|Biodegradation of PAHs in 'Pristine' Soils from Different Climatic Regions
|Uchechukwu V Okere and Kirk T Semple*
|Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK
||Kirk T Semple
Lancaster Environment Centre
University, Lancaster, LA1 4YQ, UK
|Received November 10, 2011; Accepted December 23, 2011; Published
December 26, 2011
|Citation: Okere UV, Semple KT (2012) Biodegradation of PAHs in 'Pristine' Soils
from Different Climatic Regions. J Bioremed Biodegrad S1:006. doi:10.4172/2155-
|Copyright: © 2012 Okere UV, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
|Polycyclic aromatic hydrocarbons (PAHs) are an important class of organic contaminants ubiquitously found in
soils globally. Their fate in soil varies depending on both soil properties and chemical structure; however, microbial
degradation represents the most significant means of loss. It is therefore important to understand the factors that
control PAH biodegradation in different soil environments. This review considers PAH biodegradation in “pristine”
Antarctic, temperate, tropical and hot desert soils. Pre-exposure of indigenous microbes to PAHs is important for
the development of the capacity to degrade PAHs so PAH sources to these soils are discussed. The role of PAH
bioavailability in the biodegradation of PAHs in ‘pristine’ soils from the different climatic regions is also discussed
as well as the factors that control it. Soil organic matter, water content and temperature are seen as the main
environmental factors that control PAH bioavailability in these soils. With most studies focussing on temperate soils,
there is need for more research on soils from other climatic zones.
|Bioavailability; Biodegradation; Deserts; Temperate;
|Polycyclic aromatic hydrocarbons (PAHs) are an important class
of hydrophobic organic contaminants (HOCs) widely found in the
environment. They have been studied with increasing interest for
more than twenty years because of more findings about their toxicity,
environmental persistence and prevalence [16,20,57,83,147,149].
|PAHs are produced into the environment through natural and
anthropogenic sources. Natural sources include their formation as
biogenic precursors during early diagenesis [22,76,140] and production
by plants and termites [15,75,140]. Production from anthropogenic
activities such as petroleum oil refining, combustion, transportation
and spillage and the use of domestic heaters are however more
significant [51,147]. PAHs are widely distributed in the environment as
products of incomplete combustion, and have been detected in various
environmental samples including air , water , sediments 
and soil [67,68].
|The soil environment appears to be the ultimate sink for PAHs
as, globally, soils store more than 90% of the total PAHs found in
the environment [1,147]. PAHs are delivered into background soils
mainly by atmospheric deposition [68,133,144], but the proximity
to point sources and soil properties (mainly soil organic matter
and total organic carbon) have been found to affect concentrations
[21,69]. Moreover, high concentrations of PAHs have been found
in urban and roadside soils [69,88], with even higher concentrations
found at sites impacted by organic contamination [94,103]. PAHs
have been shown to accumulate mainly in the organic layer in soils
, with a positive correlation between soil PAHs and soil organic
matter reported [94,146]. Knowledge of soil PAH concentrations can
be used to estimate PAH sources to soil. Since soils may act both as
a sink for high molecular weight (HMW) PAHs and as a source of
low molecular (LMW) PAHs to the atmosphere, this knowledge is
important [37,131,151]. In fact, PAH concentrations in soil have been
reported to significantly correspond with levels in air , house
dust  and urban street dust . PAHs differ from other groups
of organic compounds because of their unique physical and chemical
properties. These properties control their transport and behaviour in
|Physico-Chemical Properties of PAHs
|Individual PAHs differ significantly in their physical and chemical
properties (Table 1) and can be divided into low molecular weight (e.g.,
2 to 3 ring PAHs like naphthalenes, fluorenes, phenanthrenes, and
anthracenes) and high molecular weight PAHs (4 to 7 ring PAHs from
chrysenes to coronenes). Generally, LMW PAHs are more soluble,
and volatile and less hydrophobic [81,153]. As a result, they are more
easily lost from the environment. Many HMW PAHs (e.g.benzo[a]
pyrene and benzo[b]fluoranthene, benzo(e)pyrene and benzo(j)
fluoranthene) are structural isomers with the same molecular weight
but different structural formulae. Purified HMW PAHs (five benzene
rings and above) are solids and generally do not volatilize because
their melting points are greater than 100°C . HMW PAHs also
have large resonance energies which make them thermodynamically
stable though they can be photo-oxidized at various aromatic positions
to form quinones. An example is when benzo[a]pyrene undergoes
photolysis. Increase in hydrophobicity endows these chemicals with
high octanol: water partition coefficients (kow) as a result of which they
tend to adsorb to soil organic matter, are less bioavailable and therefore
are not easily degraded giving rise to greater persistence in soil. High
molecular weight PAHs are therefore naturally recalcitrant in the soil
||Table 1: Physical and chemical properties of selvected PAHs.
|Fate and Behaviour of PAHs in Soil
|When PAHs enter soil, they can be lost or removed by a number
of physical, chemical or biological processes. For example, photo- oxidation and/or volatilization to the atmosphere, leaching to ground
water, irreversible sorption to soil organic matter, uptake by plants and
microbial degradation are some of these processes [109,122] (Figure
1). The rate at which these loss processes occur and the extent to which
a PAH is affected is dependent on a number of factors including soil
type (mineral and organic matter contents), the physico-chemical
properties of the individual PAH, soil temperature and moisture,
redox potential, nutrients availability, presence and activity of
degrading microorganisms and bioavailability of the PAH to degrading
||Figure 1: Fate of PAHs in soil.
|Figure 2 shows the possible fate of a PAH in soil . Compounds
with low Kow, high water solubility and volatility are more mobile and
degradable. As a result, they are lost rapidly when they enter the soil
(A). Many contaminants however, show a biphasic behaviour where
though losses are clearly occurring, the rate and extent of the losses
decrease as contact time between the soil and contaminant(s) increase
(B). This process has been termed ageing [59,117]. C represents the loss
pattern of a recalcitrant PAH in soil . The major biotic means by
which PAHs are lost from soils is through degradation by bacteria and
bioavailability is crucial to PAH biodegradation in soil [23,30,100,123].
||Figure 2: Theoretical loss curvesT ifmore three classes of contaminants .
|Bioavailability/Bioaccessibility and Biodegradation of
PAHs in Soil
|Two steps are involved in the biodegradation of PAHs in soil. The
first is the physical uptake of the PAH by the microbial cell and the
second is the biological metabolism of the PAH . The second step
is a function of the intrinsic biological ability of the microorganism to
degrade the PAH but the first is a function of the physical availability
of the compound to the cell. Assuming the presence of an adequate
metabolic pathway, the degradation of a PAH can only proceed rapidly
if the PAH is available to the microorganism in a form (usually in
the aqueous phase) that the organism can utilise it [93,115]. Where
this is non-existent, the biodegradation of the PAH will be limited
by the bioavailability of the PAH to the microorganism. Despite its
importance to biodegradation (and by extension to contaminated land
assessment), the concept of bioavailability has struggled to progress
from a mere research topic to a practical tool for the assessment and
management of risks associated with contaminated land. This difficulty
is due mainly to confusing and disagreeing definitions (arising from
the varied use of the term in different fields) and the absence of a fast
and cost effective tool for measuring bioavailability [44,56,107,113].
Semple et al.  proposed the use of two definitions, rather than a
single term: “bioavailable” and “bioaccessible”. The authors defined the
bioavailable compound as “that which is freely available to cross an
organism’s cellular membrane from the medium the organism inhabits
at a given time” . By contrast, the bioaccessible compound is
“that which is available to cross an organism’s cellular membrane from
the environment, if the organism has access to the chemical” .
The difference between the bioavailable and bioaccessible fractions
according to their definition is that whereas the bioavailable compound
is available to cross the organisms’ cellular membrane (for degradation,
transformation or exertion of a toxic effect) now, the bioaccessible
compound can only do so when a restraint in time and/or space has
been removed. In other words, the bioaccessible compound comprises
that which is actually bioavailable now and that which is potentially
bioavailable. Figure 3 conceptualises the bioavailable and bioaccessible
fractions of a contaminant in soil . Both of these definitions
are important because bioavailability is a descriptor for potential
rates of biodegradation; whereas, bioaccessibility describes realistic
biodegradation endpoints [107,114]. The significance of defining
bioavailability by the terms suggested by the authors lies in the fact that
it raises questions about what is actually being measured by current
methods that claim to measure the bioavailable fraction.
||Figure 3: A conceptual diagram illustrating the bioavailable and bioaccessible fractions of a contaminant in soil as defined by physical location .
|The concept of ageing (Figure 2) is important to the bioavailability
of PAHs in soil . Decreased bioavailability of PAHs to degrading
microbes due to sequestration/ageing in soil has been reported and
two processes have been proposed to be responsible for this. The
first is the diffusion of PAHs through dissolution sites present in the
rubbery and glassy phases of soil organic matter. Contaminants that
diffuse through the glassy phase interact more with soil organic matter
because of the presence of more rigid cavities . The second process
occurs when surfaces within the nano- and micro-pores in soils adsorb
contaminants diffusing through them . Since soils contain many
pores with diameters less than 20 nm , any PAHs that diffuse into
such pores become unreachable by degrading bacteria, resulting in
their reduced bioavailability.
|Biodegradation of PAHs by microorganisms and the
pathways involved have been extensively reviewed elsewhere
[13,30,55,70,101,117,155]. Several bacteria capable of degrading
PAHs in soils have also been isolated and identified [49,60-62,128].
Biodegradation of PAHs in soil does not depend on the PAH properties
alone  but also on environmental conditions , soil properties
, bioavailability of the PAH  and the degradation potential of
the indigenous microorganisms .
|Temperature, soil moisture content, alkalinity, soil nutrient
content and pre exposure to PAHs are some of the environmental
and soil properties affecting PAH biodegradation in soil. Generally,
bacterial metabolic activity and PAH biodegradation increases with
increasing temperature up to an optimum temperature reported to
be around 30°C to 40°C [78,156]. Hydrocarbon utilizing bacteria can
also adapt to temperature extremes to maintain metabolic activity.
Hydrocarbon degradation has therefore been reported at temperatures
close to freezing and above 30°C [5,10]. The optimum moisture level
for the biodegradation of oil sludge reported in literature is between
30% - 90% of the soil’s water holding capacity . At higher water
contents, there is a risk of the onset of anaerobic conditions arising
from the slow rate of oxygen diffusion through water. At lower water
contents, water availability becomes a limiting factor to microbial
activity or PAH bioavailability. PAH degradation has also been shown
to be favoured in slightly alkaline soils, as PAH-degrading bacteria
become less competitive with increasing acidic conditions . Bacteria
require nutrient elements, such as nitrogen and phosphorus for
incorporation into biomass and the synthesis of cellular components.
The presence of these nutrient elements in soil is therefore critical for
the biodegradation of PAHs . The optimisation of the C:N:P ratio
is thought to be one of the most important amendments enhancing the
rates and extents of PAH biodegradation in soil. It is generally accepted that optimum ratios for PAH biodegradation is approximately 100:10:1
|PAH biodegradation in soil is inducible  but the processes
through which indigenous soil microbes develop the capacity or
“learn” to degrade PAHs are not fully understood . However, the
pre-exposure of soils to PAH concentrations higher than background
PAH levels [65,121] and a change in the genetic composition of the
indigenous soil microbial community are thought to be important
factors affecting the process . A change in the genetic composition
of the indigenous soil microbial community is necessary where
the indigenous microbes have not been exposed to either naturally
occurring or anthropogenic compounds similar to the PAH of interest.
In a study to investigate the relationship between pre exposure and
metabolism of PAHs in soil, all 14C PAHs, studied except 14C benzo[a]
pyrene, were readily mineralized in most of the pre exposed soils,
whereas in the uncontaminated soil, less than 5% of each 14C PAH
was mineralized . In a similar study to assess the development of
pyrene catabolic activity in two similar soils (pasture and woodland),
significant decreases in the lag times and significant increases in the
maximum rates and extents of 14C-pyrene mineralised were observed
with increasing soil-pyrene contact time . The workers also found
that the microbial community in the pasture soil (with lower soil organic matter) adapted to use pyrene before the community in the woodland
soil. This observation was attributed to decreased bioavailability of
pyrene in the woodland soil due to a higher soil organic matter content.
|Having noted the importance of pre-exposure to the development
of PAH degrading capacity and the dependence of PAH bioavailability
and biodegradation on soil and environmental factors, this review
aims to consider existing knowledge on PAH levels, bioavailability
and indigenous biodegradation in background soils from extreme
temperature environments, temperate environments and tropical
|Biodegradation of PAHs in Soils from Different Climatic
|Although Antarctic soils are diverse, they have all been generally
described as cold, sandy, desert soils, with minimal effective annual
precipitation, high pH, low organic matter and nutrient contents and
rare occurrences of vegetation, mostly lichens and mosses [5,27]. Mean
monthly temperatures in the continental Antarctic region where the
most extreme conditions are found are below -15°C all year round,
falling to below -30°C in winter . Less than 0.35% of the entire
terrestrial Antarctic continent is ice free, the remainder is permanently
under the cover of ice . Despite this tiny proportion of the continent
being ice-free and exposed to weathering processes, there is significant
microbial diversity in Antarctic soils. Different PAH degrading
microorganisms have been found : up to 13 MPN g-1 , 230 MPN
g-1  and 7×104 CFU g-1  of hydrocarbon degrading bacteria have
been reported to be present in uncontaminated Antarctic soils. Most
of these PAH degraders were identified as bacteria rather than fungi
, e.g. Flavobacterium spp., Corynebacterium spp., Bacillus spp. an
isolate from the family of Enterobacteriaceae , Pseudomonas spp.
or Sphingomonas spp. related to PAH degradative strains from lower
latitudes [3,17] and Acinetobacter  are some of those isolated from
Antarctic soils. These degraders were psychrotolerant, able to tolerate
temperatures >20°C and capable of growing at or near 0°C. They were
also able to degrade monoaromatic hydrocarbons, naphthalene as well
as its methyl derivatives [3,71].
|The presence of PAH-degrading bacteria in background soils is a
good indicator that the soils have been exposed to PAHs , even
at low concentrations. Long range atmospheric transport of low
molecular weight PAHS and increased human presence and activity
in the Antarctic represent the main sources of PAHs in these soils.
Human activities require hydrocarbons for heating, power generation
and to power land and air transport vehicles. Elevated levels of PAHs
in Antarctic soils have therefore been found mostly in soils with close
proximity to current or former scientific bases, fuel stores or automobile
or aircraft fuelling stations [8,125]. Accidental discharge of petroleum
products during storage and distribution from drums and pipelines is
also a significant source . PAHs in uncontaminated Antarctic soils
have either been undetectable or detected at background levels [4,5].
For instance, a study was carried out to compare PAH concentrations
at fuel spill sites to uncontaminated control sites in three Antarctic
locations: Scott Base, Marble Point and Wright Valley. The study found
PAH concentrations in the contaminated sites to range from 41 – 8,105
ng g-1 of dry soil. PAHs were, however, undetectable at the control
soils . PAH concentrations of up to 27,000 mg kg-1 have also been
detected in contaminated soils at McMurdo Station, Antarctica; the
highest concentrations were found in soils from unpaved roadbeds and
gasoline pumps . Where PAHs have been found in uncontaminated Antarctic soils, low molecular weight PAHs (phenanthrene and
fluoranthene) are usually prevalent and are present at levels consistent
with pre-industrial times . This is an indication that while local
sources are responsible for high levels of PAHs in contaminated soils,
long range atmospheric transport from populated areas of Africa,
South America, and Australia is the most likely source of PAHs to
background Antarctic soils .
|The fate of PAHs in Antarctic soils includes volatilisation, dispersal,
transformation or degradation as a result of physical, chemical and
biological processes. Lighter PAHs volatise readily from Antarctic soils
and can migrate through soil layers . Less volatile fractions tend not
to migrate far from their point of deposition . Lighter molecular
weight PAHs are expected to be available for microbial degradation
in Antarctic soils  because these soils are typically dry and have low
organic matter contents. Since the sequestration of organic compounds
in dry soils with < 4% moisture is controlled by adsorption onto
mineral surfaces and not organic matter [18,134], PAH biodegradation
in Antarctic soils should not be limited by sequestration to soil organic
matter. PAH bioavailability may however, be limited by poor water
activity . Because of the low moisture contents of Antarctic soils,
PAHs and microbes may not be able to move around sufficiently
enough to come in contact for biodegradation. Microbial degradation
of PAHs in Antarctic soils has been measured in microcosms  and
mesocosms . Low moisture content, alkaline soil pH , and low
temperature [6,41] have been identified as the limiting factors for
PAH biodegradation in Antarctic soils; however, nutrient availability
and temperature fluctuations are considered to be the most important
|When high concentrations of PAHs are introduced into Antarctic
soils, they have the potential to further deplete the already low amounts
of N and P present in the soils when they are used up by microorganisms
during biodegradation . Enhanced biodegradation of PAHs has
been achieved in Antarctic soils by the addition of fertilizers containing
N and P. For instance 14C-naphthalene was mineralised to a greater
extent when N was added as nitrate or ammonium . The addition
of nutrients to the coarse-textured, low-moisture soils prevalent in
continental Antarctica has the potential to inhibit PAH biodegradation
by decreasing soil water potentials, care must therefore be taken to
add the right amount of nutrients and there is no consensus on what
that right amount is because of the varying soil and environmental
conditions prevalent in Antarctica .
|Temperature limits the biodegradation of PAHs in Antarctic
soils by affecting the physical nature of the PAHs and the metabolic
activity of the degrading microbes . At low temperatures, PAHs
are more viscous, less volatile and less soluble resulting in reduced
bioavailability and subsequent biodegradation . Metabolic
activity of PAH-degrading microbes at low temperatures are reduced
because they follow the Arrhenius relationship, increasing with
increasing temperature , usually doubling for each 10°C increase
in temperature from 10°C to 40°C [24,36]. As a result, though PAH
biodegradation has been reported at low temperatures , increasing
the temperature has often resulted in increased rates and extents of
PAHs degradation [73,136].
|Temperate environments are characterised by their general
mildness and four distinct seasons with summer and winter roughly
of equal length. 35% - 40% of the global human population live in
the hospitable temperate environment comprising roughly 7% of the Earth’s surface area . Temperate and tropical soils share many
soil types, making the distinction between them artificial; however,
characteristics like soil temperature and soil moisture are often
different between the two across all soil types . Temperate soils are
different from tropical soils firstly because during a significant portion
of the year (winter), they are exposed to sub-optimal or freezing
temperatures resulting in reduced biological activity and changes in
the physical and chemical properties of the soils. Secondly, agricultural
soils in temperate environments are generally younger than soils from
the tropics . It is also generally believed that the residence times
of organic matter/carbon is longer in temperate soils than in tropical
soils due mainly to reduced rates of decomposition arising from lower
temperatures and moisture levels in temperate regions .
|In soils throughout the temperate zone, PAH profiles are similar
, showing correlations between individual PAH concentrations
 and a quantitative dominance of higher molecular weight
PAHs (e.g. chrysene, pyrene, fluoranthene) produced mainly due to
anthropogenic activities [43,94,138]. This similarity in PAH profiles
has been explained by the “weathering” of emissions from different
sources in the atmosphere, resulting from the transformation of PAHs
from different sources during atmospheric transport .
|Levels of PAHs in background soils from Western Europe have
been found to cover a wide range of concentrations from 8.6 to 11,200
μg kg-1 . Land use plays an important role in PAH concentrations
in temperate soils. For instance, PAHs in agricultural soils amended
with sewage sludge have been reviewed , showing the effects the
introduction of sewage sludge can have on PAH concentrations.
Sewage sludge may enhance the removal of PAHs from soils, either
by the introduction of microorganisms adapted to PAH degradation
or by increasing the leachability of PAHs through the movement of
dissolved organic matter. Conversely, the application of sewage sludge
can introduce more organic matter into the soil, resulting in greater
sorption of PAHs and subsequent retention in the soils . Long
term studies of archived UK agricultural soils amended with sewage
sludge revealed PAH levels were three times higher than unamended
control soils, particularly with higher molecular weight PAHs
[145,148]. The implication may be that the addition of sewage sludge to
temperate agricultural soil does more to enhance the retention of PAHs
than their losses.
|PAH concentrations in temperate soils also differ according to soil
type. In most cases, concentrations of persistent organic pollutants are
lower in grassland soils than in woodland/forest soils because of the
greater ability of the forest canopy to scavenge for persistent organic
pollutants . Selected Norwegian grassland soils have been found to
have generally lower PAH levels than neighbouring forest systems .
|Urban temperate soils are often associated with increased human
activity resulting in high PAH levels relative to soils from less populated
rural areas [69,74]. Distance from point sources, climatic conditions
and soil organic matter are other factors that influence levels of PAHs
in urban soils. Positive correlations have been drawn between PAH
concentrations in temperate soils and soil organic matter [67,94,147].
|PAH degraders have been found in pristine soil samples from
Norway (up to 2.4 x106 CFU g-1 soil) and Denmark (up to 6.6 x 106
CFU g-1 soil), agricultural soils (up to 9.9 x106 CFU g-1 soil), diffusely
polluted (up to 1.1 x 108 CFU g-1 soil) and industrially polluted (up to
1.7 x 107 CFU g-1 soil) soils from Denmark. This shows the ubiquitous
presence of PAH degraders following the ubiquitous presence of PAHs
in temperate soils. In fact, a correlation has been found between the total PAHs in a temperate soil and the potential for PAH mineralisation
in the soil suggesting the dependence of PAH degradation potential
on the level of PAH contamination . While studying the total
and bioaccessible amounts of PAHs present at a motorway site north
of Copenhagen and the prevalence of microbial PAH degraders as
well as the potential for PAH degradation, the highest levels of PAH
contamination were found in samples closest to the motorway. The
highest numbers of bacterial PAH degraders and the greatest potential
for PAH biodegradation were also found in the same soils. The fact that
the highest levels of PAH contamination was found in the soil with
the highest numbers of bacterial PAH degraders appear contradictory
and raises questions about PAH persistence. The authors suggested
that low bioavailability of PAHs and not a limitation in the bacterial
population was responsible for the accumulation of PAHs in the soil
with a high number of bacterial PAH degraders. A measurement of
the bioaccessible PAH fractions showed that only 1-5 % of the PAHs
were actually accessible to the soil microorganisms for degradation.
Adsorption of PAHs to soot particles released from diesel engines was
given as a possible reason for the reduced bioavailability and consequent
reduced biodegradation of the PAHs . Higher soil organic matter
content in temperate soils has also been found to result in slower
adaptation of indigenous soil microorganisms to PAH degradation
. In a study, two similar temperate soils (pasture and woodland)
were amended with 100 mg pyrene kg-1 and significant mineralisation
of 14C pyrene was observed after 8 and 76 weeks soil pyrene contact time
respectively. The addition of microbial inocula led to the mineralisation
of the previously added 14C pyrene implying that it was bioavailable
but the indigenous microflora had not adapted to its degradation. As
the only difference between the two soils was a higher organic matter
content in the woodland soil, the authors suggest that the rate of pyrene
organic matter content in the woodland soil, the authors suggest that
the rate of pyrene transfer from the soil to the microorganisms was
slower in the woodland soil due to its higher organic matter content
leading to slower adaptation of the microorganisms and subsequent
biodegradation. Despite PAH-degrading microorganisms being
ubiquitously distributed through temperate soils, the major limiting
factor for PAH degradation appears to be bioavailability as controlled
by soil type and soil organic matter content.
|Geographically, the tropics can be defined as that part of the
world located between 23.5 degrees north and south of the equator,
representing the land mass between the Tropic of Cancer and the
Tropic of Capricorn. Tropical environments are largely warmer
than temperate environments with much less temperature variations
between seasons. Approximately 87% of the global tropical areas are
usually above 25°C. Differences in amounts of precipitation and solar
radiation further distinguish between tropical and temperate climates
|The generalisations that can be drawn from the characteristics
of tropical soils have been reviewed as follows: (i) the types and
properties of clay minerals are much more varied in the tropics than
in glaciated temperate areas; (ii) many tropical soils exhibit significant
anion exchange capacity; (iii) organic matter contents in the tropics
are similar to those of the temperate region; (v) although the annual
addition of organic carbon to the soil is five times greater in tropical
udic environments than temperate udic environments, the rate of
organic decomposition is also five times greater in the tropics; (v)
in ustic environments, lack of soil moisture during the dry season
decreases organic carbon decomposition just as low temperatures do in temperate regions; (vi) the vast majority of the soils of the humid
tropics are acidic, and (vii) the vast majority of the cultivated soils of
the humid tropics are not acidic .
|In comparison to temperate soils, published data on the PAH
levels of tropical soils is considerably less . Available data show that
generally, PAH levels in most tropical soils are lower than in temperate
soils. For instance, PAH levels in soils from Lahore, Pakistan were
considerably lower than those in soils collected from Birmingham, UK
(which were representative of UK PAH levels) even though atmospheric
levels were higher . PAH levels of 5 ng g-1 dry weight have been
reported for Costa Rican soil . Σ20PAHs in 30 hydromorphic soils
of the tropical metropolis of Bangkok was found to range between 12
and 380 μg kg-1 . Higher PAH levels have been found in temperate
UK (42 and 11,200 μg kg-1) and Norway (8.6 and 1,050 μg kg-1)
background soils [69,94]. Shorter PAH accumulation time in tropical
soils and enhanced PAH losses through biodegradation, volatilisation,
photo-oxidation, and leaching into groundwater have been proposed as
the reason why PAHs levels are lower in tropical soils . Although
the ΣPAHs in tropical soils is often less than in temperate soils, low
molecular weight PAHs (naphthalene and phenanthrene) are most
prevalent and are present at levels comparable to those in temperate
|Soil-PAH fingerprints are often defined by soil type and PAH
sources into the soil. PAHs enter tropical environments mostly through
on-going biomass burning and vehicular traffic . By contrast,
fossil fuel combustion (known to release higher molecular weight four
and five ring PAHs) dominate PAH sources of PAHs into temperate
soils . Also, a longer history of industrialisation in the temperate
region implies historical PAH sources. This suggest that over time, the
more volatile, biodegradable lower molecular weight PAHs would have
been removed from temperate soils . Biologically derived PAH
sources known to produce low molecular weight PAHs have also been
identified as sources of low molecular weight PAHs into tropical soils
|Microbial diversity in soils is determined by vegetation type, carbon
and nutrient availability, soil moisture and soil pH. Soils with similar
environmental characteristics will therefore have similar bacterial
communities irrespective of geographical location . Studies where
indigenous PAH degraders are enumerated in tropical soils are rare
but tropical soils with similar properties as temperate soils can be
expected to contain similar microbial communities. The presence of
microorganisms with the necessary genes for PAH degradation in
contaminated tropical soils have been reported . For example, thirty
three distinct species of bacteria, fungi and yeast capable of degrading
PAHs were collected from petroleum polluted soils and cyanobacteria
mats from Indonesia . Also, three Pseudomonas species LP1, LP2
and LP3 were isolated from an oil polluted soil in Lagos, Nigeria. They
were able to degrade a broad range of PAHs including up to 67.79%,
66.61% and 47.09% of pyrene respectively .
|Moisture and temperature are the main factors that influence
microbial activities in which tropical and temperate soils differ. Tropical
soils have higher temperatures all year round . Rates of PAHs
degradation in tropical soils are therefore generally expected to be
higher than in temperate soils. The higher rates of organic matter turn
over observed in tropical soils support this argument . Moisture
should therefore be the controlling/limiting factor of microbial activity
in tropical soil . PAH biodegradation can be controlled by moisture either directly because microbial activity can be inhibited by poor soil
moisture or indirectly by affecting PAH transport and bioavailability .
|Desert soils can be found globally except in Europe. They are
characterised by poor organic matter and water levels, are usually
subjected to rather high temperatures (up to 50°C) and extensive
light . There is a paucity of data relating to the sources and
levels of PAHs in background hot desert soils because most studies
on background soils have been conducted using temperate soils .
However, both short and long range atmospheric transport of PAHs
and proximity of hot deserts to PAH point sources play significant
roles in the background concentration of PAHs in desert soils .
|Temperature is significant to the fate of PAHs in desert soils.
Because of the hydrophobic nature of PAHs and their consequently
reduced solubility, PAH volatilisation, solubility and bioavailability are
controlled by temperature. At high temperatures, PAHs are less viscous,
more easily distributed and can diffuse faster. Bioavailability of some
PAHs can therefore be enhanced at elevated temperatures . On the
other hand, bioavailability and biodegradation of PAHs can be inhibited
by the poor moisture content of the soils. Increased bioavailability and
subsequent biodegradation has been achieved by repeated irrigation of
polluted desert soils [105,106]. Biodegradation of PAHs in extremely
hot desert soils requires the activity of thermophilic bacteria but not
much is known about PAH degradation by thermophiles in soil [86,96].
A few microbes capable of degrading PAHs at very hot temperatures
have been isolated. For example, Bacillus thermoleovorans, which
grows at 60°C, has been isolated from a compost consisting of wooden
ties treated with lignite tar and was able to utilise naphthalene as a sole
carbon source . Nocardia otitidiscaviarum which grows optimally at
50°C has also been shown to possess PAH degrading capabilities .
Other microorganisms able to degrade naphthalene, phenanthrene
and anthracene under thermophilic conditions have also been isolated
[47,92]. Some of these microbes were found to produce metabolites
that were different from those produced during the biodegradation of
PAHs by mesophilic bacteria . A new pathway was also identified
when Bacillus thermoleovorans degraded naphthalene at 60°C .
For some time, it was assumed that at elevated temperatures, reduced
oxygen solubility limited effective aerobic degradation of PAHs but
more recent discoveries have shown that at elevated temperatures,
the oxygen transfer coefficient rises thereby compensating for the
reduction in oxygen solubility .
|PAHs are an important class of environmental contaminants
produced by both natural and anthropogenic processes. The importance
of soil as a reservoir of organic pollutants means that PAHs can now
be found in virtually all global soils irrespective of nature or location.
When PAHs enter a soil, they interact with the soil in different ways
leading to either retention or loss depending on the physical, chemical
and biological characteristics of the soil. Since global soils differ in
their characteristics, the fate of PAHs in soils also differ and it may not
always be correct to extrapolate the fate of a PAH in one soil type to
another. This makes understanding the interactions between PAHs and
soils from different environments important. The weight of available
information on these interactions leans heavily in favour of temperate
soils. More research is therefore needed into PAH-soil interactions in
tropical and desert soils.
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