Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 10

CHAPTER 10 Measurement and Interpretation of the Ecological Effects of Toxicants INTRODUCTION This chapter deals with perhaps the most difficult topic in environmental toxi-cology, how to measure and then evaluate the impact of toxicants at ecological levels of organization. The chapter starts with an evaluation of methods and ends with a discussion of the responses of ecosystems to chemical stressors. MEASUREMENT OF ECOLOGICAL EFFECTS AT VARIOUS LEVELS OF BIOLOGICAL ORGANIZATION Biomonitoring is a term that implies a biological system is used in some way for the evaluation of the current status of an ecosystem. Validation as to the predic-tions and protections derived from the elaborate series of tests and our understanding presented in previous chapters can only be done by effective monitoring of ecosys-tems (Landis 1991). In general, biomonitoring programs fall into two categories, exposure and effects. Many of the traditional monitoring programs involve the analytical measurement of a target compound with the tissue of a sampled organism. The examination of pesticide residues in fish tissues or PCBs in terrestrial mammals and birds are examples of this application of biomonitoring. Effects monitoring looks at various levels of biological organization to evaluate the status of the biological community. Generically, effects monitoring allows a toxicologist to perform an evaluation without an analytical determination of any particular chemical concen-tration. Synergistic and antagonistic interactions within complex mixtures are inte-grated into the biomonitoring response. In the biomonitoring process, there is the problem of balancing specificity with the reliability of seeing an impact (Figure 10.1). Specificity is important since it is crucial to know and understand the causal relationships in order to set management or cleanup strategies. However, an increase in specificity generally results in a focus Figure 10.1 The tug of war in biomonitoring. An organismal or community structure monitoring system may pick up a variety of effects but lack the ability to determine the precise cause. On the other hand, a specific test, such as looking at the inhibition of a particular enzyme system, may be very specific but completely miss other modes of action. on one particular class of causal agent and effects, and in many cases chemicals are added to ecosystems as mixtures. Emphasis upon a particular causal agent may mean that effects due to other materials can be missed. A tug of war exists between specificity and reliability. There is a continuum of monitoring points along the path that an effect on an ecosystem takes from introduction of a xenobiotic to the biosphere to the final series of effects (Chapter 2). Techniques are available for monitoring at each level, although they are not uniform for each class of toxicant. It is possible to outline the current organizational levels of biomonitoring: · Bioaccumulation/biotransformation/biodegradation · Biochemical monitoring · Physiological and behavioral · Population parameters · Community parameters · Ecosystem effects A graphical representation of the methods used to examine each of these levels are depicted in Figure 10.2. Many of these levels of effects can be examined using organisms native to the particular environment, or exotics planted or introduced by the researcher. There is an interesting trade-off for which species to use. The naturally occurring organism represents the population and the ecological community that is under surveillance. There is no control over the genetic background of the observed population and little is usually known about the native species from a toxicological viewpoint. Introduced organisms, either placed by the research or enticed by the creation of habitat, have the advantage of a database and some control over the source. Questions dealing with the realism of the situation and the alteration of the habitat to support the introduced species can be raised. Figure 10.2 Methods and measurements used in biomonitoring for ecological effects. A num-ber of methods are used both in a laboratory situation and in the field to attempt to classify the effects of xenobiotics upon ecological systems. Toxicity tests can be used to examine effects at several levels of biological organization and can be performed with species introduced as monitors for a particular environment. It may also prove useful to consider a measure of biomonitoring efficacy as a means to judge biomonitoring. Such a relationship may be expressed in the terms of a safety factor as E = Ui (10.1) i Where E is the efficacy of the biomonitoring methodology, Ui is the concentration at which undesirable effects upon the population or ecosystem in system i occur and Bi is the concentration at which the biomonitoring methods can predict the undesir-able effect or effects in system i. The usefulness of such an idea is that it measures the ability to predict a more general effect. Methods that can predict effects rather than observe detrimental impacts are under development. Several of the methods discussed below are developments that may have a high efficacy factor. BIOACCUMULATION/BIOTRANSFORMATION/BIODEGRADATION Much can occur to the introduced pesticide or other xenobiotic from its intro-duction to the environment to its interaction at the site of action. Bioaccumulation often occurs with lipophillic materials. Tissues or the entire organism can be analyzed for the presence of compounds such as PCBs and halogenated organic pesticides. Often the biotransformation and degradation products can be detected. For example, DDE is often an indication of past exposure to DDT. With the advent of DNA probes it may even be possible to use the presence of certain degradative plasmids and specific gene sequences as indications of past and current exposure to toxic xeno-biotics. Biosensors are a new analytical tool that also may hold promise as new analytical tools. In this new class of sensors a biological entity such as the receptor molecule or an antibody for a particular xenobiotic is bound to an appropriate electronic sensor. A signal can then be produced as the material bound to the chip interacts with the toxicant. One of the great advantages to the analytical determination of the presence of a compound in the tissue of an organism is the ability to estimate exposure of the material. Although exposure cannot necessarily be tied to effects at the population and community levels, it can assist in confirming that the changes seen at these levels are due to anthropogenic impacts and are not natural alterations. The difficul-ties in these methods lay in the fact that it is impossible to measure all compounds. Therefore, it is necessary to limit the scope of the investigation to suspect compounds or to those required by regulation. Compounds in mixtures can be at low levels, even those not detected by analytical means, yet in combination can produce eco-logical impacts. It should always be noted that analytical chemistry does not measure toxicity. Although there is a correspondence, materials easily detected analytically may not be bioavailable, and conversely, compounds difficult to measure may have dramatic effects. MOLECULAR AND PHYSIOLOGICAL INDICATORS OF CHEMICAL STRESS BIOMARKERS A great deal of research has been done recently on the development of a variety of molecular and physiological tests to be used as indicators and perhaps eventually predictors of the effects of toxicants. McCarthy and Shugart (1990) have published a book reviewing in detail a number of biomarkers and their use in terrestrial and aquatic environments. The collective term, biomarkers, has been given to these measurements, although they are a diversified set of measurements ranging from DNA damage to physiological and even behavioral indices. To date, biomarkers have not proven to be predictive of effects at the population, community, or ecosystem levels of organization. How-ever, these measurements have demonstrated some usefulness as measures of expo-sure and can provide clinical evidence of causative agent. The predictive power of biomarkers is currently a topic of research interest. Biomarkers have been demonstrated to act as indicators of exposure (Fairbrother et al. 1989). Often specific enzyme systems are inhibited by only a few classes of materials. Conversely, induction of certain detoxification mechanisms, such as spe-cific mixed function oxidases, can be used as indications of the exposure of the organism to specific agents, even if the agent is currently below detectable levels. Additionally, the presence of certain enzymes in the blood plasma, that is generally contained in a specific organ system, can be a useful indication of lesions or other damage to that specific organ. These uses justify biomarkers as a monitoring tool even if the predictive power of these techniques has not been demonstrated. The following discussion is a brief summary of the biomarkers currently under investi-gation. Enzymatic and Biochemical Processes The inhibition of specific enzymes such as acetylcholinesterase has proven to be a popular biomarker and with justification. The observation is at the most basic level of toxicant-active site interaction. Measurement of acetylcholinesterase activity has been investigated for a number of vertebrates, from fish to birds to man. It is also possible to examine cholinesterase inhibition without the destruction of the organism. Blood plasma acetyl and butyl cholinesterase can be readily measured. The drawbacks to using blood samples are the intrinsic variability of the cholinest-erase activity in the blood due to hormonal cycles and other causes. Brain cholinest-erase is a more direct measure, but requires sacrifice of the animal. Agents exist that can enhance the recovery of acetylcholinesterase from inhibition by typical organ-ophosphates, providing a measure of protection due to an organophosphate agent. Not only are enzyme activities inhibited, but they also can be induced by a toxicant agent. Quantitative measures exist for a broad variety of these enzymes. Mixed function oxidases are perhaps the best studied with approximately 100 now identified from a variety of organisms. Activity can be measured or the synthesis of new mixed function oxidases may be identified using antibody techniques. DNA repair enzymes can also be measured and their induction is an indication of DNA damage and associated genotoxic effects. Not all proteins induced by a toxicant are detoxification enzymes. Stress proteins are a group of molecules that have gathered a great deal of attention in the past several years as indicators of toxicant stress. Stress proteins are involved in the protection of other enzymes and structure from the effects of a variety of stressors (Bradley 1990). A specialized group, the heat shock proteins (hsps) are a varied set of proteins with four basic ranges of molecular weights 90, 70, 58 to 60 and 20 to 30 kDa. A related protein, ubiquitin, has an extremely small molecular weight, 7kDa. Although termed heat shock proteins, stressors other than heat are known to induce their formation. The exact mechanism is not known. Other groups of stress-related proteins also are known. The glucose regulated proteins are 100 to 75 kDa molecular weight and form another group of proteins that respond to a variety of stressors. The stress-related proteins discussed above are induced by a variety of stressors. However, other groups of proteins are induced by specific materials. Metallothioneins ... - tailieumienphi.vn