Ecotoxicology Essentials. Environmental Contaminants and their Biological Effects on Animals and Plants

478 Pages·2016·24.43 MB·English

by  Donald W. Sparling

Ecotoxicology Essentials
Ecotoxicology
Essentials
Environmental Contaminants and Their
Biological Effects on Animals and Plants
Donald W. Sparling
Cooperative Wildlife Research Laboratory and Department of Zoology,
Southern Illinois University, Carbondale, IL, USA
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Chapter 1
An Introduction to Ecotoxicology
Terms to Know
Ecotoxicology
Anthropogenic
Sublethal Effects
Lethality
Compensatory Effects
Additive Effects
Octanol/water Coefficient (K )
ow
Soil/water Coefficient (K )
oc
Persistence
Photolysis
Bioavailability
Bioassimilation
Bioconcentration
Biomagnification
Hyperaccumulate
Dissolved Organic Carbon (Matter)
Environmental Matrix
Environmentally Relevant Concentration
Median Lethal Concentration (Dose)
Median Effect Concentration (Dose)
No Adverse Effects Level
Lowest Observed Adverse Effects Level
INTRODUCTION
A good starting place in this introductory chapter is to define the basic sub-
ject of this book, ecotoxicology. Although toxicology, the science of poisons
to humans, has been studied for hundreds of years, ecotoxicology is relatively
new. It is generally accepted that Rachel Carson’s landmark book Silent Spring
(1962) awoke the country to the potential dangers of pesticides, mostly DDT
and its relatives, to the environment and that this book served as the impetus for
starting the science. The term ecotoxicology was first coined by Rene Truhaut
in 1969 to denote a natural extension of ecology and toxicology that included
Ecotoxicology Essentials. DOI:http://dx.doi.org/10.1016/B978-0-12-801947-4.00001-9
© 22001163 Elsevier Inc. All rights reserved. 3
4 SECTION | I Basic Principles and Tools of Ecotoxicology
the effects of chemical pollutants on any aspect of the environment (Truhaut,
1977). The discipline has received many definitions since then including from
Cairns (1989): “toxicity testing on one or more components of the ecosystem”
and Hoffman et al. (2003) as “the science of predicting effects of potentially
toxic agents on natural ecosystems and on nontarget species.” Virtually all other
definitions include chemicals or contaminants, effects, and ecosystem or ecol-
ogy. The overriding objective of ecotoxicology is to understand how chemicals
(usually of human origin or anthropogenic) behave in the natural environment
and how they affect organisms in that environment. Specific investigations will
have objectives that refine or limit that overriding one, but essentially all eco-
toxicological studies fall under that one umbrella.
As you might surmise after a few moments of thought, to accomplish that
one main objective requires many different disciplines. Practitioners of the sci-
ence have at least some expertise in chemistry, physiology, ecology, statistics,
risk assessment, and similar areas. Very few investigators can truly be experts in
all of these areas so in today’s era of specialization, scientists tend to focus on
one of these areas or subdisciplines.
We can identify some of these specialties knowing that any definition will
meet with exceptions and that there is some degree of overlap among subdis-
ciplines. Environmental chemists study fate and transport of contaminants in
ecosystems. This includes how the contaminants get into the ecosystem in the
first place, how they move from one compartment (ie, air, water, sediment, soil,
organisms) to another, and how they degrade or change form in these com-
partments. The study of chemicals in the environment has become increasingly
sophisticated with continual improvements in instrumentation and procedures
to detect chemicals at the parts per billion level or even lower.
Scientists who are interested in physiological effects often use controlled
studies in the laboratory or in controllable outdoor experiments. Some questions
they ask include: Do the contaminants cause cancer, disrupt endocrine systems,
retard growth or development, or affect behavior? At what point do these sublethal
effects (ie, physiological changes short of death of the organism) become lethal
and the organism dies? Statistically supported cause and effect relationships are
the goal in these investigations. Historically, these controlled experiments have
emphasized the effects of a given chemical in isolation from all other chemicals.
However, this does not reflect most real-life exposures. In actuality, organisms are
seldom exposed to only one contaminant at a time; mostly they encounter a bat-
tery of chemicals that have a variety of interactions. As a result, many studies are
now looking at the interaction between two or more chemicals.
Ecotoxicologists are mostly interested in how contaminants affect popula-
tions, communities, and ecosystems. Contaminants are often thought of as addi-
tional stressors along with predation, disease, competition, and other factors.
Changes at these higher levels of organization may occur through physiological
effects of organisms, but they also involve numerous interactions among chemi-
cals and the other stressors that organisms face under seminatural conditions
An Introduction to Ecotoxicology Chapter | 1 5
(see Chapter 2). While statistical support of hypotheses is also critical in these
tests, the complexity of chemicals interacting with other stressors may make it
difficult to ascertain that a specific contaminant is having an effect on a popula-
tion, leading to decreased population growth or health. This difficulty is com-
pounded in field studies so the most informative studies are usually those that
involve both field investigations and controlled experiments.
When studying chemical (or any other) stressors in general, there are three
possible outcomes: (1) the stressor has no apparent effect on the population;
(2) the stressor does seem to cause death or debilitation of individuals in a popu-
lation but may not affect population dynamics—the concept of compensatory
effects in population ecology (Burnham and Anderson, 1984); or (3) the stressor
is affecting individuals and has a measurable effect on population size, growth,
or health—the concept of additive effects. Under the concept of compensatory
effects, organisms that die due to the contaminant would likely die from some
other factor, so the overall net loss to the population is not increased. According
to the additive effects concept, the mortality or harm to individuals from chemi-
cal stressors is in addition to the mortality experienced from other stressors. At
times the additive effect from contaminants may exceed the effects produced
by all other stressors and contaminants become the major limiting factor on the
population. It is one thing to show that organisms in the field are dying from
exposure to contaminants, but it is often very difficult to distinguish between
compensatory and additive effects. One of the most convincing examples of
additive effects were population declines of fish-eating birds due to eggshell
thinning related to exposure to dichlorodiphenyldichloroethylene (DDE), a
derivative of dichlorodiphenyltrichloroethylene (DDT) that was used to con-
trol insect pests from the1940s to the early 1970s (see the FOCUS section in
Chapter 4 for more details).
Risk assessment assimilates information on chemical fate and transport,
dose and effect relationships, and ecological factors to develop models that pre-
dict the probability that a chemical will have deleterious effects on a population
or ecological entity of concern. Frequently this involves developing models that
encompass chemical and other factors that may occur in a contaminant scenario
and extrapolating possible outcomes by changing the values of those factors.
Often risk assessment is directed toward human health issues, but the same prin-
ciples of modeling pertain to animal and plant populations.
We will cover all of these topics and several others in this book. The primary
objective of this book is to provide students with a solid background in the disci-
pline of ecotoxicology. This includes an understanding of specific contaminants
and how these contaminants affect individuals, populations, and communities.
In addition, we hope to foster an appreciation for the nature of experimental
studies in ecotoxicology. The objective of this chapter is to provide an introduc-
tion to the book by discussing several basic concepts associated with factors in
the environment that influence fate and transport of contaminants, and describ-
ing key concepts of the responses of organisms to environmental contaminants.
6 SECTION | I Basic Principles and Tools of Ecotoxicology
CHARACTERISTICS OF CHEMICALS THAT AFFECT THEIR
PRESENCE IN THE NATURAL ENVIRONMENT
Chemicals have intrinsic characteristics that affect how they behave under natu-
ral conditions. Questions that can be asked in this regard include:
● Are they repulsed by water or do they mix well with it?
● Do they tend to adhere to soil, sediments, or suspended particles or are they
found free in the environment?
● Are they normally found as solids, liquids, or gases?
● How persistent are they in the environment?
● How likely are they to be found in the atmosphere or in any other compart-
ment of the environment?
● Do they have very similar “cousins” or isomers?
● How does their structure influence their toxicity?
Solubility
Water has been called the “universal solvent” because many organic and inor-
ganic molecules can dissolve in water, at least to some extent. However, water
solubility from one chemical to another can vary tremendously. Solubility
becomes especially important in the environment when a contaminant is pre-
sent in water. Some metals have comparatively high solubility, especially under
acidic conditions. Polychlorinated biphenyls (PCBs) with many chlorine atoms,
high molecular weight polycyclic aromatic hydrocarbons (PAHs), and some
organochlorine pesticides (OPs) are notoriously insoluble in water although
they may be soluble in oils or lipids. In contrast, PCBs with two or three chlo-
rines, PAHs with relatively low molecular weight, and some OPs are reasonably
soluble in the mg/kg or parts per million (ppm) range.
Chemists use a measure called the octanol/water coefficient (K ) to rank
ow
chemicals based on their relative ability to dissolve in water or an organic sol-
vent. The coefficient is determined by quantifying the maximum concentration
of a chemical that dissolves in octanol, an organic solvent divided by the maxi-
mum concentration of that same chemical in water at standard conditions of
temperature and pressure. Although octanol is polar due to a hydroxyl mol-
ecule, it does not mix (immiscible) with water. Water is strongly polar. Nonpolar
and weakly polar substances preferentially dissolve in octanol. The higher the
K , therefore, the more nonpolar the compound. Log K values are inversely
ow ow
related to aqueous solubility and directly proportional to molecular weight (US
EPA, 2009). For example, the PAH naphthalene has a log K of 3.3 and a water
ow
solubility of 31.8 mg/L. In comparison, another PAH, benzo[a]pyrene has a K
ow
of 6.0 and a solubility of 0.0000000055 mg/L.
Organic molecules that have a relatively high water solubility or a log K
ow
less than 3.5 are usually called hydrophilic (“water-loving”), whereas those with
An Introduction to Ecotoxicology Chapter | 1 7
log K greater than 3.5 are called lipophilic (“lipid or fat-loving”). Hydrophilic
ow
chemicals are also lipophobic (“lipid-fearing”) and lipophilic chemicals are
similarly hydrophobic. Hydrophobic chemicals, if in a body of water, are typi-
cally on the surface of the water such as PAH films which appear as an oily
sheen or sheen, are attached to particulates. Polar molecules have a difficult
time entering plant or animal cells because the outer nonpolar portion of the
cell or plasma membrane provides a barrier to these molecules unless there are
special receptors on the outer surface of the membrane (water passes through
freely because of its small molecular size). Nonpolar molecules more readily
pass through the membrane and into the interior of cells where they can cause
many problems, as we shall see in the following chapters of this book. Passage
through the plasma membrane for ions such as dissolved metals can be facili-
tated if they are attached to organic molecules such as methylmercury which is
formed by a bond between the metal mercury with a nonpolar methyl (–CH −)
3
group. Organic contaminants with moderately high K values have a potential
ow
to bioconcentrate and biomagnify because of their attraction to lipids.
Soil/Water Partition Coefficients
Contaminants in soil or water can be found attached to particulates, such as
soil, or suspended matter in the water column. In general, contaminants that are
attached to particulate matter are less available to organisms (ie, bioavailable)
and less reactive to other environmental factors such as ultraviolet radiation than
the same chemical free in water. Each molecule has a characteristic propensity
to adsorb to soil, but this propensity may change with environmental conditions
such as pH. The ratio between a contaminant in water and that attached to par-
ticulates under standard conditions of pH, temperature, and pressure is referred
to as its soil/water partition coefficient (K ). This coefficient is calculated by
oc
measuring the amount of contaminant adhering to organic carbon particles and
dividing that by the amount in the water phase. The K is primarily used for soil
oc
matrices, but the values can also be used to assess the likelihood that a contami-
nant will be free in the water column of a lake or stream rather than attached to
particles in the water. Chemicals with high K values are more likely to adhere
oc
to particles and less likely to be bioavailable than chemicals with lower values.
Most often, the log of K is used.
oc
Vapor Pressure
A general definition of vapor pressure is that it is the pressure exerted by a vapor
in thermodynamic equilibrium with its condensed phases (solid or liquid) at a
given temperature in a closed system. It is usually measured in mm Hg. Vapor
pressure is relevant to ecotoxicology because it is a reflection of the tendency
of a chemical to be present in the atmosphere. Contaminants with low vapor
pressure are not very likely to be present in air. Vapor pressure increases as a
8 SECTION | I Basic Principles and Tools of Ecotoxicology
substance is heated or if it is in a liquid rather than solid state. Vapor pressures
for metals in the solid state are near zero and their melting points are too high to
be environmentally relevant so, if they are in the atmosphere at all, it is mostly
because they are attached to windblown particles. For, PAHs, dioxins, furans,
and polybrominated diphenyl ethers (PBDEs), vapor pressures tend to decline
as molecular weight or halogenation increases. Vapor pressure for PAHs, for
example, ranges from 2.9 × 10−6 to 11.1 mm Hg and for PCBs, PBDEs, and
dioxins the range is 9.6 × 10−11 to 0.05 mm Hg. OPs have vapor pressures
between 10−5 to 10−7 mm Hg, as do organophosphates.
Melting Point
The melting point or the temperature in which a solid form of a contaminant
becomes liquid is important for a couple of reasons. If a contaminant is solid at
normal environmental temperatures (say 0°C to 40°C), it will be less mobile, less
likely to vaporize, less likely to be found in the atmosphere, and less likely to be
biologically available than if it is a liquid at those temperatures. Temperatures are
often higher than environmentally realistic for organic molecules or metals to be
gases, so that phase is not very important for most contaminants. It is axiomatic
that melting points among contaminants span a huge range. Most metals have
melting points exceeding several hundred degrees Celsius or more than 1000°F.
However, there are the exceptions of mercury, which becomes a liquid at −38°C
(−37°F), and arsenic that does not have a melting point, per se, but sublimates
or turns into a gas at 615°C (1137°F) and never becomes a liquid. Most of the
organic contaminants have low melting points at lower degrees of halogenation
or molecular weight which then increases with the degree of halogenation or
molecular weight. For example, the lightweight PAH naphthalene has a molecu-
lar weight of 128.2 g/mol and a melting point of 80.3°C (176.5°F) and the heavier
benzo[a]pyrene at 252.3 g/mol has a melting point of 495°C (923°F).
Structure
The molecular structure of a contaminant has many ramifications for its behav-
ior. In Chapter 4 on OPs, we will discuss stereoisomers, which are molecules
with the same molecular weight and same atomic composition but different
structures. These stereoisomers may have very different behavior in the envi-
ronment and different toxicities. For example, endrin and dieldrin are stereoiso-
mers of each other (Fig. 1.1), but endrin is approximately 10 times more toxic
to laboratory mammals than dieldrin.
The most toxic PCBs, dioxins, and furans are called coplanar or planar,
as opposed to nonplanar PCBs. PCBs are composed of two phenyl or benzene
rings and if the two rings lie in the same plane, they are coplanar (Fig. 1.1) and
can attach to a particular receptor at the cell level. Binding with this receptor
facilitates their entrance into the cell where they can alter protein synthesis.
An Introduction to Ecotoxicology Chapter | 1 9
FIGURE 1.1 Differences in structure can be very important in how a contaminant behaves.
(A) Dieldrin and endrin are organochlorines that are stereoisomers of each other and have different
toxicities. (B) Hg is much more toxic when attached to an organic molecule such as ethane; (C)
Planar PCBs are more carcinogenic than nonplanar PCBs, and (D) the Bay and Fjord regions of
PAHs can increase toxicity. From webbook.nist.gov.
Coplanar PCBs are dioxin like and pose a serious cancer threat to humans and
wildlife. The rings of nonplanar PCBs cannot lie in the same plane, cannot bind
with the cellular receptor, and are much less toxic than coplanar PCBs (see
Chapter 6).
Some PAHs pose serious cancer risks because one end of their molecule has
an open bay or fjord region (Fig. 1.1) that allows them to attach to a receptor on
the cell membrane and enter the cell readily. PAHs without this bay region are
less serious of a risk (see Chapter 7).
As we have already discussed in this chapter and will spend more time on
in Chapter 8, metal ions may be blocked from entering a cell by the plasma
membrane. When attached to a methyl- or ethyl- group, however, they can more
readily enter cells. Methylation can also increase the toxicity of PAHs. Strictly
speaking, the attachment of a methyl- or ethyl- group to either a metal or any
other molecule creates a different molecule so it is not just a structural change
but it has a similar connotation.
Persistence
Persistence is the ability of a contaminant to stay in the environment unchanged.
Metals, being elements, of course, are permanent although they may be buried
deep in sediments or soil and be biologically unavailable. Persistence is typi-
cally measured in half-lives. A half-life is the constant amount of time (given
constant environmental conditions) it takes half of a quantity of a contaminant
to degrade into something else. It may sound a bit surprising, but it takes the
same amount of time for 100 kg of a PCB to degrade to 50 kg as it does for 10 kg