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that are non-dispersible in air and insoluble in water, are considered as
sources of xenobiotic profile formation, since they can have chemical substances embedded in them. Various natural processes, as well as human
economic activity can significantly change the natural xenobiotic profile
of ecosystems. Chemical substances that accumulate in the environment in
unusual quantities and cause a change in the natural xenobiotic profile act
as pollutants (Table 5.1). This does not always lead to detrimental consequences for humans and wildlife. A chemical substance is only considered
an ecotoxicant, if it has accumulated in the environment in an amount sufficient to initiate a toxic process in the biocenosis at any level of biological
organization. Thus, one of the major practical objectives of ecotoxicology
is to determine quantitative parameters for the definition of pollutants as
ecotoxicants. The complexity of this objective lies in the fact that, in real
conditions, an entire xenobiotic profile of the environment acts on the biocenosis, while modifying the biological activity of individual compounds.
Table 5.1
Major chemical pollutants of air, soil and water
Air pollutants
Soil and water pollutants
Gases:
Sulphur oxides
Nitrogen oxides
Carbon oxides
Ozone
Chlorine
Hydrocarbons
Freons
Dust particles:
Asbestos
Coal dust
Silicon oxide
Metals
Metals: lead, arsenic, cadmium, mercury, etc.
Organochlorine pesticides: DDT, aldrin, dieldrin, chlordane,
etc.
Nitrates
Phosphates
Petroleum and petroleum products
Organic solvents: toluene, benzene, etc.
Low molecular weight halogenated hydrocarbons: chloroform,
carbon tetrachloride, dichloroethane, etc.
Polycyclic aromatic hydrocarbons (PAHs)
Polychlorinated biphenyls
Dioxins
Dibenzofurans
Acids
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5 . 2 . E c o t o x i c o k i n e t i c s
Ecotoxicokinetics is a branch of ecotoxicology that examines the fate
of xenobiotics (ecopollutants) in the environment. This includes their
sources, distribution in abiotic and biotic components of the environment,
transformation in habitats, and elimination from the environment. The identification of sources of pollutants is an essential element of their ecotoxicological characterization. Nevertheless, the contribution of some sources of
toxicants to the xenobiotic profile of an ecosystem is negligible. Natural
sources of bioavailable xenobiotics, according to WHO (1992), include
wind-blown dust particles, sea salt aerosol, volcanic activity, forest fires,
biogenic particles, and biogenic volatile substances. However, the importance of human activity as a source of xenobiotics in the environment is
steadily rising.
Abiotic (occurring without the participation of living organisms) and
biotic (occurring with the participation of living organisms) transformations
of chemical substances into toxicants of different characteristics constitute
a major source of xenobiotics in the environment. However, numerous abiotic and biotic processes are aimed at eliminating (removing) xenobiotics
from the environment.
In this regard, exposure time to many xenobiotics in the environment is
negligible to cause any harmful effect. Substances that are resistant to decomposition processes, and, as a result, persist in the environment for a long
time, are potentially harmful ecotoxicants (Table 5.2).
Table 5.2
Half-life of some xenobiotics in the environment
Xenobiotic
Half-life
Environment
DDT
TCDD
Atrazine
Benzoperylene
Phenanthrene
Carbofuran
Phosphorylthiocholines
Mustard gas
Sarin
10 years
9 years
25 months
14 months
138 days
45 days
21 days
7 days
4 hours
Soil
Soil
Water (рН 7.0)
Soil
Soil
Water (рН 7.0)
Soil (temp. 15 оC)
Soil (temp. 15 оC)
Soil (temp. 15 оC)
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The constant release of persistent xenobiotics into the environment leads
to their accumulation, becoming ecotoxicants for the most vulnerable (sensitive) individuals of the biosystem. Even after termination of the release of
these toxicants, they continue to remain in the environment for a long time.
For instance, the pesticide – mirex – the use of which was discontinued
in the late 1970s, was still found twenty years later in high concentrations
in Lake Ontario. In the waters of the US Air Force test site in Florida, where
in 1962 to 1964, the Agent Orange was sprayed for research purposes, even
after ten years, the sludge contained 10 to 35 ng/kg TCDD, which is hugely
over the maximum permissible concentration of 0.1 pg/kg (US standards) or
10 pg/kg (Russian standards). Substances that persist in the environment for
a long time include heavy metals, such as lead, copper, zinc, nickel, cadmium,
cobalt, antimony, mercury, arsenic, and chromium; polycyclic polyhalogenated hydrocarbons, such as polychlorinated dibenzodioxins, dibenzofurans,
polychlorinated biphenyls, etc.; and organochlorine pesticides, such as DDT,
hexachlorane, aldrin, lindane, etc.; among many others. The characteristics
and rate of the various processes of transformation of these substances determine their persistence in the environment.
Abiotic transformation of xenobiotics in the environment involve various
processes, including photolysis, hydrolysis, and oxidation. A common abiotic
transformation involves the formation of nitroso compounds in the soil. In this
regard, pesticides, such as dialkyl thiocarbamates, thiocarbamoyl disulfides,
salts of phenoxyacetic acid, etc., combine with nitrites in an acidic condition to
form nitroso compounds, which are known carcinogens. Photolysis is the process of degradation of chemical substances through the absorption of light, particularly, ultraviolet rays that break down chemical bonds in compounds.
For example, the photochemical transformations of 2,4,5-trichlorophenoxyacetic acid (Fig. 5.1), a known herbicide, can lead to the formation of a hazardous
ecopollutant – 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Photolysis takes
place mainly in the atmosphere and on the surface of soil and water. The rate
of photolysis depends on the intensity of light and the ability of the substance
to absorb it. Unsaturated aromatic compounds, such as polycyclic aromatic hydrocarbons (PAHs), are most sensitive to photolysis, due to their ability to actively absorb light energy. Moreover, light accelerates other processes of degradation, such as hydrolysis and oxidation of substances. Likewise, the presence of photo-oxidants, such as ozone, nitrogen oxides, formaldehyde, acrolein,
and organic peroxides, significantly accelerates the process of photolysis of
other xenobiotics. For instance, the photo-oxidation of parathion (Fig. 5.2) results in the formation of paraoxon, the toxicity to mammals of which is several
tens of times higher than that of parathion itself. Hydrolysis of xenobiotics is
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their reaction with water to form two or more products. This involves ionization
of water molecules and the splitting of the xenobiotic into other compounds.
For example, the ester bonds in molecules of organophosphorus compounds are
highly sensitive to the action of water, which explains the moderate resistance
of these compounds in the environment. The rate of hydrolysis is highly dependent on pH, such that, extreme pH levels accelerate the ionization of water
molecules, and thus, increasing the rate of hydrolytic reactions.
Fig. 5.1. Photolytic transformation of 2,4,5-trichlorophenoxyacetic acid (1)
and the abiotic synthesis of tetrachlorodibenzodioxin (TCDD)
Fig. 5.2. Abiotic transformation of parathion
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Biotic degradation of chemical substances usually takes place at
a higher rate than abiotic decomposition. With the participation of biota of
an ecosystem, particularly, bacteria and fungi, the process of degradation of
xenobiotics in the environment is much faster, due to the enzymatic activity
of these microorganisms. The biotransformation of chemical substances in
the environment involves processes of oxidation, hydrolysis, dehalogenation,
cleavage of cyclic molecular structures, removal of alkyl radicals (dealkylation), etc. The breakdown of a compound can result in its complete decomposition, i.e. mineralization with the formation of water, carbon dioxide, and
other simple compounds. However, some intermediate products of biotransformation can have a higher toxicity than the original xenobiotic. For instance, the conversion of inorganic mercury compounds by phytoplankton
can lead to the formation of more toxic organometallic compound – methylmercury. A practical example of this is the historical event of the Minamata
Bay in Japan in the 1950s and 60s. Biotic transformation of mercury, that
entered the water of the bay with the effluents of an industrial plant for
the production of nitrogen compounds, resulted in the formation of methylmercury. The latter was concentrated in the tissues of marine organisms and
fish, which served as food for the local population. As a result, people who
consumed fish from the bay developed a complex neurological disorder, and
newborns had birth defects. Over the years, tens of thousands of the people
of Minamata have been reported to have neurological symptoms consistent
with methylmercury poisoning.
Non-degradative elimination processes involve the removal of xenobiotics from the environment without any transformation process. This alters
the distribution of the xenobiotics in the ecosystem. A pollutant with a high
vapor pressure can easily evaporate from water and soil, and move with
the current of wind to a different environment. This phenomenon underlies
the ubiquity of relatively volatile organochlorine insecticides, such as lindane
and hexachlorobenzene. Redistribution of xenobiotics in the environment
can occur through the movement by wind and atmospheric currents of particles of soil on which the xenobiotics are adsorbed. Benzopyrene, for instance, and its related compounds of both natural (mainly volcanic) and anthropogenic sources (emissions from metallurgical, oil refining, thermal
power plants, etc.) are found in the biospheric cycle of substances, passing
from one environment to another by wind currents. These are usually in association with solid particles of atmospheric dust. In this regard, fine dust of
particles of 1 to 10 microns in size persist in the air for a long time, while
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larger dust particles quickly settle on soil and into water. The ash from volcanoes is one source of such particles, the distance of dispersion of which
increases with increasing height of eruption.
Another non-degradative process involves the sorption of xenobiotics
on suspended particles in water, followed by precipitation, leading to their
elimination from the water, while accumulating with sedimentation at
the bottom of the water.
Redistribution of water-soluble substances is facilitated by rainfall and
groundwater movement. Atrazine – a herbicide, which is easily soluble in
water, migrates to and accumulates in groundwater. It is quite persistent in
the environment, having a half-life of 25 months. Thus, atrazine is found to
be common in surface waters of several countries, where the herbicide is used
for the control of broadleaf weeds in agriculture and parks.
Bioaccumulation is the process by which living organisms accumulate
toxicants by extracting them from the abiotic phase (water, soil, air) and food
(trophic transmission). If an environmental pollutant cannot enter the body,
it usually does not pose a significant risk to the organism. However, once in
the internal environment, many xenobiotics are able to accumulate in the tissues. This has detrimental consequences for the organism that is accumulating the xenobiotic, reaching a damaging concentration in critical tissues, as
well as for other organisms that feed on the tissues with accumulated xenobiotics. Aquatic environments provide the best conditions for bioaccumulation of compounds. It is home to myriads of aquatic organisms that have
the ability to extract toxicants, that are capable of cumulation. Aquatic organisms accumulate substances in concentrations, sometimes thousands of
times higher than those contained in their habitat (Table 5.3).
The propensity of an ecotoxicant to bioaccumulate depends on
a number of factors, including its persistence in the environment. The degree of accumulation of a substance in the body is determined by its concentration in the environment. Substances that are easily eliminated from
the environment poorly accumulate in biological systems; except in
the case of chronic release of the substance into the environment, particularly, in industrial quantities. Although hydrocyanic acid is a known toxic
compound, it is not considered a potentially dangerous ecotoxicant, due to
its high volatility and thus, easy elimination from the atmosphere. However, it has not been possible to completely exclude that certain types of
diseases and pregnancy disorders in women living near gold mining
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enterprises, where cyanides are used in huge quantities, are not associated
with the chronic effect of the substance.
Table 5.3
Bioaccumulation of some xenobiotics in fish
Xenobiotic
Bioaccumulation factor
DDT
TCDD
Endrin
Pentachlorobenzene
Leptophos
Trichlorobenzene
127000
39000
6800
5000
750
183
After the entry of substances into the body, their fate is determined
by toxicokinetic processes. Fat-soluble (lipophilic) substances, that slowly
metabolize in the body, have the highest ability to bioaccumulate. Indeed,
the adipose tissue is the main site of long-term deposition of xenobiotics.
Thus, many years after exposure, a high amount of TCDD was found in
biopsy specimens of adipose tissue and blood plasma of US Army veterans, who participated in the Vietnam War. Many lipophilic substances,
however, are prone to sorption on the surfaces of various particles, that
precipitate from water and air, reducing their bioavailability. The sorption
of benzopyrene with humic acids, for instance, reduces the toxicant’s ability to bioaccumulate in fish tissues by three times. In fact, fish from water
bodies with a low amount of suspended particles in the water accumulate
greater amounts of DDT than fish from eutrophic water bodies with a high
amount of suspended materials. Substances metabolizing in the body accumulate in smaller quantities than would be expected based on their physicochemical properties (Table 5.4). The differences in bioaccumulation
factors of xenobiotics are largely determined by the characteristics of metabolism in different species. Bioaccumulation may underlie not only
chronic, but also delayed acute toxic effects. For instance, the rapid loss of
fat, in which a large amount of the substance is accumulated, leads to
the release of the toxicant into the blood with the consequence of delayed
toxic effect. Practically, mobilization of adipose tissue in animals is often
observed during their breeding season. In ecologically disadvantaged regions, this may be accompanied by mass death of animals when they reach
puberty.
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Table 5.4
Real and calculated bioaccumulation factors of some toxicants
in fish tissues
Toxicant
Intensity of
biotransformati
on
Bioaccumulation factor
Calculated
Real
Chlordane
Polychlorinated biphenyls
Mirex
Pentachlorophenol
2,3-Dibromopropyl phosphate
Low
Low
Low
High
High
47900
36300
21900
4900
4570
38000
42600
18200
780
3
Biomagnification is the phenomenon by which the concentration of
xenobiotics increases along food chains. Chemical substances can move
along food chains from “victim organisms” to “consumer organisms”.
For highly lipophilic substances, this movement may be accompanied by an
increase in the concentration of the toxicant in the tissues of each subsequent
organism – a link in the food chain. In a case of DDT used against mosquitoes
on a lake in California, the pesticide content in the water was 0.02 parts per
million (ppm). After some time, DDT was determined in plankton at a concentration of 10 ppm, in the tissues of planktivorous fish – 900 ppm, predatory fish – 2700 ppm, birds feeding on fish – 21000 ppm. Thus, the amount
of DDT in the tissues of birds not directly exposed to the pesticide was
1,000,000 times higher than in water and 20 times higher than in the body of
fish – the first link in the food chain.
5 . 3 . E c o t o x i c o d y n a m i c s
Ecotoxicodynamics is a section of ecotoxicology that entails the study
of specific mechanisms of toxic processes, caused by ecotoxicants on the biocenosis and/or the individual species in it. The mechanisms by which chemical substances can cause adverse effects in biogeocenoses are numerous and,
probably, in each case, unique. The impact of these chemical substances can
be classified as direct, indirect and mixed effects of ecotoxicants. The direct
effect of an ecotoxicant or a set of ecotoxicants of a given xenobiotic profile
is the immediate damage caused by the toxicant(s) to organisms of a particular population or several populations of a biocenosis. An example of
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substances with direct mechanism of action on humans is cadmium. This
metal accumulates in the body even at minimum concentrations in the environment, and at a critical concentration, it initiates a toxic process, that is
characterized by immunosuppression, carcinogenesis, as well as damage to
the respiratory system and kidneys. The indirect effect of the xenobiotic profile of an environment on biotic and abiotic components of a habitat is the impact on the conditions and resources of the environment, leading to their inadequacy for the optimal existence of the ecosystem. Many toxicants can
have both direct and indirect effects; referred to as a mixed effect. For example, the Agent Orange used by the US Army in Vietnam was a mixture of
2,4,5-trichlorophenoacetic acid and 2,4-dichlorophenoacetic acid with traces
of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Its widespread use in Vietnam
caused significant damage to the flora and fauna of the country and directly
to human health.
Ecotoxicity is the ability of a given xenobiotic profile of an environment to cause adverse effects in a given biocenosis. However, ecotoxicity
may refer to a single xenobiotic, if the disruption of the natural xenobiotic
profile of an ecosystem is caused by excessive accumulation of that particular
xenobiotic in the environment. Ecotoxic effects of xenobiotics can be categorized according to the levels of organization of biological systems as follows:
– at the organismic level (autotoxic effect) – observed as a decrease in
resistance to other active environmental factors, decrease in activity, illnesses, carcinogenesis, reproductive disorders, death of organism, etc.;
– at the population level (demographic ecotoxic effect) – observed as
death of a population, increase in morbidity and mortality, decrease in birth
rate, increase in number of birth defects, disruption of demographic characteristics (age and sex ratio, etc.), reduction in life expectancy, cultural degradation;
– at the level of biogeocenosis (ecosystemic toxic effect) – manifests
as change in the population spectrum of a biocenosis up to the extinction of
certain species and the emergence of new ones that are not peculiar to the particular biocenosis, disruption of inter-species relationships.
Ecosystem-level toxicity can also be categorized into acute and chronic
ecotoxicity, based on the duration of toxic effects of ecotoxicants on an ecosystem.
Acute ecotoxicity entails the toxic effect of ecotoxicants on a biocenosis as a result of accidents and disasters, accompanied by the release into
the environment of a large amount of a relatively unstable toxicant or
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improper use of the toxicant. The use of highly toxic chemicals for military
purposes has been one of the leading causes of environmental disasters. During the First World War, for instance, belligerent countries used about
120 thousand tons of toxic substances on the battlefields, resulting into more
than 1.3 million cases of poisoning, a situation that can be considered as one
of the biggest environmental disasters in human history. Acute ecotoxic effects may result in immediate death or illness in humans and other species,
or in the development of neoplasms, leading to delayed effects.
Chronic ecotoxicity of chemical substances is usually associated with
sublethal effects. Often, this implies reproductive dysfunction, breakdown of
immunity, endocrine pathology, malformations, allergies, etc. Chronic exposure to a toxicant can also lead to deaths in some species of living organisms.
The effect of ecotoxicants can be very diverse, as numerous examples of their
mechanisms of action are given to enable the assessment of the complexity
of their impact on ecosystems.
The use of effective pesticides, such as insecticides and herbicides is an
application of the strategy of direct effect of toxicants, leading to mass death
of sensitive species. Nevertheless, concomitant negative phenomena arise in
some cases of this scientific application. For example, methylmercuric dicyanamide was widely used in Sweden in the 1950s and 60s as a fungicide for
the treatment of seeds of grain crops. This resulted in the concentration of
10 mg/kg of mercury in the grain seeds. After a few years, a massive death
of pheasants, pigeons, partridges and other grain-eating birds from chronic
mercury intoxication occurred as a result of periodic pecking of treated seeds
by the birds. However, environmental assessments involve the application of
the basic principle that, sensitivity to a chemical substance varies for different species of living organisms.
Therefore, the presence of a toxicant in the environment, even in small
quantities, can be detrimental to the most sensitive species but not to others
within the same ecosystem. For instance, 0.01 mg/L of lead chloride in water
leads to death of daphnia within a period of one day, but is of little harm to
other aquatic species.
The accumulation of polychlorinated biphenyls (PCBs) in seals, resulting into a decrease in their resistance to infection is a direct effect of a xeno-
biotic, leading to the development of a toxic process. At the end of the 1980s,
about 18,000 seals died as a result of viral infections in the Baltic, North and
Irish Seas. A high concentration of PCBs was found in the tissues of the dead
seals. Like other chlorine-containing compounds, such as DDT, hexachlorobenzene and dieldrin, PCBs have an immunosuppressive effect on mammals.
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