profile at each successive time point showed
increasing extents of pyrene mineralization and reductions
in lag time, with the maximal mineralization extent reaching
at 63.1 7.8% after 95 days of incubation (Table 1, Fig. 2c).
The lag time reduced by 12-fold over the 180 days incubation period to just 1 day; furthermore, the maximum rate of
mineralization increased between the 30 and 180 days timepoints to 1.1% 14CO2 h 1 (Table 1).
Catabolism of 14C-benzo[a ]pyrene in soil
Despite measuring the loss of 14C-B[a]P in the soil over the
180 days incubation, there was no appreciable degradation
in the microcosms over the 180 days ageing period (Fig. 2a
and b). The catabolism of B[a]P was significantly different
from all the other hydrocarbons in this study (Table 1).
B[a]P was not mineralized over the duration of the study,
with a cumulative mineralization extent of o 1% at all time
points.
4
4
3
1
1
58.3 1.4
60.7 3.2
52.6 2.1
13.8 1.8
16.0 7.1
2
2
1
2
2
0
30
60
95
180
Max rate
(% 14CO2 h 1)
Lag (days)
Ageing time
1.4
1.3
1.1
0.2
0.3
Lag (days)
Catabolism of 14C-hexadecane in soil
Extent (%)
Phenanthrene
Naphthalene
Table 1. Catabolic profiles of 14C-labelled hydrocarbons in soil after 14 days respirometric assay
Pyrene
B[a]P
Extent
(%)
Hexadecane
Extent
(%)
Biodegradation of hydrocarbons in soil
FEMS Microbiol Lett 272 (2007) 120–126
Hexadecane was rapidly degraded in the nonsterile microcosms (Fig. 1a), 18.2% loss was detected after the 30 days
incubation period. This degradation continued and by 180
days, the recovery of 14C-hydrocarbon-associated activity
was found to be 42.1 3.4%. The sterile microcosms
showed no significant losses, with the level of 14C-hydrocarbon-associated activity found to be 98.6 1.5% at 180
days (Fig. 1b).
Hexadecane had a very different mineralization profile
compared with the PAHs (Table 1, Fig. 2d). Initially this
hydrocarbon was rapidly mineralized, with a lag time of
1 day and was the shortest lag time of all of the hydrocarbons. The mineralization extent of 14C-hexadecane at
time 0 was 45.4 0.6%. However, unlike PAH mineralization, the lag time increased to 3 days at the 30 days ageing
period, while the mineralization extent remained at
46.0 0.3% and the maximum rate of mineralization was
consistent with the previous time-point, reaching the maximum of 0.4% 14CO2 h 1 (Table 1, Fig. 2d). From the 60
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124
J.L. Stroud et al.
60
60
40
Cumulative
Cumulative
CO (%)
(b) 80
CO (%)
(a) 80
20
40
20
0
0
0
2
4
6
8
10
12
14
0
2
4
6
(d) 80
60
CO (%)
60
Cumulative
40
CO (%)
(c) 80
Cumulative
8
10
12
14
10
12
14
Time (days)
Time (days)
40
20
20
0
0
0
2
4
6
8
Time (days)
10
12
14
0
2
4
6
8
Time (days)
Fig. 2. Development of 14CO2 from the catabolism of 14C-hydrocarbons (50 mg kg 1) in soil for naphthalene (a), phenanthrene (b), pyrene (c) and
hexadecane (d) at () 0 days, (&) 30 days, (m) 60 days, (,) 95 days, (~) 180 days. SE of the mean (n = 3).
days ageing period, the extent of mineralization significantly
(P 0.05) decreased and remained constant, with an average of 18.6% (Table 1, Fig. 2d).
There is correlation between the maximum rate of mineralization and log Kow (Fig. 3). With increasing log Kow, lower
maximum rates of mineralization were detected. B[a]P was
not considered due to the lack of measurable mineralization.
Naphthalene has lowest log Kow in the study of 3.36, but the
fastest rate of mineralization at 1.4% 14CO2 h 1. In comparison, phenanthrene has a log Kow 4.16, and a lower maximum rate of mineralization of 1.2% 14CO2 h 1. The
maximum rate of pyrene mineralization was determined to
be 1.1% 14CO2 h 1 and has a log Kow of 5.19. Hexadecane,
the hydrocarbon with the highest log Kow in this study, had
the lowest maximum rate, which was up to four times
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Mineralisation rate (%14CO2 h –1)
Comparison of the catabolic behaviour of the
hydrocarbons in soil
1.6
1.4
1.2
H C
1.0
0.8
0.6
CH
0.4
0.2
3
4
5
6
7
8
9
10
log Kow
Fig. 3. Correlation between the maximum rate of mineralization of the
14
C-labelled hydrocarbons and the partioning coefficient (log Kow) of the
14
C-labelled hydrocarbons.
FEMS Microbiol Lett 272 (2007) 120–126
125
Biodegradation of hydrocarbons in soil
smaller than the PAHs at 0.4% 14CO2 h 1. There was a
strong negative trend with a quality of fit, r2 value of 0.99
and correlation coefficient of 0.97 (P 0.05).
Discussion
It is widely accepted that the rate and extent of the degradation of organic contaminants differs between classes of
chemicals and even between individual chemicals with
similar structures. This catabolic behaviour is controlled by
the physico-chemical properties of the contaminants and
the soil, with organic matter playing an important role
(Macleod & Semple, 2002). Differences in degradation of
both aromatic and aliphatic hydrocarbons were observed in
this study. For example, hexadecane was removed most
rapidly, followed by the PAHs, with the number of aromatic
rings influencing the persistence and the development of
catabolism in the soil, with naphthalene 4 phenanthrene
4 pyrene 4 B[a]P (Fig. 1a).
Lag time has been reported as a measure of the catabolic
potential of microorganisms for hydrocarbons (Macleod &
Semple, 2002, 2006; Reid et al., 2002; Lee et al., 2003; Kanaly
& Watanabe, 2004). In this study, the evolution of 14CO2
through the action of the soil microbial communities on the
14
C-hexadecane occurred immediately as described by the
length of the lag time in the respirometers at the beginning
of the study. Hexadecane is an n-alkane, similar in structure
to low molecular weight soil organic matter and usually
degraded through b-oxidation of the terminal methyl group
(Koma et al., 2001). Furthermore, the enzymes required for
this biotransformation are constitutive, thus catabolism is
inherent in the soil microbial community. Constitutive
biodegradation is often characterized by rapid and immediate degradation aliphatic hydrocarbon degraders known to
be common in soil (Bouchez-Naitali et al., 1999), and
perhaps even more numerous than aromatic hydrocarbon
degraders (Gogoi et al., 2003). This would explain the
consistency of n-hexadecane biodegradation over the timescale of this study (Table 1, Figs 1 and 2d).
The soil microbial communities were able to rapidly
degrade the lower molecular weight 14C-PAHs, naphthalene
and phenanthrene, as shown by the relatively short lag time
and relatively high extents of mineralization found at the
beginning of the study (Table 1), indicating that catabolic
activity was already present in the soil. This is not surprising
as PAHs are ubiquitous contaminants and are known to be
present in all soils at very low concentrations. Macleod &
Semple (2003) reported that the background PAH burdens
of two rural soils ranged between 200 and 400 mg kg 1.
Furthermore, it has been widely reported that both naphthalene and phenanthrene may be readily degraded by indigenous soil microbial communities (Reid et al., 2001; Lee et al.,
2003). Given the sigmoidal nature of the catabolic data (Fig.
FEMS Microbiol Lett 272 (2007) 120–126
2a and c), it is likely that the microorganisms capable of
degradation were increasing in biomass to biodegrade this
carbon source, causing the lag time to decrease over time
(Grosser et al., 1991). Mineralization of 14C-pyrene was not
measured until well into the study as the indigenous soil
microbial communities required more time to develop the
capacity to degrade the four-ring PAH; this is in agreement
with other studies (Macleod & Semple, 2002, 2006).
As a point of note, there were no significant losses of 14Cnaphthalene in either the sterile and nonsterile microcosms
until 95 days (Figs 1a and b). However, the 14C-respirometric data showed that the soil microbial communities
were able to mineralize the 14C-naphthalene before this,
after a 2 days lag period (Table 1, Fig. 2a), suggesting that the
microorganisms were readily able to degrade this simple
PAH, as discussed previously. Interestingly, at the same time
as the abiotic loss of naphthalene (after 95 days), there was
also a significant decrease in the extent to which 14Cnaphthalene was mineralized (Table 1, Fig. 2a). A similar
observation was noted by Reid et al. (2002), who were
investigating the development in phenanthrene catabolic
activity in Phase II mushroom compost, although the
authors were unable to provide a definitive reason for this
behaviour. Although there is no definitive evidence for this
observation in this study, there may be several reasons as to
why the abiotic loss of 14C-naphthalene had a significant
impact on the ability of the indigenous microbial communities to mineralize the 14C-PAH. This could have been due
to the amount of the PAH remaining in the soil coupled
with putative decreases in the accessibility of the naphthalene to the soil microbial communities . The sequestration of
hydrocarbons into soil over time has been shown to
significantly affect bioaccessibility, causing limited biodegradation (Hatzinger & Alexander, 1995). Finally, the degrading microbial population(s) may have decreased their
activity, perhaps through the production of a toxic metabolite during degradation. It is possible that a combination of
these possibilities contributed to the decrease in mineralization of the 14C-naphthalene after 95 days incubation; however, a decrease in the bioaccessible concentration seems a
more likely explanation.
Finally and unsurprisingly, the indigenous soil microbial
communities were not able to catabolize B[a]P in this study.
This was expected as B[a]P is known to be highly resistant to
microbial degradation and it has been reported that cometabolism may be involved in biodegradation of this high
molecular weight PAH (Kanaly et al., 1997).
An interesting finding of this study was the correlation
between the maximum rates of mineralization and the
octanol-water partition coefficient (log Kow) of the hydrocarbons, which was found to be highly significant (P 0.01). This correlation shows that the higher the log Kow
value, the slower the rates of mineralization. Given the trend
2007 Federation of European Microbiological Societies
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c
126
in physico-chemical properties, partitioning of the PAHs
within the soil may be the rate-limiting step in the biodegradation of hydrocarbons. However, caution must be
applied with regard to the experimental design of the study.
For hexadecane, it is known that desorption to the aqueous
phase may not be important for biodegradation to proceed
in soil (Huesemann et al., 2003). In this study, respirometry
was conducted under slurry conditions, maximizing the
interactions between the soil, the contaminant, the indigenous microbial communities and the aqueous phase. Therefore, the conditions in the respirometric set-up may have
impacted the measured rates and extents of 14C-hexadecane
mineralization.
The value of this study is that it investigated the catabolism of aliphatic and aromatic hydrocarbons with significantly different physico-chemical properties. This has, to the
authors knowledge, not been investigated in this way before.
The study showed that a single soil may possess the catabolic
potential for a wide range of hydrocarbon contaminants and
that biodegradation is an important loss mechanism for
organic contaminants in soil.
Acknowledgements
The authors would like to thank the Natural Environment
Research Council, UK and Remedios, for financially
supporting this work.
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