Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 2 ppsx

The site of action is the particular protein or other biological molecule that interacts with the toxicant.. Third, the interaction of the xenobiotic with a site of action at the molecul

CHAPTER 2

A Framework for Environmental Toxicology

Environmental toxicology can be simplified to the understanding of only three functions These functions are presented in Figure 2.1 First, there is the interaction

of the introduced chemical, xenobiotic, with the environment This interaction con-trols the amount of toxicant or the dose available to the biota Second, the xenobiotic interacts with its site of action The site of action is the particular protein or other biological molecule that interacts with the toxicant Third, the interaction of the xenobiotic with a site of action at the molecular level produces effects at higher levels of biological organization If environmental toxicologists could write appro-priate functions that would describe the transfer of an effect from its interaction with

a specific receptor molecule to the effects seen at the community level, it would be possible to predict accurately the effects of pollutants in the environment We are far from a suitable understanding of these functions The remainder of the chapter introduces the critical factors for each of these functions Unfortunately, we do not clearly understand how the impacts seen at the population and community levels are propagated from molecular interactions

THE CLASSICAL VIEWPOINT FOR CLASSIFYING

TOXICOLOGICAL EFFECTS

Techniques have been derived to evaluate effects at each step from the introduc-tion of a xenobiotic to the biosphere to the final series of effects These techniques are not uniform for each class of toxicant, and mixtures are even more difficult to evaluate Given this background, however, it is possible to outline the levels of biological interaction with a xenobiotic:

Chemical Physical-Chemical Characteristics Bioaccumulation/Biotransformation/Biodegradation Site of Action

Biochemical Monitoring Physiological and Behavioral Population Parameters Community Parameters Ecosystem Effects Each level of organization can be observed and examined at various degrees of resolution The factors falling under each level are illustrated in Figure 2.2 Examples

of these factors at each level of biological organization are given below

Chemical Physical-Chemical Characteristics

The interaction of the atoms and electrons within a specific molecule determines the impact of the compound at the molecular level The contribution of the physical-chemical characteristics of a compound to the observed toxicity is called quantitative structure-activity relationships (QSAR) QSAR has the potential of enabling envi-ronmental toxicologists to predict the envienvi-ronmental consequences of toxicants using only structure as a guide The response of a chemical to ultraviolet radiation and its reactivity with the abiotic constituents of the environment determines a fate of a compound

It must be remembered that in most cases the interaction at a molecular level with a xenobiotic is happenstance Often this interaction is a byproduct of the usual physiological function of the particular biological site with some other low molecular weight compound that occurs in the normal metabolism of the organism Xenobiotics often mimic these naturally occurring organisms, causing degradation and detoxifi-cation in some cases and toxicity in others

Figure 2.1 The three functions of environmental toxicology Only three basic functions need

to be described after the introduction of a xenobiotic into the environment The first describes the fate and distribution of the material in the biosphere and the organism after the initial release to the environment (f(f)) The second function describes the interaction of the material with the site of action (f(s)) The last function describes the impact of this molecular interaction upon the function of an ecosystem (f(e)).

A great deal can occur to a xenobiotic from its introduction to the environment

to its interaction at the site of action Many materials are altered in specific ways depending upon the particular chemical characteristics of the environment Bioac-cumulation, the increase in concentration of a chemical in tissue compared to the environment, often occurs with materials that are more soluble in lipid and organics (lipophilic) than in water (hydrophilic) Compounds are often transformed into other materials by the various metabolic systems that reduce or alter the toxicity of materials introduced to the body This process is biotransformation Biodegradation

is the process that breaks down a xenobiotic into a simpler form Ultimately, the biodegradation of organics results in the release of CO2 and H2O to the environment

Receptor and the Mode of Action

The site at which the xenobiotic interacts with the organism at the molecular level is particularly important This receptor molecule or site of action may be the nucleic acids, specific proteins within nerve synapses or present within the cellular membrane, or it can be very nonspecific Narcosis may affect the organism, not by interaction with a particular key molecule, but by changing the characteristics of the cell membrane The particular kind of interaction determines whether the effect is broad or more specific within the organism and phylogenetically

Figure 2.2 Parameters and indications of the interaction of a xenobiotic with the ecosystem.

The examples listed are only a selection of the parameters that need to be understood for the explanation of the effects of a xenobiotic upon an ecosystem However, biological systems appear to be organized within a hierarchy and that

is how environmental toxicology must frame its outlook upon environmental problems.

Biochemical and Molecular Effects

There are broad ranges of effects at this level We will use as an example, at the most basic and fundamental of changes, alterations to DNA

DNA adducts and strand breakages are indicators of genotoxic materials, com-pounds that affect or alter the transmission of genetic material One advantage to these methods is that the active site can be examined for a variety of organisms The methodologies are proven and can be used virtually regardless of species However, damage to the DNA only provides a broad classification as to the type of toxicant The study of the normal variation and damage to DNA in unpolluted environments has just begun

Cytogenetic examination of meiotic and mitotic cells can reveal damage to genetic components of the organism Chromosomal breakage, micronuclei, and various trisomys can be detected microscopically Few organisms, however, have the requisite chromosomal maps to accurately score more subtle types of damage Properly developed, cytogenetic examinations may prove to be powerful and sensi-tive indicators of environmental contamination for certain classes of material

A more complicated and ultimately complex system, directly affected by damage

to certain regions of DNA and to cellular proteins, is the inhibition of the immuno-logical system of an organism — immunoimmuno-logical suppression Immunoimmuno-logical sup-pression by xenobiotics could have subtle but important impacts on natural popula-tions Invertebrates and other organisms have a variety of immunological responses that can be examined in the laboratory setting from field collections The immuno-logical responses of bivalves in some ways are similar to vertebrate systems and can be suppressed or activated by various toxicants Mammals and birds have well documented immunological responses although the impacts of pollutants are not well understood Considering the importance to the organism, immunological responses could be very valuable in assessing the health of an ecosystem at the population level

Physiological and Behavioral Effects

Physiological and behavioral indicators of impact within a population are the classical means by which the health of populations is assessed The major drawback has been the extrapolation of these factors based upon the health of an individual organism, attributing the damage to a particular pollutant and extrapolating this to the population level

Lesions and necrosis in tissues have been the cornerstone of much environmental pathology Gills are sensitive tissues and often reflect the presence of irritant mate-rials In addition, damage to the gills has an obvious and direct impact upon the health of the organism Related to the detection of lesions are those that are tumor-agenic Tumors in fish, especially flatfish, have been extensively studied as indicators

of oncogenic materials in marine sediments Oncogenesis also has been extensively studied in Medaka and trout as means of determining the pathways responsible for

tumor development Development of tumors in fish more commonly found in natural communities should follow similar mechanisms As with many indicators of toxicant impact, relating the effect of tumor development to the health and reproduction of

a wild population has not been as closely examined as the endpoint

Reproductive success is certainly another measure of the health of an organism and is the principal indicator of the Darwinian fitness of an organism In a laboratory situation it certainly is possible to measure fecundity and the success of offspring

in their maturation In nature these parameters may be very difficult to measure accurately Many factors other than pollution can lead to poor reproductive success Secondary effects, such as the impact of habitat loss on zooplankton populations essential for fry feeding will be seen in the depression or elimination of the young age classes

Mortality is certainly easy to assay on the individual organism Macroinverte-brates, such as bivalves and cnideria, can be examined and since they are relatively sessile, the mortality can be attributed to a factor in the immediate environment Fish, being mobile, can die due to exposure kilometers away or because of multiple intoxications during their migrations By the time the fish are dying, the other levels

of the ecosystem are in a sad state

The use of the cough response and ventilatory rate of fish has been a promising system for the determination and prevention of environmental contamination Pio-neered at Virginia Polytechnic Institute and State University, the measurement of the ventilatory rate of fish using electrodes to pick up the muscular contraction of the operculum has been brought to a very high stage of refinement It is now possible

to monitor continually the water quality as perceived by the test organisms with a desktop computer analysis system at a relatively low cost

Population Parameters

A variety of endpoints have been used, including number and structure of a population, to indicate stress Population numbers or density have been widely used for plant, animal, and microbial populations in spite of the problems in mark recapture and other sampling strategies Since younger life stages are considered to

be more sensitive to a variety of pollutants, shifts in age structure to an older population may indicate stress In addition, cycles in age structure and population size occur due to the inherent properties of the age structure of the population and predator–prey interactions Crashes in populations, such as those of the stripped bass

in the Chesapeake Bay, do occur and certainly are observed A crash often does not lend itself to an easy cause–effect relationship, making mitigation strategies difficult

to create

The determination of alterations in genetic structure, i.e., the frequency of certain marker alleles, has become increasingly popular The technology of gel electrophore-sis has made this a seemingly easy procedure Population geneticists have long used this method to observe alterations in gene frequencies in populations of bacteria, protozoans, plants, various vertebrates, and the famous Drosophilla The largest

drawback in this method is ascribing differential sensitivities to the genotypes in question Usually a marker is used that demonstrates heterogeneity within a partic-ular species Toxicity tests can be performed to provide relative sensitivities How-ever, the genes that have been looked at to date are not genes controlling xenobiotic metabolism These genes have some other physiological function and act as a marker for the remainder of the genes within a particular linkage group Although with some problems, this method does promise to provide both populational and biochemical data that may prove useful in certain circumstances

Alterations in the competitive abilities of organisms can indicate pollution Obviously, bacteria that can use a xenobiotic as a carbon or other nutrient source

or that can detoxify a material have a competitive advantage, with all other factors being equal Xenobiotics may also enhance species diversity if a particularly com-petitive species is more sensitive to a particular toxicant These effects may lead to

an increase in plant or algal diversity after the application of a toxicant

Community Effects

The structure of biological communities has always been a commonly used indicator of stress in a biological community Early studies on cultural eutrophication emphasized the impacts of pollution as they altered the species composition and energy flow of aquatic ecosystems Various biological indices have been developed

to judge the health of ecosystems by measuring aspects of the invertebrate, fish, or plant populations Perhaps the largest drawback is the effort necessary to determine the structure of ecosystems and to understand pollution-induced effects from normal successional changes There is also the temptation to reduce the data to a single index or other parameter that eliminates the dynamics and stochastic properties of the community

One of the most widely used indexes of community structure has been species diversity Many measures for diversity are used, from such elementary forms as species number to measures based on information theory A decrease in species diversity is usually taken as an indication of stress or impact upon a particular ecosystem Diversity indexes, however, hide the dynamic nature of the system and the effects of island biogeography and seasonal state As demonstrated in microcosm experiments, diversity is often insensitive to toxicant impacts

Related to diversity is the notion of static and dynamic stability in ecosystems Traditional dogma stated that diverse ecosystems were more stable and therefore healthier than less rich ecosystems May’s work in the early 1970s did much to question these almost unquestionable assumptions about properties of ecosystems

We certainly do not doubt the importance of biological diversity, but diversity itself may indicate the longevity and size of the habitat rather than the inherent properties

of the ecosystem Rarely are basic principals, such as island biogeography, incor-porated into comparisons of species diversity when assessments of community health are made Diversity should be examined closely as to its worth in determining xenobiotic impacts upon biological communities

Currently it is difficult to pick a parameter that describes the health of a biological community and have that form a basis of prediction A single variable or magic number may not even be possible In addition, what are often termed biological communities are based upon human constructs The members of the marine benthic invertebrate community interact with many other types of organisms, microorgan-isms, vertebrates, and protists that in many ways determine the diversity and per-sistence of an organism Communities also can be defined as functional groups, such

as the intertidal community or alpine forest community, that may more accurately describe functional groupings of organisms

Ecosystem Effects

Alterations in the species composition and metabolism of an ecosystem are the most dramatic impacts that can be observed Acid precipitation has been documented

to cause dramatic alterations in both aquatic and terrestrial ecosystems Introduction

of nutrients certainly increases the rate of eutrophication

Effects can occur that alter the landscape pattern of the ecosystem Changes in global temperatures have had dramatic effects upon species distributions Combina-tions of nutrient inputs, utilization, and toxicants have dramatically altered the Chesapeake Bay system

AN ALTERNATIVE FRAMEWORK INCORPORATING

COMPLEXITY THEORY

The framework presented above is a classical approach to presenting the impacts

of chemicals upon various aspects of biological and ecological systems It is possible that an alternative exists that more accurately portrays the fundamental properties

of each aspect of these systems

Such a framework is in the initial stages of development and has been recently published in outline form (Landis et al 1995, 1996) The basic format of this framework is straightforward There are two distinctly different types of structures that concern risk assessment (Figure 2.3)

Organisms have a central core of information, subject to natural selection, that can impose homeostasis (body temperature) or diversity (immune system) upon the constituents of that system The genome of an organism is highly redundant, a complete copy existing in virtually every cell, and directed communication and coordination between different segments of the organism is a common occurrence Unless there are changes in the genetic structure of the germ line, impacts to the somatic cells and structure of the organism are erased upon the establishment of a new generation

Nonorganismal or ecological structures have fundamentally different properties There is no central and inheritable repository of information analogous to the genome that serves as the blueprint for an ecological system Furthermore, natural selection

is selfish, working upon the phenotype characteristic of a genome and its close relatives, and not upon a structure that exists beyond the confines of a genome The lack of a blueprint and the many interactions and nonlinear relationships within an ecosytem means that the history of past events is written into the structure and dynamics The many nonlinear dynamics and historical nature of ecosystems confer upon the system the property of complexity

Complex, nonlinear structures have specific properties (Çambel 1993) A few that are particularly critical to how ecosystems react to contaminants include:

1 Complex structures are neither completely deterministic or stochastic and exhibit both characteristics.

2 The causes and effects of the events the system experiences are not proportional.

3 The different parts of complex systems are linked and affect one another in a synergistic manner.

4 Complex systems undergo irreversible processes.

5 Complex systems are dynamic and not in equilibrium; they are constantly moving targets.

These properties are especially important in the design, data analysis, and interpre-tation of multispecies toxicity tests, field studies, and environmental risk assesssment and will be discussed in the appropriate sections This alternate approach rejects the smooth transition of effects and recognizes that ecosystems have fundamentally different properties and are expected to react unexpectedly to contaminants

Figure 2.3 Organismal and nonorganismal framework As the information is passed on to the

complex structure, it becomes part of the history of the ecosystem.

SPATIAL AND TEMPORAL SCALES

Not only are there scales in organization, but scales over space and time exist

It is crucial to note that all of the functions described in previous sections act at a variety of spatial and temporal scales (Suter and Barnthouse 1993) Although in many instances these scales appear disconnected, they are in fact intimately inter-twined Effects at the molecular level have ecosystem level effects Conversely, impacts on a broad scale affect the very sequence of the genetic material as evolution occurs in response to the changes in toxicant concentrations or interspecific inter-actions

The range of scales important in environmental toxicology range from the few angstroms of molecular interactions to the hundreds of thousands of square kilome-ters affected by large-scale events Figure 2.4 presents some of the organizational aspects of ecological systems with their corresponding temporal and spatial scale The diagram is only a general guide Molecular activities and degradation may exist over short periods and volumes, but their ultimate impact may be global

Figure 2.4 The overlap of spatial and temporal scales in environmental toxicology Not only

are there scales in organization, but scales over space and time exist Many molecular activities exist over short periods and volumes Populations can exist over relatively small areas, even a few square meters for microorganisms, thou-sands of square kilometers for many bird and mammal populations Although often diagrammed as discrete, each of these levels are intimately connected and phase one into another along both the space and time scales.

Perhaps the most important example of a new biochemical pathway generating

a global impact was the development of photosynthesis The atmosphere of Earth originally was reducing Photosynthesis produces oxygen as a by-product Oxygen, which is quite toxic, became a major constituent of the atmosphere This change produced a mass extinction event, yet also provided for the evolution of much more efficient metabolisms

Effects at the community and ecosystem level conversely have effects upon lower levels of organization The structure of the ecological system may allow some individuals of populations to migrate to areas where the species are below a sus-tainable level or are at extinction If the pathways to the depleted areas are not too long, the source population may rescue the population that is below a sustainable level Instead of extinction, a population may be sustainable or even increase due

to its rescue from a neighboring population If the structure of the ecological land-scape provides few opportunities for rescue, localized extinctions would be more likely

As the effects of a toxicant can range over a variety of temporal scales, so can the nature of the input of the toxicant to the system (Figure 2.5) Household or

Figure 2.5 The overlap of spatial and temporal scales in chemical contamination Just as

there are scales of ecological processes, contamination events also range in scale Pesticide applications can range from small-scale household use to large-scale agricultural applications The addition of surplus nutrients and other materials due

to agriculture or human habitation is generally large scale and long lived Acid precipitation generated by the tall stacks in the midwestern United States is a fairly recent phenomena, but the effects will likely be long term However, each of these events has molecular scale interactions.