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Reviews in Fisheries Science
, 17(4):478–484, 2009
Copyright C
Taylor and Francis Group, LLC
ISSN: 1064-1262 print
DOI: 10.1080/10641260903067852
The Micronucleus Assay in Fish
Species as an Important Tool for
Xenobiotic Exposure Risk
Assessment—A Brief Review and an
Example Using Neotropical Fish
Exposed To Methylmercury
CARLOS ALBERTO MACHADO DA ROCHA,1,2RAQUEL ALVES DOS
SANTOS,3MARCELO DE OLIVEIRA BAHIA,2LORENA ARA ´
UJO DA CUNHA,1
HELEM FERREIRA RIBEIRO,2and ROMMEL MARIO RODR´
IGUEZ
BURBANO2
1Coordenac¸˜
ao de Recursos Pesqueiros e Agroneg´
ocio, Insituto Federal de Educac¸˜
ao, Ciˆ
encia e Tecnologia do Par´
a, Brazil
2Laborat´
orio de Citogen´
eticaHumanaeGen
´
etica Toxicol´
ogica, Universidade Federal do Par´
a, Brazil
3Laborat´
orio de Mutagˆ
enese, Faculdade de Medicina de Ribeir˜
ao Preto, Universidade de S˜
ao Paulo, Brazil
Micronucleus (MN) assay has been extensively used in the evaluation of DNA damage. Mutagenesis and genotoxicity
studies employed this methodology to evaluate possible genotoxic risk due to exposition to hazardous xenobiotics in different
organisms, including aquatic sentinel organisms. MN assay in such species is sensitive, fast, and an important biomarker
of mutagenic exposure in the environment. The use of bioassays, considering the toxic effects of isolated or combined
contaminants, is also important since the environmental variants are minimized. The aim of this study is to gather and
evaluate published data on the use of fish MN assay in biomonitoring and genotoxicity assays. In addition, we show the
results of both micronuclei and other nuclear abnormalities in erythrocytes from Colossoma macropomum, exposed to
methylmercury. Specimens (n=9) were subjected to 2 mg/L of methylmercury, with an equal control group. Chi-square test
was performed to compare the frequencies of nuclear abnormalities between control and treatment groups. The contingence
table of χ2test showed a significant increase of altered cells in the exposed group. Our results support the importance of MN
test as an effective indicator for genotoxicity in fishes, which can be used with exposition bioindicators of human populations
exposed to chemical pollutants of consuming water.
Keywords micronucleus assay, fishes, biomonitoring, genotoxicity
INTRODUCTION
Micronuclei (MN) were first described in the cytoplasm
of erythrocytes more than a century ago and were called
“fragment of nuclear material” by Howell or “intraglobu-
laries corpuscules” in the terminology of Jolly in the late
Address correspondence to Rommel Mario Rodr´
ıguez Burbano, Campus
Universit´
ario do Guam´
a, Av. Augusto Correa, 01, Universidade de S˜
ao Paulo,
Bel´
em 66075-110, Brazil. E-mail: rommel@ufpa.br
18th century and early 1900. These structures are known by the
hematologists as “Howell-Jolly bodies” (Kirsch-Volders et al.,
2003).
Micronuclei (MN) are citoplasmatic chromatin masses that
look like small nuclei as a result of lesions at the chromosomes
or DNA strains, or at the level of proteins directly involved in
chromosome segregation; formation of MN originating from
chromosome fragments or chromosome loss events requires a
mitotic or meiotic division (Heddle et al., 1983; Al-Sabti and
Metcalfe, 1995). Scoring MN in interphase nuclei is technically
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MICRONUCLEUS ASSAY IN FISH: BRIEF REVIEW AND EXAMPLE USING NEOTROPICAL FISH 479
easier and faster than scoring of chromosomal aberration in
metaphase nuclei (Al-Sabti and Metcalfe, 1995).
Salvadori and colleagues (2003) highlighted that, whatever
the type of DNA damage, MN are originated during cell divi-
sion. Then, DNA damage due to exposure to mutagenic agents is
expressed in micronulceus form just after one cycle of cell divi-
sion and is dependent on the proportion of cells under division.
The increased interest in environmental genotoxicity studies
went ahead with the development of a great number of tests
to evaluate genotoxic effects in aquatic environments. Consid-
ering this, micronucleus assay (MN), one of the most popular
and promising tests on ecotoxicology, represents a cytogenetic
indicator of DNA damage for over 30 years (Fenech et al., 2003).
The advantage in using fish models includes the facility by
which teleostei, especially the small species, can be maintained
and handled inside the laboratory under experimental conditions
of toxic exposure. Besides, fishes frequently respond to chemi-
cal exposure as superior vertebrates, which validate this model
to study potential teratogenic and carcinogenic compounds in
humans (Al-Sabti and Metcalfe, 1995).
The genotoxic effect caused by some chemicals on fish
genome has been the object of many studies, especially those
aiming to assess the genetic responses to environmental stimuli
(B¨
ucker et al., 2006). Fishes can, in fact, be sentinel organ-
isms that indicate the risk of human exposure to drinking water
contaminated with genotoxic chemicals. Furthermore, fish and
crustaceous species are among the most important vectors of
human contamination once feeding is one of the major routes
of exposure to toxicants in human populations (Al-Sabti and
Metcalfe, 1995).
TECHNICAL ASPECTS AND ANALYSIS
A large number of studies using fish peripheral erythrocytes
showed increase in MN frequency after laboratory exposure to
different pollutants. The fish MN assay, as it has been actually
called, is quick and simple: a drop of blood is smeared on a
clean slide and dried at room temperature; slides are then fixed
in absolute ethanol for 20 min and stained with Giemsa 10%
(phosphate buffer pH 6.8); finally, the slides are analyzed with
conventional microscopy to evaluate and quantify the presence
of micronuclei (Beninc´
a, 2006).
Two slides per sample are recommended for preparation, in
which 4,000 erythrocytes are analyzed under 100×objective
and scored for presence of both typical micronuclei and nu-
clear alterations manifested as changes in the normal elliptical
shape of the main nuclei (Ayllon and Garcia-Vazquez, 2000).
Therefore, the micronuclei test also accounts for nuclear abnor-
malities, as the occurrence of lobulated, segmented, and kidney
aspect cells.
Although the significant differences between controls and
exposed groups is usually reported using the Chi-square test
(Ferraro et al., 2004; Lopez-Poleza, 2004; Cavas et al., 2005),
other statistical analysis using non-parametric tests such as
Mann-Whitney test (Lopes-Poleza, 2004; Grisolia et al., 2005;
B¨
ucker et al., 2006; Vanzella, 2006; Andreik¨
enait¨
e et al., 2007),
or Kruskal-Wallis test (Beninc´
a, 2006; Matsumoto et al., 2006;
Vanzella, 2006) are valid and recommended according to the
data distribution.
Micronuclei are circa 1/20 to 1/10 smaller than the main
nucleus (Ribeiro, 2003). However, in fish erythrocyte samples,
they seem to vary from 1/30 to 1/10 since the chromosome
size is shorter than in mammals (Al-Sabti and Metcalfe, 1995;
Ayllon and Garcia-Vazquez, 2000).
ECOLOGICAL BIOMONITORING
The most accepted definition of biomonitoring is the use of
systematic responses of a living organism to evaluate environ-
mental changes, mostly trigged by human action. The biomon-
itoring programs are generally used to detect and control a
problem and can be considered a promising tool for identifi-
cation of pollutants that may affect human and environmen-
tal health; mostly with organisms (bioindicators) exposed to
these pollutants, using biological system assays (biomarkers)
(Da Silva et al., 2003). The use of biomarkers to measure bi-
ological responses in the affected organisms is very useful to
simplify and lower costs of biological monitoring, especially in
aquatic environments (Ramsdorf et al., 2008). These biomarkers
consist of the formation of DNA adducts, chromosomal aberra-
tions, DNA break and micronuclei frequency, and other nuclear
abnormalities (Bombail et al., 2001).
The impact of toxic agents on the DNA integrity and function
has been extensively investigated under environment conditions
(Bombail et al., 2001). Considering endemic species, monitor-
ing the presence of xenobiotic agents in aquatic environments
can improve environmental quality and human health. Fishes
are one of the most indicated organisms for the monitoring of
aquatic environments (Van Der Oost et al., 2003). Localization
and determination of pollutant concentration ensures the sur-
vival and health of these organisms in a way they can be used by
human populations to supply their nutritional and leisure needs
(Nicareta, 2004).
Micronucleus assay, originally developed in mammalian
species, has been extensively used to evaluate the genotoxic
risk of a large number of chemical agents (Heddle et al., 1983).
Differing from other organisms belonging to the trophic chain,
fishes are sensitive to relatively low concentrations of environ-
mental pollutants (with possible mutagenic effects) and, there-
fore, are considered excellent bioindicators of environmental
biomonitoring (Minissi et al., 1996).
In the last 10 years, the number of biomonitoring in situ stud-
ies using the MN assay in fish species has increased significantly.
Some examples are presented in Table 1.
In Brazil, biomonitoring studies using aquatic species have
also increased in the last years. Grisolia and Starling (2001)
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480 C. A. MACHADO DA ROCHA ET AL.
Tab l e 1 In vivo biomonitoring studies using the micronucleus assay in fish species
Organism Cell type Localization Contaminant Reference
Pholis gunnellus Erythrocytes Firth of Forth (Scotland) Metals, hydrocarbons,
and organochlorates
Bombail et al., 2001
Tilapia rendalli,Oreochromis niloticus
and Cyprinus carpio
Erythrocytes Lago Parano´
ainBras
´
ılia
(Brazil)
Domestic sewage Grisolia and Starling, 2001
Anguilla Anguilla, Phoxinus phoxinus,
and Salmo trutta
Erythrocytes North Rivers of Spain Metals, hydrocarbons,
pesticides
Rodriguez-Cea et al., 2003
Mugil cephalus Erythrocytes and gill cells Mediterranean (Turkey) Aromatic hydrocarbons Cavas and Ergene-Gozukara,
2005
Geophagus brasiliensis Erythrocytes Santa Catarina (Brazil) Domestic sewage Beninc´
a, 2006
Trematomus newnesi Erythrocytes Brazilian Antarctic Research
Station “Comandante
Ferraz”
Diesel oil and sewage Campos, 2007
Astyanax sp. B and A. Altiparanae Erythrocytes, hepatocytes,
and kidney cells
Paran ´
a (Brazil) Agrotoxics Ramsdorf, 2007
Clarias gariepinus, Oreochromis
niloticus,Oreochromis aureus, and
Tilapia zilli
Erythrocytes River Nile, Abou Homos,
Kafr Eldawar, and Lake
Mariout (Egypt)
Heavy metals Ali et al., 2008
Astyanax jacuhiensis Erythrocytes Rio Grande do Sul (Brazil) Petrochemicals Lemos et al., 2008
evaluated the ability of wastewater from two municipal sewage
treatment plants that debouch into Lake Parano´
a to cause ge-
netic damage using the MN test. They did not find significant
differences between control and hypertrophic areas; however,
cyclophosphamide and mitomycin C, used to test the sensitivity
of the biological assay, significantly increased the MN frequency
in Tilapia rendalli,Oreochromis niloticus, and Cyprinus carpio.
In this study, T. rendalli was the most sensitive specie to both
clastogens and C. carpio the most resistant. Lemos et al. (2008)
performed a genotoxic evaluation in the Bom Jardim brook, a
small stream that flows through an area under the influence of
a petrochemical complex in Rio Grande do Sul, Brazil, using
the MN assay in Astyanax jacuhiensis. The study found in-
creased micronuclei frequencies and nuclear abnormalities in
the exposed group when compared to the control; however, no
differences were observed between samples collected in differ-
ent sites from the brook (two ponds upstream from the indus-
trial area). They showed that sites exposed to petrochemical
influence were under higher genotoxic impact and that A.
jacuhiensis was a sensible bioindicator.
GENOTOXICITY ASSAYS
The use of bioassays to evaluate the toxic effects of associ-
ated or isolated contaminants reduces significantly the influence
of different environmental variables. Even though such results
cannot be directly extrapolated to the environment, they are im-
portant to maintain of databases that can help to understand
the interfering factors in the organism health and/or altering the
environmental balance (Ramsdorf, 2007).
Genotoxicity assays using fish species can be performed in
vitro and/or in vivo.Inthein vivo assays, the tested agents
are injected into the fishes or added to water or food (Cotelle
and Ferard, 1999). Table 2 shows some examples of detection
of genotoxic agents in an aquatic environment using the MN
assay.
In fish species, the micronucleus assay is usually performed
in erythrocytes since these cells contain a nucleus (Al-Sabti and
Metcalfe, 1995). Erythrocyte samples are easily acquired and
no cellular dissociation is required (Belpaeme et al., 1998).
The majority of genotoxicity studies with fish species are
usually performed using micronucleus assay and/or the comet
assay, which denotes the importance of discriminating what is
genotoxic and what is mutagenic and also helps to evaluate low-
dose effects (B¨
ucker et al., 2006). Ferraro et al. (2004) evaluated
the mutagenic potential of tributyltin (TBT) and inorganic lead
(PbII) in Hoplias malabaricus using micronucleus assay, comet
assay, and analysis of chromosomal aberrations. They observed
that lead was highly mutagenic in all endpoints analyzed. How-
ever, genotoxicity of TBT was positive in the micronucleus
assay and chromosomal aberrations but not in the comet assay,
at least in the cellular type and concentrations tested.
Lopes-Poleza (2004) evaluated the genotoxic effects of
methylmercury (CH3Hg+)inHoplias malabaricus, using chro-
mosomal aberrations (anterior kidney), micronuclei, and DNA
damage by the comet assay in erythrocytes. The results showed
that genotoxic effects of methylmercury are due to its accu-
mulation in tissues and not in the circulating blood; thus, the
erythrocytes were not a good bioindicator to demonstrate the
hazardous effects of this compound in low concentrations.
B¨
ucker et al. (2006) exposed Eingenmannia virescens to ben-
zene (50 ppm) during different periods. Although no significant
results with the micronucleus were found, the comet assay sug-
gested the genotoxicity of benzene once the authors observed
a gradual increase in the number of damaged nucleoids in a
dose-dependent response. These results suggest that the comet
assay was more sensible than the micronucleus assay.
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MICRONUCLEUS ASSAY IN FISH: BRIEF REVIEW AND EXAMPLE USING NEOTROPICAL FISH 481
Tab l e 2 Genotoxicity studies using the micronucleus assay in fish species
Organism Cell type Contaminant Reference
Oreochromis niloticus and Tilapia rendalli Erythrocytes Mitomycin C and cyclophosphamide Palhares and Grisolia, 2002
Oreochromis niloticus Erythrocytes and gill cells Textile mill effluent Cavas and Ergene-Gozukara,
2003
Hoplias malabaricus Erythrocytes TBT and lead Ferraro et al., 2004
Hoplias malabaricus Erythrocytes Methylmercury Lopes-Poleza, 2004
Cyprinus carpio, Carassius gibelio, and
Corydoras paleatus
Erythrocytes, gill cells, and
liver cells
Cadmium chloride and copper sulphate Cavas et al., 2005
Oreochromis niloticus and Tilapia rendalli Erythrocytes Domestic sewage Grisolia et al., 2005
Scophthalmus maximus Erythrocytes Dialkyl phthalate, bisphenol A and
tetrabromodiphenyl ether
Bolognesi et al., 2006
Eigenmannia virescens Erythrocytes Benzene B¨
ucker et al., 2006
Prochilodus lineatus Erythrocytes Diesel oil Vanzella, 2006
Oreochromis niloticus Erythrocytes Chromo Matsumoto et al., 2006
Oncorhynchus mykiss Erythrocytes Mixture of heavy metals Andreik¨
enait¨
e et al., 2007
Trematomus newnesi Erythrocytes Diesel oil and sewage Campos, 2007
Prochilodus lineatus Erythrocytes Aluminum Galindo, 2007
Gambusia affinis Erythrocytes Residual hydrocarbons Caliani et al., 2008
Carassius auratus auratus Erythrocytes and epithelial of
gill and fin
Mercury chloride and lead acetate Cavas, 2008
Vanzella (2006) evaluated the genotoxicity of the soluble
fraction of diesel (SFD) in Prochilodus lineatus using the
comet and micronucleus assays. The animals were submitted
to acute (6, 24, and 96 hr) and sub-acute (15 days) exposure
to SFD 50% in water. The results demonstrated clastogenic
and aneugenic effects of this tested fraction. Consequently,
the combination of these two methodologies was adequate and
advantageous.
Matsumoto et al. (2006) studied samples of water from
Catfish Brook in Franca, a city in the Brazilian state of S˜
ao
Paulo. Erythrocytes from Oreochromis niloticus were submit-
ted to comet and micronucleus assays. Samples from a tan-
ning region with chrome effluent exhibited the highest level
of DNA damage, supporting the hypothesis that this metal is
genotoxic.
Galindo (2007) used RAPD, comet assay, and micronucleus
assay to evaluate the genotoxic effect of aluminum in acid
medium (pH 5.0) using the neotropical specie Prochilodus
lineatus. The genotoxic effect was demonstrated with comet
assay and RAPD. However, the level of micronuclei frequency
was not increased and stayed near to control values. The authors
concluded that, in the concentrations tested, aluminum was not
mutagenic.
ASSESSMENT OF MUTAGENIC EFFECTS OBSERVED
IN NEOTROPICAL FISH EXPOSED TO
METHYLMERCURY
Fish Specimens
Colossoma macropomum Cuvier, 1818 (Characidae), well
known as tambaqui, is an onivorous fish that belongs to the
Amazon region in the Orinoco river and its effluents. This specie
is abundant and has high aquaculture potential because they
can be cultured and reproduced in captivity. It is the second
biggest specie with squama and the biggest Characiforme in
the Solim˜
oes/Amazonas Rivers, reaching 100 cm in native en-
vironment, weighting approximately 30 kg (Ara´
ujo-Lima and
Goulding, 1998). With its tasty meat, it is widely appreciated in
northern Brazil, with economic value in six of seven states in
this region (Val et al., 2000).
Methylmercury
Mercury, like some other metals and several organomercu-
rial compounds, has demonstrated mutagenic properties. This
toxicity occurs on tubulin, the structural subunit of cellular mi-
crotubules, playing a role in cytoplasmic organization and for-
mation of spindle fibers, interfering with the tubulin polymeriza-
tion and causing contraction of the chromosomes in metaphase,
delayed division of the centromere and reduced anaphasic move-
ment (Cassidy and Furr, 1978; Thier et al., 2003).
Methylmercury (MeHg) is a compound classified as group
2B by IARC (International Agency for Research on Cancer),
being indicated as a possible carcinogenic to humans (Hal-
lenbeck, 1993) and, due to its property of integrating them-
selves in trophic chains, accumulates to a much greater ex-
tent when compared to other forms of mercury (Azevedo,
2003).
Neurotoxicity induced by MeHg increases the emergency of
reactive radicals and accelerates free radical reactions. Oxida-
tive stress on CNS can produce damage by several interacting
mechanisms, including mitochondrial damage with increase in
intracellular free Ca2+, activation and inhibition of enzymes,
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482 C. A. MACHADO DA ROCHA ET AL.
release of excitatory amino acids, metallothioneins expression,
and microtubule disassembly (Nascimento et al., 2008).
METHODS
In the present study, we used young C. macropomum species
obtained from Pisciculture Station of Federal Rural Amazon
University in Castanhal City, Par´
a, Brazil (between 01◦1802
S and 01◦2243 S and 48◦0505 W and 48◦1546 W).
Acclimatization to laboratory conditions for one month was
done using dechlorinated tap water with the following physic-
ochemical characteristics: temperature =26 ±1.3◦C, pH =
6.5 ±0.29, dissolved oxygen =2.78 ±0.55, total hardness =
6–49 mg/L (as CaCO3), and conductivity =12–97µS/cm.
Specimens were housed at a density of three specimens in a
30-L aquaria under constant aeration and 12 hr light/dark pho-
toperiod. Preliminary experiments determined the concentra-
tion or maximum tolerated dose (MTD), at which the animals
showed no reduction in survival and food uptake. A sublethal
concentration of methylmercury (2 mg/L) was tested, and nine
specimens were used as a test group with an equal control group.
Blood samples were collected of each group after five days of
treatment.
The blood was spread onto a microscope slide accord to
Beninc´
a (2006). Slides were observed for MN score in a trans-
mission light microscope, using 1,000×magnification. Two
thousand erythrocytes were analyzed per slide, and 4,000 per
animal, including those cells with micronucleus and modifica-
tions in the nuclear shape. Only cells with well-preserved cy-
toplasm were considered. Coded and randomized slides were
scored using a blind analysis by a single scorer. Statistical
analysis considered the difference between groups that was
<0.05 in the Chi-square test (BioEstat 5.0) (Ayres et al.,
2007).
MNs were defined as round or oval intracytoplasmatic bod-
ies not linked or connected to the main nucleus, with a diameter
1/30–1/10 of the major nucleus and on the same optical plane
(Al-Sabti and Metcalfe, 1995; Ayllon and Garcia-Vazquez,
2000). Three other nuclear abnormalities were still considered:
buds, lobes, and invaginations (Ayllon and Garcia-Vazquez,
2000; Bolognesi et al., 2006).
RESULTS AND DISCUSSION
In each animals kept in the fish tank with 2 mg/L of
methylmercury, we observed 4,000 blood cells (Figure 1), with
1.02 ±0.30% mean cells with alterations (Figure 2). Control
animals presented a count of 0.85 ±0.34% mean red blood
cells with alterations. In animals subjected to methlymercury
2 mg/L, an increase in clastogenic or aneugenic accidents was
observed, yielding the formation of micronuclei and other nu-
clear abnormalities.
Figure 1 Photomicrograph (1,000×) of erythrocytes from C. macropomum
treated with methylmercury 2 mg/L, showing a micronucleated erythrocyte.
In widespread informal gold mining in the Amazon Basin,
mercury is used to capture the gold particles as amalgam. Mer-
cury releases into the environment, resulting in contamination
of freshwater fish with methylmercury (Grandjean et al., 1999).
Methylmercury is an organic neurotoxic form of mercury, the
one that is easier to bioaccumulate in organisms.
Spontaneous levels of micronuclei in fish species are rela-
tively low (Ferraro et al., 2004). Our results demonstrated low
frequencies of micronuclei in controls as in the exposed group.
When we considered morfonuclear alterations and micronuclei
altogether, exposed group and controls were significantly dif-
ferent.
MN assay in fish erythrocytes is a mutagenicity assay, being
less sensitive than comet assay, which demonstrates genomic
lesions that can be repaired (Russo et al., 2004; B¨
ucker et al.,
2006; Ramsdorf, 2007), reducing the number of stable lesions
present in DNA molecule.
Figure 2 Number of cells with micronuclei and morphological alterations in
their nuclei (per 4,000 cells). Significant difference was observed between the
control and treated animals (χ2test; p<0.05).
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MICRONUCLEUS ASSAY IN FISH: BRIEF REVIEW AND EXAMPLE USING NEOTROPICAL FISH 483
On the other hand, the observation of nuclear abnormalities.
besides the presence of MN, also can be considered a useful
indicator of genotoxic and cytotoxic effects of contaminants in
aquatic organisms (Cavas and Ergene-Gozukara, 2003; Dailia-
nis et al., 2003; Ferraro et al., 2004, Cavas et al., 2005; Barˇ
sien¨
e
et al., 2006; Matsumoto et al., 2006).
Our results corroborate the importance of MN in mutagenic
exposure assessment in fishes, which can be used as sentinel
organisms for indicating the potential for human exposures to
genotoxic chemicals in drinking water.
GENERAL CONCLUSION
When compared to other DNA damage detection techniques,
micronucleus assay has some advantages: (1) it can be per-
formed rapidly; (2) it is not complex; (3) it presents low costs;
(4) its preparation and analysis are simpler and faster than chro-
mosomal aberrations. Although the MN assay cannot give in-
formation about the type of chromosomal breakage, it is in-
formative when the exposure causes aneugenic effects. Taken
together, all the above-mentioned aspects render this method-
ology high applicability in the routine of mutagenesis studies.
Research on environmental biomonitoring requires fast results
and reproducibility. Exploration of the MN assay in fish species
is welcome in order to standardize and improve the assessment
of genotoxicity in target tissues. We assessed the mutagenic
potential of methylmercury 2 mg/L using samples of the fish
Colossoma macropomum (commonly called tambaqui) through
piscine micronucleus test. Our results confirm the mutagenic ef-
fect of methylmercury. The sensibility to this compound and the
economic relevance of C. macropomum show that such species
can be used to monitor acute effects of metallic pollutant spilled
in freshwater of Amazonic ecosystems.
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