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Biomagnification

Biomagnification

Biomagnification and food-web accumulation

Biomagnification of inorganic chemicals

Biomagnificaiton of chlorinated hydrocarbons

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Biomagnification (or bioaccumulation) refers to the ability of living organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, non-living environment, or in the case of animals, in the food that they eat. Organisms accumulate any chemical needed for their nutrition. The major focus of biomagnification, however, is the accumulation of certain non-essential chemicals, especially certain chlorinated hydrocarbons that are persistent in the environment. These compounds are insoluble in water, but highly soluble in fats. Because almost all fats within ecosystems occur in the living bodies of organisms, chlorinated hydrocarbons such as 4,4-(2, 2, 2-trichloroethane-1, 1-diyl)-bis(chlorobenzene) (DDT) and polychlorinated biphenyls (PCBs) tend to selectively accumulate in organisms. This can lead to ecotoxicological problems, especially for top predators at the summit of ecological food webs, who ingest the toxic prey.

Biomagnification and food-web accumulation

Organisms are exposed to a myriad of chemicals in their environment. Some of these chemicals occur in trace concentrations in the environment, and yet they may be selectively accumulated by organisms to much larger concentrations that can cause toxicity. This tendency represents biomagnification.

Some of the biomagnified chemicals are elements such as selenium, mercury, nickel, or organic derivatives such as methylmercury. Others are in the class of chemicals known as chlorinated hydrocarbons (or organo-chlorines). These are extremely insoluble in water, but are freely soluble in organic solvents, including animal fats and plant oils (these are collectively known as lipids). Many of the chlorinated hydrocarbons are also very persistent in the environment, because they are not easily broken down to simpler chemicals through the metabolism of microorganisms, or by ultraviolet radiation or other inorganic processes. Common examples of bioaccumulating chlorinated hydrocarbons are the insecticides DDT and dieldrin, and a class of industrial chemicals abbreviated as PCBs.

Food-web accumulation is a special case of biomagnification, in which certain chemicals occur in their largest ecological concentration in predators at the top of the food web. An ecological food web is a complex of species that are linked through their trophic interactions, that is, their feeding relationships. In terms of energy flow, food webs are supported by inputs of solar energy, which is fixed by green plants through photosynthesis. Some of this fixed energy is used by the plants in their own respiration, and the rest, as plant biomass, is available to be passed along to animals, which are incapable of metabolizing any other type of energy. Within the food web, animals that eat plants are known as herbivores. These are eaten by carnivores, which in turn may be eaten by other carnivores. Top predators (examples include wolves, bears, and seals) occur at the summit of the food web. In general, food webs have a pyramidal structure, with plant productivity being much greater than that of herbivores, and these being more productive than their predators. Top predators are usually quite uncommon. Within food webs, biomagnifying chemicals such as DDT, dieldrin, and PCBs have their largest concentrations, and cause the greatest damage, in top predators.

Biomagnification of inorganic chemicals

All of the naturally occurring elements occur in the environment. Some occur at very low concentration, while others are more abundant. This contamination is always detectable, as long as the analytical chemistry method of detection is sensitive enough to detect even trace amounts of the target chemical. About 25 of the elements are required by plants and/or animals, including the micronutrients copper, iron, molybdenum, zinc, and rarely, aluminum, nickel, and selenium. However, under certain ecological conditions these micronutrients can biomagnify to very large concentrations, and even cause toxicity to organisms.

One example is serpentine soil and the vegetation that grows in it. Serpentine minerals contain relatively large concentrations of nickel, cobalt, chromium, and iron. Soils derived from this mineral can be toxic to plants. However, some plants grown on serpentine soils are physiologically tolerant of these metals, and can bioaccumulate them to very large concentrations. For example, the normal concentration of nickel in plants is about 1-5 ppm (parts per million, a concentration equivalent to mg/kg). However, on sites with serpentine soils much larger concentrations of nickel occur in plant foliage and other tissues. Nickel concentrations as large as 16% occur in tissues of a plant in the mustard family, Streptanthus polygaloides, in California, and 11-25% nickel occurs in the blue-colored latex of Sebertia acuminata on the island of New Caledonia in the Pacific Ocean. It is common for plants growing on serpentine soils to have nickel concentrations of thousands of parts per million, which is usually considerably larger than the concentration in soil.

Another case of biomagnification occurs on some sites in semiarid regions in which the soil is contaminated by selenium, which may then be hyperaccumulated (i.e., extremely accumulated) by specialized species of plants. These plants are poisonous to grazing livestock and other large animals, causing a toxic reaction called blind staggers. The most important selenium-accumulating plants in North America are milk vetches in the genus Astragalus, in the legume family. There are 500 species of Astragalus in North America, of which 25 are accumulators of selenium. The foliage of these plants can contain thousands of parts per million (ppm; equivalent to 1 milligram per liter) of selenium, to a maximum of about 15,000 ppm, much larger than the concentration in soil. Sometimes, accumulator and non-accumulator Astragalus species grow together, as in the case of a place in Nebraska with 5 ppm selenium in soil, and 5,560 ppm in Astragalus bisulcatus, but only 25 ppm in A. missouriensis.

Mercury can also be biomagnified from trace concentrations in the environment. In this case, trace concentrations of mercury in water can result in large contaminations of fish and other predators. For example, fish species known to bioaccumulate mercury in offshore waters of North America include Atlantic swordfish, Pacific blue marlin, tunas, and halibut, among others. These fish can accumulate mercury from trace concentrations in seawater (less than 0.1 ppm) to concentrations in flesh that commonly exceed 0.5 ppm of the fresh weight of the fish, the maximum acceptable concentration in fish for human consumption. The contamination of oceanic fish by mercury is probably natural, and is not only a modern phenomenon. Studies have found no difference in mercury contaminations of modern tuna and museum specimens collected before 1909, or concentrations in feathers of pre-1930 and post-1980 seabirds collected from islands in the northeast Atlantic Ocean. In this phenomenon of mercury biomagnification, there is a tendency for larger, older fish to have relatively large concentrations. In a study of Atlantic swordfish, for example, the average mercury concentration of animals smaller than 51 lb (23 kg) was 0.55 ppm, compared with 0.86 ppm for those 51-99 lb (23-45 kg) in weight, and 1.1 ppm for those heavier than 45 kg. Large concentrations of mercury also occur in fish-eating marine mammals and birds that are predators at or near the top of the marine food web.

Biomagnificaiton of chlorinated hydrocarbons

Chlorinated hydrocarbons such as some insecticides (examples include DDT, dieldrin, and methoxychlor), PCBs, and dioxin have a low solubility in water. In other words, they tend not to dissolve in water to forma solution. As a result, these chemicals cannot be diluted into the larger volume of water. However, chlorinated hydrocarbons are highly soluble in lipids. Because most lipids within ecosystems occur in biological tissues, the chlorinated hydrocarbons have a strong affinity for living organisms, and they tend to biomagnify by many orders of magnitude from vanishingly small aqueous concentrations. Furthermore, because chlorinated hydrocarbons are persistent in the environment, they accumulate progressively as organisms grow older, and they accumulate into especially large concentrations in top predators, as described previously. In some cases, older individuals of top-predator animals such as raptorial birds and fish-eating marine mammals have been found to have thousands of ppm of DDT and PCBs in their fatty tissues. The toxicity caused by these animals accumulated exposures to DDT, PCBs, and other chlorinated hydrocarbons is a well-recognized environmental problem.

The biomagnification and food-web accumulation characteristics of DDT are especially well known. Typically, DDT has extremely small concentrations in air and water, and, to a lesser degree in soil. However, concentrations are much larger in organisms, especially in animals at or near the top of their food web, such as humans and predatory birds. The food-web biomagnification of DDT can be illustrated by the case of Lake Kariba, Zimbabwe. Although banned in most industrialized countries since the early 1970s, DDT is still used in many tropical countries for agriculture purposes and to control insect vectors of human diseases. The use of DDT in agriculture was banned in Zimbabwe in 1982, but DDT continues to be used to control mosquitoes and tsetse flies, insects that spread malaria and diseases of livestock. The concentration of DDT in the water of Lake Kariba was less than 0.002 ppb, but concentrations in sediment were 0.4 ppm (because sediment contains a relatively large concentration of organic matter, it contains much more DDT than the overlying water). Planktonic algae contained 2.5 ppm. A filter-feeding mussel had 10 ppm (values for animal tissues are for DDT in fat), while two species of plant-eating fish contained 2 ppm, and a bottom-feeding fish contained 6 ppm. A predatory fish and a fish-eating bird, the great cormorant, contained 5-10 ppm. The Nile crocodile is the top predator in Lake Kariba (other than humans), and it had 34 ppm. Therefore, the data for Lake Kariba illustrates a substantial biomagnification of DDT from water, and to a lesser degree from sediment, as well as a marked food-web accumulation from herbivores to top carnivores.

The widespread occurrence of food-web biomagnification of DDT and other chlorinated hydrocarbons caused chronic, ecotoxicological damage to birds and mammals of many species, even in habitats remote from sprayed sites. In some species, effects on predatory birds were severe enough to cause large declines in abundance beginning in the early 1950s, and resulting in local or regional losses of populations. Prominent examples of North American birds that suffered population decreases because of exposure to chlorinated hydrocarbons include bald eagle, golden eagle, peregrine falcon, osprey, brown pelican, and double-crested cormorant, among others. However, since the banning of the use of DDT in North America in the early 1970s, these birds have increased in abundance. In the case of the peregrine falcon, this increase was enhanced by a captive-breeding and

KEY TERMS

Biomagnification Tendency of organisms to accumulate certain chemicals to a concentration larger than that occurring in their inorganic, nonliving environment, such as soil or water, or in the case of animals, larger than in their food.

Ecotoxicology The study of the effects of toxic chemicals on organisms and ecosystems. Ecotoxicology considers both direct effects of toxic substances and also the indirect effects caused, for example, by changes in habitat structure or the abundance of food.

Food-web accumulation Tendency of certain chemicals to occur in their largest concentration in predators at the top of the ecological food web. As such, chemicals such as DDT, PCBs, and mercury in the aquatic environment have their largest concentrations in predators, in comparison with the non-living environment, or with plants and herbivores.

Hyperaccumulation A syndrome in which a chemical is bioaccumulated to an extraordinary degree.

release program over much of its former range in eastern North America.

In some African countries where malaria is a problem, the use of DDT to control mosquitoes (which can transfer the malaria-causing microorganism from person-to-person as they obtain their blood meal) has been advocated. If implimented, DDT spraying programs would have to be controlled, so as not to contaminate ground- and surface-water supplies.

See also Food chain/web.

Resources

BOOKS

Botkin, Daniel B. and Edward A. Keller. Environmental Science: Earth as a Living Planet. New York: John Wiley & Sons, 2004.

Frumkin, Howard. Environmental Health: From Global to Local. New York: Jossey-Bass, 2005.

Suzuki, David and Amanda McConnell. The Sacred Balance: Rediscovering Our Place in Nature. Vancouver: Greystone Books, 2006.

Bill Freedman

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Biomagnification

Biomagnification


The bioaccumulation of chemicals in organisms beyond the concentration expected if the chemical was in equilibrium between the organism and its surroundings. Biomagnification can occur in both terrestrial and aquatic environments, but it is generally used in relation to aquatic situations. Most often, biomagnification occurs in the higher trophic levels of the food chain/web , where exposure to chemicals takes place mostly through food consumption rather than water uptake.

Biomagnification is a specific case of bioaccumulation and is different from bioconcentration. Bioaccumulation describes the accumulation of contaminants in the tissue of organisms. Typical examples of this include the elevated levels of many chlorinated pesticides and mercury in fish tissue. Bioconcentration is used to describe the concentration of a chemical in an organism from water uptake alone. This is quantitatively described by the bioconcentration factor, or BCF, which is the chemical concentration in tissue divided by the chemical concentration in water, expressed in equivalent units, at equilibrium. The vast majority of chemicals that bioaccumulate are aromatic organic compounds, particularly those with chlorine substituents. For organic compounds, the mechanism of bioaccumulation is thought to be the partitioning or solubilization of chemical into the lipids of the organism. Thus the BCF should be proportional to the lipophilicity of the chemical, which is described by the octanol-water partition coefficient, Kow. The latter is a physical-chemical property of the compound describing its relative solubility in an organic phase and is the ratio of its solubility in octanol to its solubility in water at equilibrium. It is constant at a given temperature. If one assumes that a chemical's solubility in octanol is similar to its solubility in lipid, then we can approximate the lipid-normalized BCF as equal to the Kow. This assumption has been shown to be a reasonable first approximation for most chemicals accumulation in fish tissue.

However, animals are exposed to contaminants by other routes in addition to passive partitioning from water. For instance, fish can take up chemicals from the food they eat. It has been noted in field collections that for certain chemicals, the observed fish-water ratio (BCF) is significantly greater than the theoretical BCF, based on Kow. This indicates that the chemical has accumulated to a greater extent than its equilibrium concentration. This is defined as biomagnification. This condition has been documented in aquatic animals, including fish, shellfish, seals and sea lions , whales , and otters, and in birds, mink, rodents, and humans in both laboratory and field studies.

The biomagnification factor, BMF, is usually described as the ratio of the observed lipid-normalized BCF to Kow, which is the theoretical lipid-normalized BCF. This is equivalent to the multiplication factor above the equilibrium concentration. If this ratio is equal to or less than one, then the compound has not biomagnified. If the ratio is greater than one, then the chemicals biomagnified by that factor. For instance, if a chemical's Kow were 100,000, then its lipid normalized BCF should be 100,000 if the chemical were in equilibrium in the organism's lipids. If the fish tissue concentration (normalized to lipids) were 500,000, then the chemical would be said to have biomagnified by a factor of five.

Biomagnification in the aquatic food chain often leads to biomagnification in terrestrial food chains, particularly in the case of bird and wildlife populations that feed on fish. Consider the following example that demonstrates the results of biomagnification. The concentrations of the insecticide dieldrin in various trophic levels are determined to be the following: water, 0.1 ng/L; phytoplankton , 100 ng/g lipid; zooplankton , 200 ng/g lipid; fish, 600 ng/g lipid; terns, 800 ng/g lipid. If the Kow were equal to one million, then the phytoplankton would be in equilibrium with the water, but the zooplankton would have magnified the compound by a factor of 2, the fish by a factor of 6, and the terns by a factor of 8.

The mechanism of biomagnification is not completely understood. To achieve a concentration of a chemical greater than its equilibrium value indicates that the elimination rate is slower than for chemicals that reach equilibrium. Transfer efficiencies of the chemical would affect the relative ratio of uptake and elimination. There are many factors that control the uptake and elimination of a chemical from the consumption of contaminated food, and these include factors specific to the chemical as well as factors specific to the organism. The chemical properties include solubility, Kow, molecular weight and volume, and diffusion rates between organism gut, blood, and lipid pools. The organism properties include the feeding rate, diet preferences, assimilation rate into the gut, rate of chemical's metabolism , rate of egestion, and rate of organism growth. It is thought that the chemical's properties control whether biomagnification will occur, and that it is the transfer rate from lipid to blood that allows the chemical to attain a lipid concentration greater than its equilibrium value. Thus it follows that the chemicals that biomagnify have similar properties. They typically are organic; they have molecular weights between 200 and 600 daltons; they have Kows between 10,000 and 10 million; they are resistant to metabolism by the organism; they are non-ionic, neutral compounds; and they have molecular volumes between 260 and 760 cubic angstroms, a cross sectional width of less than 9.5 angstoms and a molecular surface area between 200 and 460 square angstroms. The latter dimensions allow them to more easily pass through lipid bilayers into cells but perhaps do not allow them to leave the cell easily due to their high lipophilicity. Since this disequilibrium would occur at each trophic level , it results in more and more biomagnification at each higher trophic level. Because humans occupy a very high trophic level, we are particularly vulnerable to adverse health effects as a result of exposure to chemicals that biomagnify.

[Deborah L. Swackhammer ]

RESOURCES

BOOKS

Connell, D. W. Bioaccumulation of Xenobiotic Compounds. Boca Raton: CRC Press, 1990.

PERIODICALS

Bierman Jr., V. J. "Equilibrium Partitioning and Biomagnification of Organic Chemicals in Benthic Animals." Environmental Science and Technology 24 (September 1990); 140712.

Sijm, D., W. Seinen, and A. Opperhuizen. "Life Cycle Biomagnification Study in Fish." Environmental Science and Technology 26 (November 1992): 216274.

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