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Challenges in RIK I. L. EGGEN RENATA BEHRA PATRICIA BURKHARDT-HOLM BEATE I. ESCHER NINA SCHWEIGERT SWISS FEDERAL INSTITUTE FOR ENVIRONMENTAL SCIENCE AND TECHNOLOGY (EAWAG) Challenges in ECOTOXICOLOGY Mechanistic understanding will help overcome the newest challenges. Mechanistic understanding will help overcome the newest challenges.

Peer Reviewed: Challenges in Ecotoxicology

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Page 1: Peer Reviewed: Challenges in Ecotoxicology

Challenges in

R I K I . L . E G G E N

R E N A T A B E H R A

P A T R I C I A B U R K H A R D T - H O L M

B E A T E I . E S C H E R

N I N A S C H W E I G E R TS W I S S F E D E R A L I N S T I T U T E F O R E N V I R O N M E N T A L S C I E N C E A N D T E C H N O L O G Y ( E A W A G )

Challenges inECOTOXICOLOGY

Mechanistic understanding

will help overcome the

newest challenges.

Mechanistic understanding

will help overcome the

newest challenges.

Page 2: Peer Reviewed: Challenges in Ecotoxicology

Modern environmental management practice has significantly re-

duced the risk of acute and high-volume pollution in ecosystems

in industrialized countries (1). Concentrations of many well-known

pollutants have decreased because new, less toxic, and less persis-

tent substances with low bioaccumulative potential are being used;

their use has been optimized; recycling

processes have been introduced; and

wastewater treatment technology has

improved. Nevertheless, ecosystems are

still threatened by low concentrations of

a steadily increasing number of pollu-

tants that cause subtle and chronic ef-

fects. Ecotoxicology, however, has not yet

adapted accordingly and is still based on

testing methods and concepts that deal

with and avoid acute environmental

chemical pollution.

Pollutants rarely occur alone; they are

usually mixtures or combined with non-

chemical stressors. The potential threat

to ecosystems by multiple stressors—

ranging from radiation to alterations in

temperature and CO2 concentrations—is

increasingly recognized (2). As a conse-

quence, we are faced with changing and

increasingly complex challenges in eco-

toxicology (see “Overview and examples

of current challenges in ecotoxicology” below). Dealing with these challenges will

require new concepts, tools, and approaches.

In this article, we will illustrate how molecular approaches could change ecotox-

icology by providing a better understanding of underlying processes. This knowl-

1

2

3

4

5

Low concentrations ofpollutants and long exposure times (chroniceffects)

Multiple effects bysingle pollutants

Complex mixtures ofpollutants

Multiple stressors

Ecosystem complexity

Endocrine disruptionDNA damage/mutagenesisDeficiencies in the immune

systemNeurological effects

Multiple target sites and multiplemodes of toxic action

Time- and tissue-dependenteffects

Wastewater treatment planteffluents

Field runoffPollutants and their degradation

productsComplexes of chemical

compounds

UV and pollutantsTemperature and pollutantsPathogens and pollutants

Variations in species sensitivitiesEffect of propagation from

organisms to populations andecosystems

Identification of thestressor–effect relationship

Examples of current challenges in ecotoxicology

Challenges in Examplesecotoxicology

© 2004 American Chemical Society FEBRUARY 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 59A

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60A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 1, 2004

edge is essential for establishing mechanism-basedassessments of the risks and effects of pollutants, de-veloping powerful bioanalytical tools, and identify-ing causative agents and their effects on wildlife(Figure 1). Challenges in ecotoxicology will be pre-sented in more detail, starting with single pollutants.Multiple effects by single compounds, chemical mix-tures, and multiple stressors are addressed as well asecosystem complexity. Finally, we will provide exam-ples of how molecular approaches can be used toconfront these challenges.

Challenge 1: Low-concentration and long-termexposureLong-term, chronic exposure to low concentrationsof pollutants may impair the fitness and viability oforganisms, populations, or ecosystems. Recent dis-cussions about the nature of the dose–response curvehave stressed the importance of mitigating low-con-centration effects (3). Research on endocrine disrup-tion, which influences developmental processes,sexual differentiation, and reproduction, has impres-sively shown that routinely tested endpoints forgrowth and survival are not sufficient to detect thesechronic effects. With the currently used ecotoxico-logical effect assessment tools, we have to wait a longtime for visible and significant reactions in ecosystemsto occur. The discrepancy, shown in Figure 2, betweenecotoxicological effect assessment in practice (most-ly relatively high concentrations and short exposuretimes) and effects under realistic scenarios (mostlylow concentration and low exposure times) is not onlyrestricted to endocrine disruption but is of a moregeneral nature. For example, other, much less stud-

ied toxic mechanisms underlying possible chronic ef-fects include influences on the neurological system,which affect developmental and differentiationprocesses or behavior; effects on the immune sys-tem, which alter susceptibilities for infectious dis-eases; and oxidative and photo-oxidative stress andincreased radical production, which cause importantbiomolecules such as chromosomes to change ordegrade.

We do not fully understand the effects of low con-centrations of pollutants at the molecular level, thelink between chronic effects on organisms and ef-fects on populations, and the ecotoxicologicalprocesses that lead to chronic effects. Furthermore,we do not know if we can extrapolate meaningful re-sults from effects caused by high doses and short ex-posure times to effects caused by low doses and longexposure times. Both exposure scenarios may resultin different effects with possibly different conse-quences (4).

We also do not yet have enough validated, effect-specific, sensitive, and fast bioanalytical tools thatwould allow us to assess the potential for chronic ef-fects within practically acceptable time frames. A pos-sible solution to this challenge would be to focus oninitial triggers of toxicity instead of trying to observesubtle secondary effects at the population and ecosys-tem levels. Initial events like the onset of defensemechanisms or deviations from cell homeostasis canbe meaningful indicators, if not of the actual effectsthen at least of the hazard potential of a givenpollutant.

Acut

e to

xic

effe

cts

Chronic toxic effects

Time

DiscrepancyDose

F I G U R E 2

Relationship between dose and exposuretimeCurrent ecotoxicological effect assessment toolsrequire waiting for long time periods until visible andsignificant reactions in ecosystems occur. High doseand short exposure times are the most commonecotoxicological tests in practice, whereas low doseand long exposure times would be the more realisticexposure scenario.

Molecular approaches in ecotoxicology

Mechanistic knowledge of underlying processes

Identification of molecular targets and key processes

Development of specific and powerful bioanalytical and risk-assessment tools

Support for the modeling of effects

Mechanism-based effect

assessment

Process-orientedrisk assessment

of chemicals

In situ effect assessmentToxicant identification

F I G U R E 1

Understanding ecotoxicology on a molecular levelMechanistic knowledge is the basis for the development ofbioanalytical and risk assessment tools, modeling of effects,mechanism-based effect assessment, identification of thecausative toxic agent, assessment of effects in wildlife (in situ),and risk assessment of chemicals.

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FEBRUARY 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 61A

Challenge 2: Multiple effects induced by singlecompoundsThe physicochemical characteristics of a pollutantdetermine the type and degree of interaction with bi-ological target molecules (5). Multiple types of inter-actions or interactions with different biological targetscause multiple biological effects.

For example, chemically reactive pollutants, suchas electrophiles, react with different nucleophilic bi-ological molecules. Depending on its electrophilici-ty, an electrophilic pollutant may prefer reactions with“soft” nucleophiles, such as thiol groups in proteinsand peptides, or with “harder” nucleophiles, such asnucleotides in DNA. A reaction with peptides andproteins interferes with the cellular-reducing capac-ity through conjugation with glutathione or will in-terfere with enzyme activities, while DNA damageleads to mutations. The two types of effects are alsorelevant on different time scales. Although direct en-zyme inhibitions may lead to acute effects, DNA dam-age is mainly relevant under chronic exposureconditions because of the increasing probability offailures in the efficiency of the DNA repair systemover time.

Another example involves hydrophobic organicpollutants. Such pollutants accumulate primarily inmembranes and will influence their permeability,the proton gradient, and/or the activity of enzymes,such as the ATP-generating ATPase, embedded inthese membranes. Heavy metals, such as copper oriron, can also disturb the structure and function ofmembranes, block active enzyme centers, or formcomplexes with organic pollutants. These complex-es may exhibit different effects than their con-stituents (6).

Cells react to pollutants and their effects by spe-cific, fine-tuned defense and repair systems aimed atprotecting a balanced metabolic network and cellu-lar functioning. These reactions may occur by the ac-tivation of already available enzymes to remove orconvert pollutants, or when stress-specific genes areactivated and stress gene-encoded protein levels rise.Molecular regulation of the biological response is de-termined by the physiological status of the cells, thephysiological status and function of the various ex-posed organs, and the developmental phase of theorganisms. Thus, depending on a pollutant’s chemi-cal characteristics and the site and time of exposure,a single pollutant can activate a different array ofgenes in various organs and life stages.

These multiple and subtle effects cannot be iden-tified if reduced growth, death, or reproduction areused as integrative toxicological endpoints. They aremissed if the exposure occurs at the wrong time win-dow or organism age. They can also be missed or mis-understood if single biomarker or gene responses aremeasured in a selected cell type or tissue, particular-ly if the biological context and relevance of these re-sponses are unknown.

To examine the principles of multiple effects bysingle compounds, we propose to use relatively sim-ple biological systems, such as subcellular systems orunicellular organisms, in combination with biomol-ecular tools such as the ones described in the pro-

ceeding sections. Using this approach, we have in-deed been able to examine multiple effects underwell-defined experimental conditions in low-com-plexity systems (7–9). On the basis of that acquiredknow-how, we could subsequently study biologicalsystems with increasing complexity.

Challenge 3: Complex mixturesOrganisms are continuously exposed to chemical mix-tures made up of various pollutants, different chem-ical forms (chemical species), metabolites, orpollutants in complexes. The general paradigmfor assessing mixture effects stems initially frompharmacological research but has found wideapplication in ecotoxicology (10). This paradigmis depicted in Figure 3. If chemicals in a mixturehave different molecular target sites, do not interact,and have different mechanisms, they act indepen-dently. If chemicals do not interact, but act at thesame target site and have similar mechanisms, theeffects are typically concentration-additive. If chem-icals in a mixture interact on their way to theirtarget site, their mixture effect cannot easi-ly be predicted. These situations requirecase-by-case investigations to identify pos-sible antagonistic or synergistic effects. Theapplicability of these concepts in ecotoxicology hasbeen convincingly demonstrated (11, 12) and illus-trates that the correct assignment of the relevantmechanisms is a prerequisite for the effect assess-ment of mixtures. The previously proposed mecha-nism-based approach is also suitable for classificationaccording to toxic mechanisms.

Concentrationaddition

Independentaction

Detailedinvestigation

Yes

Yes Yes

No

No

No

Sametargetsite?

Interaction?Same

mode ofaction?

F I G U R E 3

Addressing mixture toxicity inecotoxicologyAnswering the questions in the diamonds guidesresearchers to the conceptual model and/or researchactivities required to address or analyze therespective mixture toxicity issue.

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Challenge 4: Multiple stressor effectsOrganisms are simultaneously or sequentially exposedto multiple stressors—for example, pollutants and non-pollutant stressors such as rising temperatures, acidrain, altered CO2 concentrations, or increasing radia-tion (1). Changes caused by the combination of nutri-ents, such as phosphate or nitrate, with pollutants canalso be considered as multiple stressor effects.Organisms exposed to ultraviolet radiation encounterdirect effects, such as the formation of thymidinedimers in DNA, or they suffer oxidative and photo-oxidative stress, which leads to increased concen-trations of reactive oxygen species that damagebiomolecules, injure cells, or even cause cell death (13).Higher temperatures influence the biological metabo-lism, including enzyme-dependent defense activitiesagainst pollutant effects. Temperature also influencesinteractions between organisms and pathogens in hu-mans (14) and in aquatic organisms (15). Nonchemicalstressors can also directly affect chemical pollutants.For example, ultraviolet radiation induces degradationprocesses, which result in mixtures of chemical pollu-tants and their metabolites. Acidification caused byacid rain, for example, changes the chemical speciationand thus the bioavailable concentrations of pollutants(16).

We believe the best way to address this issue is touse a systematic approach: First, we must understandthe mechanisms for the effects of each stressor alone,and then we can increase the complexity by com-bining stressors.

Challenge 5: Ecosystem complexityAnother challenge is that, rather than single organ-isms, populations of diverse species—each having its

own metabolic capacities for handling stres-sors, different susceptibilities, and

susceptible time windows—areexposed to various combi-

nations of pollutant andnon-pollutant stressors.

Analyzing the toxic re-sponses of each spe-

cies in different timewindows to all (mixtures

of) pollutants is practically andfinancially impossible. It is, how-ever, reasonable to expect that

different organ-isms will havecommon end-

points and thatsome general prin-

ciples, studied in a selec-tion of model systems, can be

transferred to other organisms.Ecosystem complexity further encompasses

processes such as changes in genetic diversity, thecommunication between organisms, or the interac-tions of organisms in the food web. These processescan be influenced by pollutant and nonpollutantstressors and will vary between ecosystems.

To improve ecological risk assessment procedures,we need a better understanding of species sensitivi-

ties and population responses, as well as their effectson communities and ecosystems (4). Insight into suchcomplex interactions in ecosystems will require sys-tematically examining processes to understand theunderlying mechanisms of the various levels of bio-logical organization—within organisms (molecule toorganism), between organisms (differences in fitness,reproduction rates, and susceptibilities of the vari-ous species), and within ecosystems (food webinteractions).

Implementing molecular approachesUsing the example of endocrine-disrupting chemi-cals (EDCs), we will illustrate how to approach thechallenges just described. We will focus on estro-genic compounds and their effects on fish in aquat-ic ecosystems—an area in which much work hasbeen recently reported (17, 18)—to elucidate the po-tential of molecular approaches for risk assessmentand the development of bioanalytical tools. We willalso demonstrate the need for a mechanistic un-derstanding of ecotoxicological processes.

Estrogenic compounds are a structurally diversegroup consisting of natural and synthetic steroids,phytoestrogens, pesticides, and bulk industrialchemicals (19). Most of the currently known estro-genic compounds enter the aquatic ecosystems viawastewater treatment plants. After dilution in thereceiving waters, these compounds occur at nano-gram-per-liter concentrations for steroids and up tomicrogram-per-liter concentrations for industrialchemicals.

Results from in vitro and in vivo studies indicatethat the steroid estrogenic compounds are most prob-ably the primary cause for the estrogenic effects ob-served in wild fish. However, additional effects fromestrogenic industrial chemicals cannot be excluded.

Estrogenic effects described in fish include in-creased plasma vitellogenin (VTG) levels; inhibitedtesticular growth and ovotestis, which probably occuras a consequence of male fish being exposed duringa critical period of gonadogenesis (20); and reducedreproduction and fertility (21). Researchers hypoth-esize that fish populations became even smaller as aconsequence of endocrine disruption (22). Despitesignificant work on estrogenic effects in the environ-ment, major knowledge gaps do exist.

These gaps have raised numerous questions: Howcan we detect the hazard of estrogenic compoundsoccurring at low concentrations? Where in the or-ganism does endocrine disruption take place? Whendoes endocrine disruption occur—both from a sea-sonal point of view and from a developmental biol-ogy point of view? Which multiple effects occur in thevarious tissues, and what are the underlying mecha-nisms? What are the principal endocrine-active com-pounds, and what are the effects of mixtures of EDCs?Do nonchemical stressors have additional effects onendocrine disruption? What are the population-levelconsequences of effects that have been mainly ob-served in individual fish?

Initially, pollutant interactions occur at the cellu-lar level. Genes are activated in a fine-tuned and co-ordinated way to maintain cellular homeostasis.

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General stress and stress-specific responses mayoccur simultaneously. To assess the hazard potentialof estrogenic compounds, either in pure form or inan environmental sample, researchers must focus onearly biological effects. Hence, they have to analyzeprimary molecular events or immediate subsequentbiological reactions.

Detailed examination of the molecular, biochem-ical, and cellular reactions are necessary to identifythe relevant and stress-specific molecular processes,key regulatory components, and target genes. Theseearly reactions at the molecular level can be used todevelop specific bioanalytical tools and to assess thehazard potential of chemical or environmental sam-ples. Taking other classes of compounds and biolog-ical effects into the broader perspective requires abattery of mechanism-based assays that cover thevarious biological effects and can be used for eco-toxicological risk assessment and environmentalscreening purposes (23).

Already, single estrogenic compounds can havemultiple effects on cells, depending on the organ andorganism. Steroid homeostasis, for example, encom-passes steroid synthesis, transport, and clearing.These processes are influenced by the physiologicalstatus of the cells and the tissue, time of exposure, andcell- and tissue-specific signaling pathways, whichcan subsequently lead to different local expressionlevels of the estrogen target genes. Multiple effects invarious tissues are thus to be expected.

A good example is the enzyme cytochrome P450aromatase (CYP19), which converts androgens to es-trogens and hence plays a dominant role in hormonalbalance. Recently, two aromatase encoding genes(cyp19a and cyp19b) have been characterized in fishand their tissue distribution and expression profilesfollowing exposure to endocrine-active compounds(24–26). The cyp19a gene is expressed primarily ingonads and cyp19b primarily in the brain.

Both genes are regulated differently during de-velopment and are induced upon exposure to EDCs,albeit with varying kinetics and responses (27 ). Theseresults clearly illustrate the multiple effects of a sin-gle compound on only two genes coding for func-tionally identical proteins in two tissues. Thesefindings also show how important it is to understandthe relevance of gene expression and protein syn-thesis in a biological context. To use the expressionof the aromatase gene or the enzymatic aromataseactivity for bioanalytical purposes, we must knowwhen during development, at what exposure time,where in the tissue, and in which tissue to conductthe measurement.

In situ detection methods, genomics, and pro-teomics make it possible to assess multiple effects ofpollutants in different tissues of one organism.Genomics and proteomics are based on the ever-increasing number of known genome sequences andallow the simultaneous analysis of the expression ofall genes from a genome (genomics) or the analysisof all proteins encoded by the genome (proteomics)(28). Therefore, ecotoxicologists can obtain a muchmore holistic and integrative view of chemical ef-fects. Genomics and proteomics, combined with a

functional approach for understanding the physio-logical role and relevance of the genes and proteins,represent great strides in understanding multipleeffects.

However, novel technologies can also have prob-lems and pitfalls (29). It will take time to realize thefull potential of genomics in ecotoxicology. Manyquestions must be answered: Which species will bemodel organisms? What are the concentrations of pol-lutants used? When are we going to analyzegene expression profiles, and where in the or-ganism are they found? What are the differ-ences in global gene expression betweenindividuals of the same species? A specific chal-lenge will be to link gene expression profiles withwhole-organism effects.

The most promising approach to studying thefunction of genes in whole organisms is the combineduse of genetic and molecular tools with which geneexpression levels can be modified, localized,and quantified, and with which their phys-iological role can be studied.

Do molecular approaches help to copewith the problem of mixture effects? Do weneed mechanistic knowledge? Mixture effects in liv-ing organisms are difficult to predict and need case-by-case investigation. Only in the case of identicaltarget sites and similar mechanisms can mixtures ofchemicals act additively.

Estrogens and several xenoestrogens were found toact as concentration additives in in vitrotest systems like the yeast estrogenscreen, which is basically a measureof the binding to the human estro-gen receptor. In this test, the encod-ing gene is introduced to a yeast celland coupled to a marker gene (30, 31). Re-cently, mixture effects have been examined invivo in juvenile rainbow trout by measuring VTGlevels in the blood of fish exposed to mixturesof estrogenic compounds. Combinations ofestradiol and nonylphenol (32), as wellas combinations of steroidal estrogens(33), showed the expected additivi-ty. Combinations of estradiol andmethoxychlor exhibited antagonis-tic effects, presumably because of ametabolic conversion of metoxychlorto an antagonist of the estrogen receptor (32).

Therefore, we suggest a systematic approach tomixture toxicity studies in environmental toxicologythat is similar to proposals for human health (34;Figure 3). We also suggest integrating molecularapproaches in mixture effect studies by using,for example, reporter gene assays as the yeastestrogen screen, mutant strains in which re-sponses upon exposure to mixtures can be ana-lyzed more specifically, or genomics and proteomicstechniques for a more integrated picture of theresponses.

Can molecular approaches also help to addresstoxicological processes that occur in complex ecosys-tems? In our opinion, yes. Endocrine-disrupting ef-fects have mainly been analyzed on the level of

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individual organisms—effects such as increased VTGlevels in males or increased intersex ratios. Effectsthat might have impacts on population level (fecun-dity, hatching success, male reproductive health) areknown (21, 35, 36). However, it is almost always un-clear whether such influences result from primary ef-fects, like VTG induction, or are generated by othermeans. Extrapolating effects from the molecular level

to the cellular level or from individuals to groupsof many individuals and species is a major ob-jective in ecotoxicology that has yet to beachieved. This issue was the topic of a recent

feature in ES&T by Calow and Forbes (4).For this problem, we again recommend a system-

atic approach aimed at understanding each step inthe propagation of effects, identifying and under-standing the responsible toxicological mechanism atthe various levels of biological organization, and the

interactions between these levels. For exam-ple, by studying mutant organisms and theirphenotypes, clues about the importance ofa particular pathway are uncovered. The

gene expressions during specific develop-mental phases can be deregulated, and the effects ofthese “disruptions” on developmental processes andconsequences for reproduction can be examined. Onthe basis of detailed mechanistic understanding, crit-ical molecular targets can be identified and used tostudy these targets in vivo in organisms being ex-posed in the environment (37, 38). By using genetic

approaches, we can examine the ge-netic composition of populationsand changes following environ-mental stress (39). Hence, even on

the highest level of ecosystem com-plexity, mechanistic knowledge and molecular

approaches will be beneficial.

Approach for the futureModern molecular and genetic tools should beapplied to mechanistic studies in ecotoxicol-ogy. The challenges in ecotoxicology also call

for novel bioanalytical test systems forroutine screening. For their develop-ment and validation, we need to usevarious tools in the exploratory phase

and reduce them to simple, straight-forward test systems for risk and hazard assessment ap-plications. We expect that a sound scientific basecombined with molecular biological approaches willbe the key to success. At the same time, however, re-

searchers need to know how subtle changes at themolecular level affect whole organisms and ecosys-tems. By using the strategies and approaches de-scribed in this article, we are convinced that we can

meet these challenges in ecotoxicology.

Rik I. L. Eggen is a molecular biologist and ecotoxicolo-gist, Renata Behra is a population ecotoxicologist, BeateI. Escher is an environmental chemist and ecotoxicolo-gist, and Nina Schweigert is an ecotoxicologist at EAWAG.Patricia Burkhardt-Holm is currently an ecologist at theUniversity of Basel in Switzerland. Address correspon-dence to Eggen at [email protected].

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