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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
TOXIC EFFECTS OF LINEAR ALKYLBENZENE SULFONATE, ANTHRACENE AND
THEIR MIXTURE ON GROWTH OF A MICROBIAL CONSORTIUM ISOLATED
FROM POLLUTED SEDIMENT
Gabriel PINEDA FLORES, Carmen MONTERRUBIO BADILLO, Manuel HERNÁNDEZ CORTÁZAR,
Cirilo NOLASCO HIPÓLITO, Rocío SÁNCHEZ PÉREZ and Ignacio GARCÍA SÁNCHEZ
Instituto Politécnico Nacional, Centro Mexicano para la Producción más Limpia. Av. Acueducto s/n, La laguna
Ticomán, Gustavo A. Madero, México D.F. 07340, México. gpineda@ipn.mx
(Recibido junio 2008, aceptado junio 2009)
Key words: toxicity, LAS, anthracene, mixture, microbial consortium, sediment, pollution
ABSTRACT
The aim of this study was to determine the effect of linear alkylbenzene sulfonate
(LAS), anthracene and a LAS-anthracene mixture on the growth of a microbial con-
sortium isolated from polluted sediment. The microbial consortium was grown in a
sterile glass bottle with mineral medium containing 1 g/L of glucose. Microbial growth
inhibition produced by LAS, anthracene and combinations of LAS and anthracene was
determined by viable count in nutritive agar; inhibitory concentration 50 (IC
50
) was
calculated. The concentrations evaluated were 0.16, 0.8, 1.6, 16 and 160 mg/L of LAS
or anthracene. The LAS-anthracene mixtures were prepared by ±xing either LAS or
anthracene at 0.16 mg/L while increasing the other compound at the above concentra-
tions. Microbial growth was sensitive to LAS at an IC
50
of 8.22 and to anthracene at
an IC
50
of 5.2 mg/L. In the LAS-anthracene combination, if LAS concentration was
±xed and anthracene concentration varied, IC
50
(5.92 mg/L) was similar to IC
50
for
anthracene alone. In contrast, the inhibition effect was diminished when anthracene
remained constant and LAS concentration was increased (IC
50
: 70.11 mg/L). The sedi-
ment microbial populations were capable of degrading the LAS-anthracene mixture if
the concentration of both compounds were at 0.16 mg/L.
Palabras clave: toxicidad, LAS, antraceno, mezcla, consorcio microbiano, sedimento, contaminación
RESUMEN
El objetivo de este trabajo fue determinar el efecto del sulfonato de alquilbenceno
lineal (SAL), antraceno y su mezcla sobre el crecimiento de un consorcio microbia-
no aislado de sedimento contaminado. El crecimiento del consorcio se obtuvo en
botellas de vidrio con medio mineral estéril más 1 g/L de glucosa. La inhibición del
crecimiento microbiano, producida por SAL, antraceno o la combinación de ambos,
fue determinada por cuenta viable en agar nutritivo y se determinó la concentración
inhibitoria 50 (CI
50
). Las concentraciones evaluadas fueron 0.16, 0.8, 1.6, 16 y 160
mg/L de SAL o antraceno. Las mezclas SAL-antraceno fueron preparadas mantenien-
do constante la concentración de SAL o antraceno en 0.16 mg/L mientras se aumentó
la concentración del otro compuesto a las mismas concentraciones mencionadas. El
Rev. Int. Contam. Ambient. 26 (1) 39-46, 2010
G. Pineda Flores
et al.
40
crecimiento microbiano fue sensible al SAL y antraceno a una CI
50
de 8.22 mg/L y 5.2
mg/L respectivamente. Cuando la concentración de SAL y antraceno fue evaluada, si
la concentración de SAL se mantuvo ±ja y la concentración de antraceno varió, la CI
50
(5.92 mg/L) fue muy similar a la CI
50
para antraceno solo. En contraste, el efecto de
inhibición disminuyó cuando el antraceno permaneció constante con concentraciones en
incremento de SAL (CI
50
: 70.11 mg/L). Por otra parte, se observó que las poblaciones
bacterianas del sedimento son capaces de biodegradar la mezcla SAL/antraceno cuando
la concentración de ambos compuestos fue de 0.16 mg/L.
INTRODUCTION
Pollution of freshwater sediment ecosystems
is generally due to the presence of a mixture of
chemical compounds (Alexander 1997). Detergents
and polycyclic aromatic hydrocarbons are organic
pollutants that accumulate in freshwater sediment,
constituting pollutant mixtures (Smulders and Krings
1990, Aboul-Kassim 1992). Anthracene and linear
alkylbenzene sulfonate
(
LAS) are some of the many
pollutants present in different aquatic ecosystems
(Comber
et al
. 2006, Hamdi
et al
. 2006). The toxic-
ity of chemical compounds on aquatic organisms
depends on concentration in both the sediments and
the water, as well as in processes related to their
bioavailability. Bioconcentration, biodegradation,
desorption and solubilization processes that occur
in these substrata determine the quantity of free
compounds that will reach toxic levels in the organs
of aquatic organisms.
Anthracene is a tricyclic aromatic hydrocarbon;
its molecules fate in nature is of great environmental
concern due to their potential toxicity, mutagenicity
and carcinogenicity (Li
et al
. 2008). Anthracene is
a hydrophobic substance which has been shown to
be toxic to ±sh and algae (Moody
et al
. 2001). An
increasing number of studies have been conducted on
anthracene biodegradability to examine its elimina-
tion from ecosystems. An interesting and important
observation made by Cerniglia and Heitkamp (1989)
was that eukaryotic microbes such as
Cunninghamel-
la elegans
use a cytochrome P-450 monooxygenase
system or lignin peroxidase to break down aromatic
hydrocarbon rings into the detectable product cis-
dihydrodiol (Pickard
et al
. 1999).
LAS is a surfactant produced in large amounts
used around the world in detergent and personal
care products (Jiménez
et al
. 1991, Schleheck
et al
.
2004). LAS has been shown to affect the ²ora and
fauna of aquatic ecosystems. It has been observed
that this compound denatures proteins in the cell
membrane, altering the permeability of the mem-
brane to nutrients and other chemical substances
(Kimerle 1989). Due to its surfactant properties, LAS
is adsorbed preferentially onto sediments (Sanderson
et al
. 2006).
The initial enzymatic attack in LAS biodegrada-
tion occurs by omega oxidation of the terminal carbon
of the alkyl side chain. The enzymes involved in this
reaction, although not yet identi±ed, are probably as-
sociated with cell membranes. This enzymatic attack
results in a carboxylated alkyl chain or sulfophenyl-
carboxylate, which is further biodegraded through
beta oxidation. Once beta oxidation has taken place,
the molecule loses its surfactant properties because
it no longer has a hydrophobic side chain. Follow-
ing complete mineralization of the alkyl chain, the
benzene ring is desulfonated and cleaved (White and
Russell 1994, Schleheck
et al
. 2004).
Laboratory studies have demonstrated that these
toxic organic compounds do not routinely biodegrade,
as many of them are resistant to microbial degrada-
tion. This is due to microbial biodegradation is usually
accessible only when they are dissolved in aqueous
solution or at least in direct contact with water (Sun-
daram
et al
. 1994). Fu and Alexander (1995) found
that the desorption or solubilization of petroleum
hydrocarbons can be accelerated with the addition of
surfactants. As a result, bioavailability and therefore
biodegradation of anthracene may be increased in
the presence of LAS in bodies of water polluted with
these compounds.
According to Ventullo and Larson (1986), cationic
surfactants can produce alterations in heterotrophic
activity in limnetic microbial populations. Therefore
it is possible that the chemical structure and toxicity
of LAS and anthracene would have the same effect
on sediment microbial populations.
The hypothesis is that the LAS-anthracene mix-
ture can inhibit the growth of the microbial consor-
tium in sediment. It is possible that anthracene is
more toxic than LAS, and that when they are mixed,
having made the anthracene more soluble by increas-
ing the surfactant concentration, the mixture will
TOXICITY OF THE LAS-ANTHRACENE MIXTURE ON A MICROBIAL CONSORTIUM OF SEDIMENT
41
be more toxic than the separate compounds. On the
other hand, it is expected that at a low concentra-
tion, microbial degradation of the mixture will be
possible, since there will not be an inhibition effect
on microbial growth.
It was determined the toxicity and kinetics of
biodegradation of the LAS-anthracene mixture by
microbial populations of natural ecosystems, with
particular emphasis on microorganisms found in
sediment, where these compounds are concentrated.
The objective was to evaluate the acute toxic ef-
fect of LAS, anthracene and the mixture of the two
compounds on the growth of a microbial consortium
isolated from sediment, and to determine the potential
of the LAS-anthracene mixture to be degraded by
isolated microbial populations.
MATERIALS AND METHODS
Sampling area
The sampling zone was located in the municipal-
ity of Tlahuelilpan de Ocampo in the state of Hidalgo,
México. Samples were collected in Irrigation District
63, located in the southeast part of the state to the
north of México City, between 15° 44’ and 20° 29’ N
and 98° 57’ and 99° 21’ W, with an average elevation
of 1895 meters above sea level.
The sediments are in continuous contact with
domestic, agricultural and probably industrial
wastewaters. These wastewaters did not receive any
prior treatment; thus microorganisms present in the
sediments are in direct contact with a great variety of
chemical pollutants. The water is subsequently used
for irrigating agricultural crops; however, this water
contains toxic hydrocarbons and other by-products
associated to their breakdown by the microorgan-
isms contained in the sediments, which will pollute
the crops.
Screening and maintenance of the microbial
consortium
The microbial consortium was isolated from sedi-
ment obtained at the same time as the wastewaters
were sampled. One kilogram of sediment was col-
lected and placed in a sterile carrier bag. The sedi-
ment was stored at 4 ºC until required.
To maintain the microbial population, the ISO-9439
system was employed (Pineda-Flores
et al
. 2004). One
gram of sediment was placed in a 250 mL sterile glass
bottle, which contained 100 mL of mineral medium
(composition in mg/L: KH
2
PO
4
0.085, K
2
HPO
4
0.22,
Na
2
HPO
4
•2H
2
O 0.33, NH
4
Cl 0.05, MgSO
4
•7H
2
O
0.023, CaCl
2
0.028, FeCl
3
•6H
2
O 2.5×10
4
) plus 1 g of
glucose as a carbon and energy source.
The microorganisms that constitute the microbial
population were identi±ed as
Pseudomonas men-
docina
,
Flavobacterium
breve
and
Corynebacterium
favescens
by microscopy, colony morphology and
various biochemical tests following the schemes of
Weaver and King (Weyant
et al
. 1996) and Bergey’s
manual (Sneath
et al
. 1986).
The culture was maintained at 25 ºC and agi-
tated by magnetic stirring (150 rpm). The microbial
population was maintained by introducing weekly
subcultures of 1 mL of the microbial population into
100 mL of fresh sterile mineral medium containing
1 g of glucose. Aseptic technique was used for all
transfers.
Toxic effects of LAS and anthracene on microbial
growth
Once the microbial population had reached the
exponential phase (with an absorbance of 0.09 at 650
nm), 1 mL of culture was serially diluted from 10
-
3
to 10
-
6
in assay tubes with 9 mL of sterile distilled
water (Espigares
et al
. 1990). Each tube was added
with 0, 0.16, 0.8, 1.6, 16 or 160 mg/L of LAS or an-
thracene (Aldrich Chemical Co., 98 and 96 % purity
respectively, both sterilized by 0.45 µm membrane
±ltration). The samples were stirred for 5 seconds,
then a 0.2 mL aliquot was taken and streaked onto
nutritive agar obtained from Becton Dickinson and
Company, Cockeysville, MD. Each set of plates was
incubated at 25 ºC for 48 hours and the colonies enu-
merated. The experiment was replicated ±ve times.
To establish the interval of concentrations for the
tests, growth of the consortium was evaluated with
1, 4 and 16 mg/L of each compound as described
above. With 16 mg/L of each compound, a reduction
in the growth of the consortium was observed (data
not shown). For this reason, the choice of the range
of concentrations was based on decimal reductions
and decimal increases of 16 mg/L (0.16, 1.6, 16 and
160 mg/L). The 0.8 g/L concentration was included
to complete the ±ve concentrations needed as mini-
mums to determine IC
50
.
The effect of the LAS-anthracene mixture was
quanti±ed following the method described above,
except for the following difference: each assay tube
contained either a ±xed concentration of LAS (0.16
mg/L) plus 0.16, 0.8, 1.6, 16 or 160 mg/L of anthra-
cene or 0.16 mg/L anthracene plus 0.16, 0.8, 1.6, 16
or 160 mg/L of LAS. This enabled the determination
of the toxicity of the LAS-anthracene mixtures. Five
repetitions were performed for each concentration.
G. Pineda Flores
et al.
42
IC
50
for LAS, anthracene and the mixture was ob-
tained using linear regression by plotting the number
of colony-forming units against the logarithmic
concentration of LAS or anthracene.
Microbial degradation of LAS-anthracene mixture
The production of carbon dioxide by microbial
degradation of the LAS-anthracene mixture was evalu-
ated using the device and methodology described by
Pineda-Flores
et al
. (2004). A sterile 250 mL glass
bottle containing 100 mL of mineral medium was in-
oculated with 1 mL of microbial consortium adjusted
to Abs
652
= 0.09. The concentrations of LAS and an-
thracene used in the mixture were as described above.
Carbon dioxide was measured every 12 hours for 48 h.
The concentrations used for the LAS and anthracene
mixture were 10-0.1, 12.58-1, 15.84-10, 19.95-100 and
23.98-200 mg/L. Three repetitions were performed for
each treatment. In order to characterize degradation of
the LAS-anthracene mixture, the maximum reaction
velocity (Vmax), Michaelis constant (Km) and af±nity
constant (AC) were calculated from the Michaelis-
Menten equation (Conn
et al
. 1987).
RESULTS
Figure 1
shows the toxic effect on the growth of
the microbial population exposed to different concen-
trations of LAS and anthracene. The growth of the
isolated microbial population was not inhibited with
0.16 mg/L of LAS or anthracene alone. When the LAS
and anthracene concentrations were increased, toxic
effects on the microbial population also did. CFU/mL
declined as LAS and anthracene concentrations were
increased from 0.8 to 160 mg/L. The LAS-anthracene
mixtures produced a notable growth inhibition effect,
as seen when
±gure 2
is compared to
±gure 1
. The de-
cline in CFU/mL was also correlated with an increase
in LAS and anthracene concentration. The greatest
Fig. 1.
Toxic effect of LAS (a), and anthracene (b) on the growth of isolated microbial consortium. CFU of microbial con-
sortium without LAS or anthracene concentration was 1.25×10
6
² 2.12×10
5
CFU/mL. The bars represent standard
deviation of ±ve repetitions
CFU/ml
LAS concentration (mg/L)
Anthracene concentration (mg/L)
a)
3.60E+06
3.00E+06
2.40E+06
1.80E+06
1.20E+06
6.00E+05
0.00E+00
1.50E+06
1.20E+06
9.00E+05
6.00E+05
3.00E+05
0.00E+00
0.16
0.8
1.6
16
160
0.16
0.8
1.6
16
160
b)
Fig. 2.
Toxic effect of LAS-anthracene mixture increasing the anthracene concentration from 0.16 to 160 mg/L (a), and
increasing the LAS concentration from 0.16 to 160 mg/L (b). CFU of microbial consortium without LAS-anthracene
mixture was 4.48×10
6
² 3.88×10
5
CFU/mL. The bars represent standard deviation of ±ve repetitions
Anthracene concentration (mg/L)
LAS concentration (mg/L)
CFU/ml
a)
7.50E+06
6.00E+06
4.50E+06
3.00E+06
1.50E+05
0.00E+00
7.20E+06
6.00E+06
4.80E+06
3.60E+06
2.40E+06
1.20E+06
0.00E+00
0.16
0.8
1.6
16
160
0.16
0.8
1.6
16
160
b)
TOXICITY OF THE LAS-ANTHRACENE MIXTURE ON A MICROBIAL CONSORTIUM OF SEDIMENT
43
inhibition, with 100 % of the microbial population
killed, was observed with the mixture containing 0.16
mg/L of LAS and 160 mg/L of anthracene.
Table I
shows that IC
50
for anthracene was 5.2
mg/L, 3 mg/L lower than the IC
50
for LAS. The IC
50
of anthracene was similar to the IC
50
of the LAS-
anthracene mixture, where anthracene concentrations
increased from 5.2 to 5.92 mg/L. However, the dif-
ference was much more pronounced than for the IC
50
of LAS-anthracene mixture, whose 70.11 mg/L was
almost 8.5 times higher than the IC
50
of LAS without
anthracene (8.22 mg/L).
The kinetics of mineralization by the LAS-
anthracene mixture is presented in
fgure 3
. The
maximum concentration of carbon dioxide produced
in the system was observed at 24 hours. After this
time, the mineralization fell abruptly and remained
constant up to 48 hours.
Table II
shows the results of the LAS-anthracene
mixture degradation. It is to be noted that 20.45 %
of the mixture was biodegraded by the microbial
population within 24 hours.
DISCUSSION
There have been few studies of the effect of or-
ganic pollutants on sediment microbial populations.
Toxicity data of chemical pollutants on microorgan-
isms are scarce due to the considerable ability of
microbes to resist or biodegrade organic chemicals.
Some microbial populations are very sensitive to low
concentration of LAS. Brandt
et al
. (2001) showed
that ammonia-oxidizing bacteria isolated from soil
are inhibited by 5-9 mg/L of LAS; inhibition was
shown by its effect on microbial growth, speci±c
growth rate and CO
2
±xation. García
et al
. (2006)
demonstrated that for LAS, an EC
50
of 14 mg/L can
be considered a toxic concentration for anaerobic
microorganisms, and that the addition of LAS homo-
logues to anaerobic digesters at surfactant concentra-
tions higher than 5-10 g/kg of dry sludge gave rise to
partial or total inhibition of methanogenic activity.
The microbial population isolated from sediment
was previously exposed to high concentrations of LAS
contained in the polluted water. Therefore it could be
assumed that the polluted water would selectively
isolate microbial populations which could grow in en-
vironments with a high LAS concentration. However,
in contrast to the prediction, the microorganisms in this
study were sensitive to low concentrations of LAS and
the LAS-anthracene mixture, with IC
50
values of 8.22
and 5.92 mg/L, respectively. Therefore, in this case
prior exposure to polluted water did not contribute to
an increase in microbial resistance. Jensen
et al
. (2007)
demonstrated that concentrations of LAS in untreated
sludge can range from 400 to 14,000 mg/kg dw.
TABLE I.
DETERMINATION OF INHIBITING CONCEN-
TRATION 50 (IC
50
) FOR LAS, ANTHRACENE
AND THEIR MIXTURE ON GROWTH OF ISO-
LATED MICROBIAL CONSORTIUM
Compound(s)
IC
50
(mg/L)
LAS
8.22
Anthracene
5.2
LAS-anthracene
(increasing anthracene concentration)
1
5.92
LAS-anthracene
(increasing LAS concentration)
2
70.11
1
LAS concentration was kept constant at 0.16 mg/L, anthracene
concentration was increased from 0.16 to 160 mg/L
2
Anthracene concentration was kept constant at 0.16 mg/L, LAS
concentration was increased from 0.16 to 160 mg/L
TABLE II.
CHARACTERIZATION OF LAS-ANTHRACENE
MIXTURE DEGRADATION
Variable
Magnitude
Vmax
4.99 µMol CO
2
Km
0.5235 µMol CO
2
/h
Af±nity constant
0.2 1/µMol CO
2
Biodegradation percentage
1
20.45 %
1
The biodegradation percentage was calculated 24 hours after
inoculation
Fig. 3.
Mineralization kinetics of LAS-anthracene mixture for
the microbial consortium isolated from sediment. LAS
and anthracene concentrations were both 0.16 mg/L. The
bars represent standard deviation of three repetitions
μMol CO
2
30
25
20
15
10
5
0
0
12
24
36
48
Time (h)
G. Pineda Flores
et al.
44
The dose-response curves (
Figs. 2
and
3
) indi-
cated that microbial growth was not inhibited at
very low concentrations of LAS, anthracene and
LAS-anthracene mixtures (0.16 mg/L). Increasing
the concentration of all compounds and mixtures
decreased microbial growth. This may be due to the
LAS concentrations evaluated being less than the
critical micelle concentration reported (410 mg/L,
Brandt
et al
. 2001); therefore, it is not possible to
suggest that there were surfactant-micelle interac-
tions. The results suggest that the LAS toxicity may
have been due to direct interactions of LAS mono-
mers with the cell structure, causing an increase in
membrane permeability, dissipation of ion gradients
and membrane potential or leakage of essential cell
constituents. Sartoros
et al
. (2005) reported that 20
mg/L of the surfactant Tergitol NP-10 may disrupt
cell membranes by interacting with lipid structural
components of bacterial cells isolated from soil pol-
luted with polycyclic aromatic hydrocarbons.
Anthracene has a high octanol-water partition
coef±cient (log K
ow
= 4.1), and readily partitions
into organic phases such as phospholipids, which
are also found in bacterial membranes. This inter-
action provokes a hydrophobic region inside the
bacterial membrane, which can act as a reservoir
for anthracene accumulation (Bugg
et al
. 2000). It
is suggested that the toxic effect of anthracene on
isolated microbial populations in this study is due to
the accumulation and mutagenic activity of anthra-
cene, similar to the effect produced by polycyclic
aromatic hydrocarbons on
Salmonella
strain YG1041
(Kummrow
et al
. 2006).
Evaluation of the effect of the mixtures showed
clearly that there was no synergy between the com-
pounds, and that the toxicity of the mixture decreases
as the concentration of LAS is increased, contrary to
the initial hypothesis. Martínez-Tabche
et al
. (1997)
report a similar phenomenon with a mixture of crude
petroleum and sodium dodecyl sulfonate: when the
concentration of the latter was varied, toxicity of
the petroleum on
Moina macrocopa
acetylcholines-
terase activity was reduced approximately 100-fold
(antagonistic effect).
Sundaram
et al
. (1994) note that adding a ten-
soactive agent to a polyaromatic hydrocarbon can
delay biodegradation as the micelles of the former
“protect” the latter by delaying or preventing its
breakdown.
Laha and Luthy (1992) report that mineraliza-
tion of phenanthrene is completely inhibited by the
addition of 0.2 % of various surfactants. Similarly,
Fu and Alexander (1995) state that mineralization of
phenanthrene by soil microorganisms was inhibited
after the addition of the anionic surfactant Neodol
25-35. Considering these reports, the increase in IC
50
of the LAS-anthracene mixture when LAS concentra-
tion was increased was attributed to a reduction of
anthracene bioavailability promoted by its adsorption
of LAS. Because LAS concentration was below the
critical micelle concentration, the LAS molecules did
not form micelles. However, it is suggested that the
LAS molecules interact with anthracene molecules,
producing an interaction so strong that it reduces the
bioavailability of anthracene, thus avoiding direct
contact with the microorganisms and causing its
toxicity to diminish. Johnsen and Karlson (2004)
demonstrated that
Novosphingobium subartica
LH128 and
Mycobaterium
spp. VM572 only express
their biological response to phenanthrene, ²uorene,
fluoranthene and pyrene when they are directly
attached to crystals of these polycyclic aromatic
hydrocarbons. A similar process has been described
by Stelmack
et al
. (1999): when
Mycobacterium
and
Pseudomonas
strains are in direct contact with a
nonionic surfactant-hydrocarbon mixture, they do not
establish contact with the mixture and avoid its toxic
effect. Jiang
et al
. (2005) demonstrated that 200 mg/L
of LAS inhibited mineralization of phenanthrene
(by 7 to 12 %) and its toxic effect on phenanthrene-
degrading microorganisms in a water-lava-plant-air
model ecosystem.
Since the degradation of the LAS-anthracene
mixture is at a maximum at 24 hours (
Fig. 3
), it
allowed the establishment of the optimum time at
which samples should be taken during this study.
Sampling was therefore performed 24 hours after
inoculation. According to Ringelberg
et al
. (2001),
the microbial populations present in sediment pol-
luted with polycyclic aromatic hydrocarbons (PAH)
are capable of degrading anthracene and other
three-ring PAH in a bioslurry treatment system.
The three-ring PAH were biodegraded from 115
³ 5.7 mg/kg to 56 ³ 3.8 mg/kg in four months by
the microbial populations present in the system.
The microbial population isolated in this study
had a greater capacity for breaking down the LAS-
anthracene mixture, with degradation within 48
hours; however, the concentrations evaluated were
low (0.16 mg/L of both compounds). In contrast,
microorganisms isolated from soil or fresh water
were able to degrade the single compounds up to
92 % for anthracene (Moody
et al
. 2001) and up to
90 % for LAS (Nishihara
et al
. 1997, Ying 2007);
nevertheless, these results refer exclusively to the
separate compounds.
TOXICITY OF THE LAS-ANTHRACENE MIXTURE ON A MICROBIAL CONSORTIUM OF SEDIMENT
45
Mineralization of the LAS-anthracene mixture
was achieved by the microorganisms present in the
polluted sediment. The capacity of LAS and anthra-
cene to mineralize microbial populations is well
known (Marchesi
et al
. 1994, Mutnuri
et al
. 2005,
Perales
et al
. 2007). Still, it is important to consider
that there can be non-cultivable microbial popula-
tions in the polluted sediment that make an important
contribution to mineralization of the mixture.
CONCLUSIONS
The results showed that growth of the microbial
population isolated from sediment was sensitive to
LAS, anthracene and a mixture of the two. The toxic-
ity of the LAS-anthracene mixture, expressed as IC
50
,
diminished as the LAS concentration was increased,
indicating that direct contact between microorganisms
and the chemical is important to the toxic effect. The
isolated microbial population also has the capacity to
degrade the LAS-anthracene mixture if the concen-
trations of both compounds are low. When analyzing
pollutant interactions in sediments, it is important to
consider the chemical structure and concentration of
the pollutants involved, as the mixture formed may be
toxic or capable of being eliminated by the microbial
populations present in the aquatic ecosystems.
ACKNOWLEDGEMENT
To Dr. Andrew Steele, Academic Haematology,
Royal Free and UCMS, UK, for a valuable critical
reading of this manuscript.
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