Biomagnification

DEFINITION: Accumulation of toxic contaminants in organisms as they move up through the food chain

Recognition of the detrimental environmental effects of biomagnification led to the adoption of several procedures to prevent the accumulation of toxic materials in higher organisms along the food chain. Some pesticides were banned outright, and others were modified to prevent their accumulation in the environment.

As members of each level of the food chain are progressively eaten by those organisms found in higher levels of the chain, the potential grows for increased concentrations of toxic chemicals to accumulate within the tissues of the higher organisms. Not all chemicals, potentially toxic or not, are equally likely to undergo this accumulation, known as biomagnification. However, molecules susceptible to biomagnification have certain characteristics in common. They are resistant to natural microbial degradation and therefore persist in the environment. Many pesticides, insecticides, pharmaceuticals, and plastics are known as persistent organic pollutants (POPs) because they resist degradation.

These substances are also lipophilic—that is, they tend to accumulate in the fatty tissue of organisms. In addition, chemicals must be biologically active in order to have effects on the organisms in which they are found. Such compounds are likely to be absorbed from food or water in the environment and stored within the membranes or fatty tissues.

The process of biomagnification usually begins with the spraying of pesticides for the purpose of controlling insect populations. Industrial contamination, including the release of heavy metals, can be an additional cause of such pollution. Biomagnification results when these chemicals contaminate the water supply and are absorbed into the lipid membranes of microbial organisms. This process, often referred to as bioaccumulation, results in the initial concentration of the chemical in the organisms in a form that is not naturally excreted with normal waste material. The levels of the chemical in the organisms may reach anywhere from one to three times the level found in the surrounding environment. Since the nature of the chemical is such that it is neither degraded nor excreted, it remains within the organisms.

As the organisms on the bottom of the food chain are eaten and digested by members of the next level in the chain, the concentration of the accumulated material significantly increases; at each subsequent level, the concentration may reach one order of magnitude (a tenfold increase) higher. Consequently, the concentrations of the pollutant at the top of the environmental food chain, such as in fish or carnivorous birds, and potentially even humans, may be as much as 1 million times as high as the original, presumably safe, levels in the environment. For example, studies of Dichloro-diphenyl-trichloroethane (DDT) levels in the 1960s found that zooplankton at the bottom of the food chain had accumulated nearly 1,000 times the level of the pollutant in the surrounding water. Ingestion of the plankton by fish resulted in a concentration by another factor of several hundred. By the time the fish were eaten by predatory birds, the level of DDT was concentrated at a factor of more than 200,000.

Pollutants Subject to Biomagnification

DDT is characteristic of most pollutants subject to potential biomagnification. It is relatively stable in the environment, persisting for decades. It is soluble in lipids and readily incorporated into the membranes of organisms. While DDT represents the classic example of biomagnification of a toxic chemical, it is by no means the only representative of potential environmental pollutants. Other pesticides with similar characteristics include aldrin, chlordane, parathion, and toxaphene. In addition, cyanide, polychlorinated biphenyls (PCBs), and heavy metals—such as selenium, mercury, copper, lead, and zinc—have been found to concentrate within the food chain.

Some heavy metals are inherently toxic or may undergo microbial modification that increases their toxic potential. For example, mercury does not naturally accumulate in membranes and was therefore not originally viewed as a significant danger to the environment. However, some microorganisms are capable of adding a methyl group to the metal, producing methylmercury, a highly toxic material that accumulates in fatty tissue and membranes.

Since pesticides are, by their nature, biologically active compounds, which reflect their ability to control insects, they are of particular concern if subject to biomagnification. DDT remains the classic example of how bioaccumulation and biomagnification may have effects on the environment. Initially introduced as a pesticide for the control of insects and insect-borne disease, DDT was not thought to be particularly toxic. However, biomagnification of the chemical was found to result in the deaths of birds and other wildlife. In addition, DDT contamination was found to result in the formation of thin eggshells in birds, which greatly reduced the number of birds successfully hatched. Before the use of DDT was banned in the 1960s, the population levels of predatory birds such as eagles and falcons had fallen to a fraction of the levels found before the use of the insecticide. Though it was unclear whether there was any direct effect on the human population in the United States, the discovery of elevated levels of DDT in human tissue contributed to the decision to ban the use of the chemical.

Several procedures have been adopted since the 1960s to prevent the biomagnification of toxic materials. In addition to outright bans, pesticides are often modified to prevent their accumulation in the environment. The chemical structures of most synthetic pesticides are easily degraded by microorganisms found in the environment. Ideally, a pesticide should survive no longer than a single growing season before being rendered harmless by the environmental flora. Often, such chemical changes require only simple modification of a compound’s basic structure.

Microplastics

A growing concern is the abundance of plastics in the environment, especially in the oceans. As plastic waste gets into waterways, it washes into the oceans, where it poses dangers of many types to aquatic creatures. The plastic breaks down into ever smaller particles, eventually reaching the stage at which it is classified as microplastic. Particles smaller than 0.2 inch (5 millimeters) in length are considered microplastics.

Phytoplankton, microscopic photosynthetic organisms, are at the base of the food web. As they absorb plastics and other POPs from the seawater, these substances accumulate in their bodies. Zooplankton, which are slightly larger organisms, consume the phytoplankton and absorb the POPs at a higher concentration. This process continues up the food web, and POPs concentrate in increasingly larger organisms.

The Arctic orca is one apex predator that is affected by bioaccumulation and biomagnification. High levels of PCBs have been found in the animals' blubber, and researchers have discovered that young orcas are consuming the contaminants in their mothers' milk.

Microplastics are present in many varieties of seafood consumed by humans. These contaminants have been found in the human body, including the placenta (in both the fetal and maternal sides) and in semen.

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