Unit 1

Question 1

Define ecotoxicology and explain how it is different from mammalian toxicology (its parent field).

Answer 1

Ecotoxicology is the science of contaminants in the biosphere and their effects on constituents of the biosphere, including humans.

Mammalian toxicology doesn’t consider all the chemicals involved in ecotoxicology to be toxicants, as they do not directly poison individuals. For example, excessive amounts of phosphorus or nitrogen may does not directly affect a species, though they will inadvertently have an effect on their health, as the ecological community subsequently deteriorates. Another example may be seen in the excessive rates of carbon dioxide and methane globally. While the large atmospheric concentrations are not directly toxic to humans, the high rates will result in negative impacts on the earth’s ecosystems, therefore impacting the species that rely on its stability for survival.

Question 2

Clearly describe the similarities and differences between the three goals of ecotoxicology (scientific, technical and practical).

Answer 2

Scientific:
● Generally, to organize knowledge based on explanatory principles about contaminants in the biosphere
○ Goals are based on the development of scientific method
● Working hypothesis
○ Never accepted as true
○ Provides a focus for the falsification procedure
○ Weakness: Tends to favor a central theory
● Multiple working hypothesis
○ Consider all plausible ideas simultaneously
○ Useful in questions with multiple interactions
○ Although a hypothesis is never assumed true, survival tends to enhance its status
○ Must avoid weak testing of the hypotheses
○ Must avoid imprecise or biased measurements
● Eventually testing leads to a hypothesis becoming a paradigm.
○ Defined as a generally accepted concept that have survived vigorous testing
■ Act as nuclei for further testing
■ Not “truth”
■ Can create controversy when test results go against the paradigm.
● Two types of investigative behaviors can occur:
○ Normal science:
■ Incremental increase in facts and ideas which reaffirm, revise or replace paradigms
■ Methodical fact gathering.
○ Innovative science:
■ Questions paradigms and formulates new ones
■ Requires normal science
■ Young fields tend more towards normal science, but a balance is required
Technological:
● Overall: to develop and apply tools and methods to acquire a better understanding of contaminant fate and effects in the biosphere.
● Benefits tend to be more immediately apparent.
● May include development of:
a) Analytical instrumentation
b) Standard methods
c) Computational/analytical methods
● Examples:
○ Application of biomarkers
■ Using changes in biochemistry as early warnings or to detect pollutants
○ GIS systems for study of nonpoint source contamination over large areas
● Qualities aimed for: effectiveness, precision, accuracy, appropriate sensitivity, consistency, clarity of results, and ease of use.
Practical:
● Overall: the application of available knowledge, tools and procedures to solving or documenting specific problems.
● Goal is not more complete understanding, rather to address a specific problem.
● Relevant terms:
○ Criteria: estimated [toxicant] based on current literature, that are considered protective for organisms or a defined purpose, if not exceeded.
○ Standards: legal limits thought sufficient to protect environment.
● Value is placed on: effectiveness, precision, accuracy, sensitivity, consistency, clarity and ease of use
○ Value is also placed on unambiguous results, safety and clear documentation of progress
● Ideally, there should be great overlap between the scientific, technical and practical goals for ecotoxicology.
○ Practically, a given lab/situation/study will emphasize one or another
Though the scientific, technological and practical goals are overlapping, they are distinct. Sometimes there is inappropriate or inadequate integration of these goals in application. For example, present regulations remain biased toward single species tests done in the laboratory, yet our scientific knowledge clearly indicates that results from multiple species tests are at least as valuable to understanding risk. Recognizing the continual need for reintegration of knowledge, lawmakers have wisely incorporated periodic review and revision into major legislation and associated regulations. Further, new or improved technologies are continually drawn into our scientific efforts, e.g., new molecular technologies applied to assaygenetic damage and identify causes of adverse effects. The scientific foundations of the field should also expand and come into balance with technology and practice.

Question 3

Dilution paradigm

Answer 3

The solution to pollution is dilution.

● Failed with clearly unacceptable consequences to human health and ecological integrity.

Question 4

Boomerang paradigm

Answer 4

what you throw away can come back and hurt you.

● Replaced dilution paradigm gradually after WWII DDT use.

Question 5

Explain how/why we have moved from the dilution paradigm to the boomerang paradigm.

Answer 5

The boomerang paradigm makes it unacceptable to discards toxicants into the environment. This paradigm was adopted during the latter half of the 20th century after several well-publicized events captured the public’s attention. Notable among these events was the discovery that the pesticide DDT was accumulating in birds at the top of the food web.

Question 6

Explain what levels of hierarchical organization (e.g. subcellular to global) are used in ecotoxicology, and what the strengths of each level are.

Answer 6

● While all levels are equally important, their contribution differs in efforts and understanding of ecotoxicology.
○ Lower levels of the conceptual hierarchy, such as, biochemical effects, tend to be more tractable and have more potential for easy linkage to a specific cause than do effects at higher levels such as the biosphere. Effects at lower levels of the ecological hierarchy are used more readily in a proactive manner than are those at higher levels. They can indicate the potential for emergence of an adverse ecotoxicological effect
○ Effects at higher levels are useful in documenting or prompting a regulatory reaction to an existing problem. Although highly tractable and sensitive, the ecological relevance of effects at lower levels is much more ambiguous than effects at higher levels of organization. Most reasonable biologists would agree that a 50% reduction in species richness is a clear indication of diminished health of an ecological community. But a 50% increase in metallothionein in adults of an indicator species provides an equivocal indication of the health of species populations contributing to the associated community.
○ Relative to those at higher levels of biological organization, effects at lower levels tend to occur more rapidly after the stressor appears and to disappear more quickly after it is removed. Considering all of these points, it is clear that information from all relevant levels of biological organization should be used together.

Question 7

Differentiate between a contaminant and a pollutant.

Answer 7

Contaminant: a substance released by human activity

Pollutant: a substance that occurs in the environment at least in part due to human activities and which has a deleterious effect on living organisms

Question 8

Explain what contaminant partitioning is and why it is important in ecotoxicology.

Answer 8

Partitioning: Depending on its chemical properties, a charged or uncharged chemical substance will preferentially associate with (partition between) different environmental phases. The relative tendency of a substance to associate with one phase or another eventually produces an equilibrium distribution of concentrations in the available phases. The distribution or partition coefficient (or its logarithm) is used to quantify partitioning between phases. It is important in ecotoxicology because the varying states may have the capacity to effect bioavailability and toxicity in exposed organisms.

Question 9

Explain what a POP is?

Answer 9

Persistent organic pollutant – known to disperse widely and accumulate to high concentrations in wildlife and humans in regions far removed from their points of release. The most prominent are polychlorinated biphenyl, dioxins, furans, and nine pesticides. Organochlorine pesticides classify as POPs, as they are resistant to degradation in the environment and tend to increase in concentration with movement up through food webs. They persist at high concentrations in biota and many can disperse globally.

Question 10

Describe why the World Health Organization interested in POP’s?

Answer 10

The WHO is interested in POPs for two reasons… The first is the global spread of POPs worldwide, and the effect it has on bioaccumulation in humans, and the lasting effects. The other is that DDT, a POP, is an extremely important tool for disease control throughout the world. As such, DDT is currently used to fight malaria in regions with high malaria rates. The WHO now prescribes careful reintroduction of DDT for malaria control in developing countries.

Question 11

Describe the major classes of contaminants and give an example of each class.

Answer 11

Organic Compounds
●	Composed predominantly of the nonmetal elements from groups 1, 4–7A of the periodic table, however, there are important instances in which metals are integrated into biomolecules such as the cytochromes and hemoglobin, or contaminants such as methylmercury and organotins.
●	Intentional poisons
○	Insecticides
○	Herbicides
○	Fungicides
○	Wood preservatives
○	They become a problem when nontarget species come into contact with sufficiently high concentrations of them. Other organic contaminants are unintentional poisons, e.g., degreasers, solvents, and various industrial by-products. Some organic compounds such as those in personal care products (e.g., detergents and musks) or pharmaceuticals (e.g., drugs, antibiotics, and birth control substances) are designed to be directly beneficial to an individual’s well-being yet still cause problems after release into the environment.

● Include those used as poisons and wastes, by-products and products of industrial processes
○ CFC’s (refridgerants)
○ Organochlorine alkenes (solvents, degreasers)
○ Chlorinated phenols (wood preservatives)
○ Chlorination products (disinfection)
○ Organochlorine pesticides (e.g. DDT)
○ Organophosphate insecticides inhibit acetylcholine esterase (e.g. malathion)
○ Carbamate insecticides
○ Pyrethroid insecticides (e.g. allethrin) - more like plant compounds
○ Herbicides
○ Dioxins
○ PAH
○ PCB
○ PCDF
○ Petroleum related compounds
○ Oxygen demanding compounds, e.g. sewage effluents
Inorganic Compounds
● Inorganic contaminants are composed of intentional and unintentional poisons. Some are released for a very specific purpose (e.g., sodium arsenate as a pesticide), but others are released by a wide range of human activities such as the lead used in batteries, plumbing, gas additives, and many other products. Inorganic contaminants, such as nitrite in drinking waters, become a problem only if our activities elevate their concentrations to abnormal levels. Indeed, several metals are essential to life but, above certain concentrations, become harmful.
● Inorganic gases
○ CO2, NOx, SO2
● Metals and metalloids
○ Al - acid ppt or mine drainage
○ As - product of gold, lead mining
○ Cd - alloys, electroplating, batteries
○ Cr - alloys, pigments, catalysts, preservatives
○ Cu - wiring, plumbing, biocontrol agent
○ Pb - gasoline, batteries, solder, caulk, pipes, ammunition
○ Hg - electronics, amalgams, gold mining, paints
○ Ni - stainless steel, nickel plating
○ Se - electronics, glass, pigments, alloys
○ Zn - galvanizing, alloys
● Nutrients
○ Nitrogen species
○ Phosphate species

● Organometals
○ Tin - antifouling paints
■ TBT - tributyltin - low levels can cause shell abnormalities
■ TMT - trimethyltin neurotoxicants
■ TET - triethyltin
○ Lead - tetra alkyl lead - anti knock additive in gasoline
○ Mercury - methyl mercury - also as biocides for seed coatings
○ Radionuclides - weapons production/testing and Research/medical uses

Question 12

Clearly distinguish between bioaccumulation and bioconcentration.

Answer 12

Bioaccumulation: The net accumulation of a contaminant in (and in some occasional instances on) an organism from all sources including water, air, and solid phases of the environment. Solid phases include food sources.

Bioconcentration: The net accumulation in (and, in some cases, on) an organism of a contaminant from water only.

Question 13

Clearly delineate exposure routes for an organism.

Answer 13

● To be toxic – all compounds must come in contact with their site of action at a sufficient concentration
● Generally uptake is through food, water, dermal, pulmonary surfaces (lungs, gills), gut for animals and roots, stomata or epidermal for plants.
● 3 general routes into animal cells
○ Lipid route
■ Lipophilic contaminants and small uncharged polar molecules diffuse through lipid bilayer
○ Aqueous route
■ Involves membrane transport proteins
● Channels may be gated or ungated with porins (barrel shaped proteins)
● Carrier proteins may involve active transport mechanisms
● May be symporters or antiporters

● Endocytotic: taken up by endocytosis into vesicles and processed

Question 14

Clearly distinguish between steady state and equilibrium.

Answer 14

Steady state: A constant concentration in an organism resulting from processes (e.g., uptake, elimination, and internal exchange among compartments) including those requiring energy.

Equilibrium: concentration resulting from chemical equilibrium processes do not require energy to be maintained.
Steady-state concentrations resulting from bioaccumulation can be considerably higher than those predicted for chemical equilibrium.

Question 15

Distinguish between diffusion, facilitated diffusion, and active transport.

Answer 15

Diffusion: The movement of a contaminant down an electrochemical gradient that requires no
energy. It might be simple diffusion of a charged ion through a channel protein or passage of a lipophilic molecule through the lipid route.

Facilitated diffusion: Diffusion down a gradient not requiring energy, but occurring at a rate faster than expected by simple diffusion alone. Facilitated diffusion occurs down an electrochemical gradient, requires a carrier protein, does not require energy, but is faster than predicted for simple diffusion.

Active Transport: Movement of a substance up an electrochemical gradient that requires a carrier molecule and energy.

Question 16

Describe the relationship between biotransformation and activation with examples.

Answer 16

Once a contaminant enters the organism, it becomes available for possible biotransformation, the biologically mediated transformation of a chemical compound to another. Most biotransformations involve enzymatic catalysis and, as a consequence, can be subject to saturation kinetics and competitive inhibition. Biotransformation can lead to enhanced elimination, detoxification, sequestration, redistribution, or activation. With activation, the adverse effect of a contaminant is made even worse by biotransformation or an inactive compound is converted to one with an adverse bioactivity. As an example of activation, the organophosphorus pesticide, parathion, undergoes oxidative desulfuration to form the very potent paraoxon.

Question 17

Explain what metallothionein is and what it is used for.

Answer 17

Metallothioneins are now found to be ubiquitously distributed among both prokaryotes and eukaryotes, and to occur in organisms as diverse as bacteria, fungi, invertebrates, and vertebrates. In animals ranging from lower invertebrates to humans, they are found in highest concentrations in the liver or functionally equivalent organs, (e.g., the hepatopancreas), kidneys, lung or gills, and intestines. They have been of considerable interest for their role in the ecotoxicology of metals and, particularly, for application as a biomarker for environmental metal contamination.
● Small proteins with 25-30% cysteine that may bind to 6-7 metal atoms per molecule
● Induced by toxic metals, enhances elimination by kidneys
● Metallothioneins function in the uptake, internal compartmentalization, sequestration, and excretion of essential (e.g., copper and zinc) and nonessential (e.g., cadmium, mercury, and silver) metals.
General features include:
1. Low-molecular weight with a high metal content.
2. High cysteine content, and no aromatic amino acids or histidine.
3. Unique amino acid sequence, especially regarding cysteine placement.
4. Metal-thiolate clusters.

Question 18

Differentiate between the roles of Phase I and Phase II reactions.

Answer 18

Phase I reactions involve hydrolysis, reduction, and oxidation of xenobiotics. Functional groups are produced or exposed during Phase I reactions, rendering the compound more reactive and perhaps producing a biotransformation product that could be acted on in Phase II detoxification reactions. Many Phase I products might also be more water soluble than the original xenobiotic. Phase I products can enter into Phase II reactions but some are eliminated effectively without further Phase II biotransformation. “A commonality of biotransformation reactions is the conversion of hydrophobic xenobiotics into more polar, more easily excreted compounds”. Cells of different tissues differ in their capacities for Phase I and II reactions.

Phase II enzymes might also be induced by xenobiotics and used in biomarker studies. They involve conjugation, that is, “the addition to foreign compounds of endogenous groups which are generally polar and readily available in vivo”. The readily available compounds that are bound to the toxicants (or their metabolites) can be carbohydrate derivatives, amino acids, glutathione, or sulfate. The result is production of a conjugated compound that is more polar and consequently more readily eliminated. In some cases, the conjugation permits ready recognition by specific elimination processes of the conjugated toxicant or metabolite, i.e., the glucuronic acid portion of the conjugate is recognized by the transport process

Question 19

Explain how enterohepatic circulation can increase the damage a toxin may do.

Answer 19

Enterohepatic circulation refers to the circulation of biliary acids, bilirubin, drugs, or other substances from the liver to the bile, followed by entry into the small intestine, absorption by the enterocyte and transport back to the liver. Enterohepatic circulation also means that some molecules which would not otherwise be very toxic can become extremely hepatotoxic as they reach unexpectedly high hepatic concentrations. Drugs and toxins may remain in the enterohepatic circulation for a prolonged period of time as a result of this recycling process.

Question 20

Explain how bioavailability from two different exposure routes can be determined.

Answer 20

Bioavailability is the extent to which a contaminant in a source is free for uptake.
● comparisons of bioavailability for different sediments can be done by measuring the amount of a contaminant in sediments and the amount in organisms inhabiting the sediments, or by measuring uptake clearance rates of organisms placed in the various sediment types.
● assimilation efficiency was used to quantify bioavailability from food. Assimilation efficiency or percentage retention (efficiency expressed as a percentage) may be determined by feeding known amounts of contaminant, often using a radiotracer, and measuring the increase in contaminant within the fed individual. The mass incorporated into tissue divided by the mass fed to the individual is calculated assuming that all of the contaminant measured in the organism has become interspersed in or incorporated into its tissues. Much effort is made in designing this type of experiment to minimize the amount of unassimilated contaminant in the gut, hepatopancreas, or other similar sites. Without this precaution, the estimated assimilation efficiency would be biased upward from the true assimilation efficiency.

Question 21

Describe the chemical qualities that can alter the bioavailability of a toxicant from water for organic and for metal toxicants.

Answer 21

Water chemistry affects bioavailability by changing the chemical species present and the functioning of uptake sites. For example, pH has an obvious effect on the equilibrium, NH3 + H+ ↔ NH4+. The resulting distribution of ammonia species is important to understand because the neutral NH3 passes much more readily through the cell membrane than does the charged NH4+, and consequently, is the more bioavailable or poisonous form of ammonia.
The bioavailability of a dissolved metal and metalloid can also be affected by chemical speciation. Metal cations compete with other cations for dissolved ligands,* that is, anions or molecules that form coordination compounds and complexes with metals. Ligands forming complexes with metals include dissolved organic compounds and inorganic species. Natural organic ligands such as humic and fulvic acids have a wide range of relevant functional groups. Among the most important in complexation are carboxylic and phenolic groups.
As a general rule, bioavailability or toxicity is correlated with the free metal concentration.
ORGANICS - Bioavailability of organic compounds from water and other sources has been described with structure–activity relationships (SARs) that use molecular qualities of the compounds to predict activity or availability. Often such qualitative relationships predict changes in activity of a drug or toxicant with structural changes such as the addition of a chloride atom to or removal of a methyl group from a parent molecular structure. If expressed quantitatively, SARs become quantitative structure–activity relationships (QSARs). A QSAR is a quantitative—often statistical— relationship between molecular qualities and bioactivity such as bioavailability or toxicity. Molecular qualities include measures of lipophilicity, steric conformation, molecular volume, ionization, polarity, or reactivity. But, in ecotoxicology, the most commonly used are measures of lipid solubility of organic compounds, such as Kow (Figure 4.7). Partitioning between n-octanol and water has been the most common procedure used to reflect partitioning between water and lipids in organism.
In the simple Kow approach, the organism is envisioned as a membrane enveloped pool of emulsified lipids. In this conceptual model, uptake and elimination are controlled by permeation of the membrane and/or permeation through aqueous phases (Connell, 1990) with the predominant process being dictated by the qualities of the specific compound in question. Uptake of small, hydrophilic molecules is strongly influenced by membrane permeation but large, hydrophobic compounds are controlled more by permeation through aqueous phases. Very large molecules (e.g., molecular size >9.5 Å) (Landrum and Lydy, 1991) will not pass through the lipid membrane. Uptake across the membrane is dictated by Fick’s Law.
Below approximately log Kow of 3, bioaccumulation of the most water soluble compounds is controlled by permeation of the membrane. Bioaccumulation is determined primarily by diffusional processes between log KOW values of 3–6. Above this range, the relationship is strongly influenced by inhibiting effects of increased molecular size on diffusion, and accordingly, bioconcentration factors drops with increasing log KOW. Other factors contribute to bioavailability of organic compounds from water. The above model would not apply for compounds undergoing extensive biotransformation. Also, bioavailability of ionizable organic compounds would be influenced by pH, as already described and soon to be described in more detail for ingested, ionizable contaminants.

Question 22

Describe what chemical qualities influence the bioavailability of a metal or organic toxicant from solid phases.

Answer 22

METALS Boethling and MacKay (2000) suggest that absorption of contaminants in lungs is highest for those compounds with water solubilities higher than their lipid solubilities. They also indicate that polar compounds tend to be more bioavailable in inhaled aerosols than nonpolar compounds. The bioavailability of inorganic contaminants in aerosols, food, sediments, soils, and other solid phases of the environment is difficult to predict accurately; however, some general themes do emerge from the literature. The direct availability from solid phases is only one part of the story. For example, availability of a metal from a particular solid phase such as sediments can be determined by its capacity to partition into the interstitial waters. The bioavailability of metals or metalloids in solid aerosols* suspended in air is determined not only by their chemical forms in the solid but also by the size of the particulates and the distribution of the element within the particulates. The depth of passage and consequent bioavailability of contaminants in inhaled particles are related to particle size and other factors.
● Lead halides in automobile exhaust dissolves more readily in the lung after inhalation than lead in road dust that has weathered to compounds such as lead sulfate
● arsenic tends to condense onto outer layers of smaller coal fly ash particles as they move up the smoke stacks of coal burning power plants and, because of this surface deposition, the arsenic is more available than if it were uniformly distributed throughout small to large ash particles
Bioavailability of contaminants in food is a function of many factors. Just as with inhaled particulates, the size of a food particle can determine bioavailability as well as the chemical form of the affiliated contaminants. Particle size of materials passing though the human gut can modify bioavailability of some contaminants and drugs. Literature describing the bioavailability of elements ingested by humans provides several telling examples of additional factors modifying assimilation. Diet can have a strong impact on bioavailability.
Bioavailability of metals and metalloids in sediments is difficult to estimate (Luoma, 1989); however, it is thought to be determined by concentrations in interstitial water and concentrations in different solid phases. The solid phase concentrations influence bioavailability by dictating the concentrations of metal in the interstitial waters surrounding the biota in addition to defining the amounts of solid phase metal available for direct ingestion by benthic species.* Because total metal concentration in sediments can be a poor indicator of available metal (Tessier et al.1984), many estimates of bioavailable metals depend on partial extractions such as a 1 N HCl extract (Krantzberg, 1994) or a series of sequential extractions of sediments thought to grossly separate particular metal-binding fractions of the solid sediments.
ORGANICS - For ionizable contaminants, gastric pH can influence availability with the direction of effect being determined by the pKa of the contaminant.* (The pKa is –log10 of the ionization constant (Ka) for a weak, Brønsted acid, where Ka = ([H+][X−])/[HX].) Weakly acidic, organic compounds with pKa values greater than 8 are unionized in the human gastrointestinal tract but bioavailability of those with pKa values between 2.5 and 7.5 is pH sensitive.According to the pH-partition hypothesis (Shore et al. 1957; Wagner, 1975), bioavailability is determined by diffusion of the unionized form from the gastrointestinal lumen across the “lipid barrier” created by the gut lining and into the tissues as determined by pKa and pH.
The KOW influences bioavailability of lipophilic compounds in food. Spacie et al. (1995) noted a maximum availability of contaminant in food at log KOW values of 6 with uptake being lower for very hydrophobic and large compounds. They discuss several studies indicating low bioavailability to fish of several lipophilic organic contaminants in food. Donnelly et al. (1994) indicated that organic compounds with log KOW values of 4–7 such as PCBs are quickly absorbed to soils and, consequently, are not readily available to terrestrial plants. In contrast, compounds such as many pesticides with log KOW values of approximately 1–2 are taken up more easily by plants.
Molecular weights beyond 500 generally result in poor intestinal permeability as does a calculated log KOW above five. In addition, Lipinski et al. (2001) found that compound bioavailability was related to the number of NH and OH bonds, and the number of N and O atoms it possesses.
In sediments, bioavailability to benthic species usually—but not always—decreases with increasing log KOW (Landrum and Robbins, 1990). This is likely due to the enhanced partitioning of nonpolar organic compounds to the sediment solid phases with consequent low concentrations in the interstitial waters. Any increase in sediment organic carbon content can diminish the bioavailability of nonpolar organic compounds much as AVS decreases bioavailability of metals in anoxic sediments.* A maximum bioavailability has been noted at a log KOW of approximately 6 for some series of chlorinated hydrocarbons.

Question 23

Describe how temperature may alter bioavailability.

Answer 23

Temperature is perhaps the most widely studied and important factor affecting the general physiology of individual organisms. This being the case, it should be no surprise that temperature influences biochemical and physiological processes associated with bioaccumulation.
Generally, increases in temperature within normal physiological ranges have been shown to increase bioaccumulation, e.g., mercury in mayfly nymphs (Odin et al. 1994), cadmium and mercury in mollusks (Tessier et al. 1994), cadmium in Asiatic clams (Graney et al. 1984), and DDT in rainbow trout (Reinert et al. 1974). Cesium (134Cs) uptake was highest at temperatures optimal for food consumption and growth of rainbow trout (Gallegos and Whicker, 1971). The biological half-life (t1/2) for elimination of 134Cs from rainbow trout increased as water temperatures increased according to an exponential relationship*, t1/2 = (Constant) e−0.106t where t = temperature in °C (Ugedal et al. 1992). For the rainbow trout, retention of methylmercury was approximately 1.5 times longer at 0.5°C to 4.0°C than at water temperatures of 16°C to 19°C.

Question 24

Define allometry and describe its influence on bioavailability.

Answer 24

Allometry, the study of size and its consequences (Huxley, 1950), can also be important to consider for bioaccumulation. Metabolic rate and a myriad of other anatomical, physiological and biochemical qualities of organisms change with size and, in so doing, modify uptake, transformation and elimination rates. The commonly observed consequence is size-dependent bioaccumulation. However, because age and size are correlated in most species, allometric effects are often confused with age or exposure duration effects in surveys of bioaccumulation. Regardless, many studies detail allometric effects on contaminant uptake, biotransformation, elimination, and general bioaccumulation.

Question 25

Articulate the difference between a biologically determinant and indeterminant element.

Answer 25

Many essential elements and some of their analogs are defined as biologically determinant; their concentrations in organisms remain relatively constant over a wide range of environmental concentrations. Other elements are biologically indeterminant and their concentrations in organisms are directly proportional to environmental concentrations.

Question 26

Explain the types of situations that may be mistaken for biomagnification.

Answer 26

k

Question 27

Explain the twin tracer technique and how it can be used to measure assimilation from food

Answer 27

Assimilation can be measured using this technique, whereby a radiotracer of the substance to be assimilated simultaneously with an inert radiotracer to which assimilation is compared. The inert tracer is not assimilated to any appreciable amount and the retention of the assimilated tracer is quantified when all (or most) of the inert tracer has passed through the organism. For example 14C-labelled organic compound may be compared with the amount of notionally inert 51Cr fed simultaneously to a zooplankton species. The assimilation efficiencies from various foods and members of the food web are pieced together to predict trends in trophic transfer.

Question 28

Explain how d15N values can be used to determine trophic levels.

Answer 28

Isotopic discrimination of light elements such as C,N and S occurs during trophic transfers, providing an opportunity for quantifying trophic status in natural communities. Isotopic discrimination tends to reduce the amount of lighter isotopes (12C, 14N, or 32S) in organisms relative to the heavier isotopes (13C, 15N, or 34S) during trophic exchange. The lighter isotopes are eliminated from the organism more readily than the heavy isotopes.
Changes in nitrogen isotopes (14N and 15N) in organisms within a trophic web are quantified relative to the trophic ratio in the air. The equation is as follows:

Question 29

Explain whether or not metals can biomagnify with examples.

Answer 29

Metals can biomagnify. Cd, Cu, Ni, and Zn may biomagnify in specific marine food chains consisting of bivalves, herbivorous gastropods, and barnacles.

Heavy metals such as mercury, specifically methylmercury also have the ability to biomagnify as demonstrated by numerous examples including the South River in Virginia and Minimata Bay.

Question 30

Explain what Kow is and how it relates to biomagnification.

Answer 30

Bioconcentration factors can also be related to the octanol-water partition coefficient, Kow. The Kow represents the octanol-water partition coefficient which represents the ratio of the concentration of octanol to the concentration of chemical in the water. The octanol-water partition coefficient (Kow) is correlated with the potential for a chemical to bioaccumulate in organisms. A wide range of behaviours is exhibited by organic contaminants relative to trophic transfer but in general, biomagnification of compounds not subject to metabolism seem to be predictable in aquatic food webs with the log Kow. Biomagnification is possible if the log Kow values were above approximately 4. A log of Kow values greater than 8 is the approximate upper limit for biomagnification in the aquatic food web.