Bioaccumulation

Bioaccumulation is the gradual build up over time of a chemical in a living organism. This occurs either because the chemical is taken up faster than it can be used, or because the chemical cannot be broken down for use by the organism (that is, the chemical cannot be metabolized).

Bioaccumulation need not be a concern if the accumulated compound is not harmful. Compounds that are harmful to health, such as mercury, however, can accumulate in living tissues.

Chemical pollutants that are bioaccumulated come from many sources. Pesticides are an example of a contaminant that bioaccumulates in organisms. Rain can wash freshly sprayed pesticides into creeks, where they will eventually make their way to rivers, estuaries, and the ocean. Anther major source of toxic contaminants is the presence of compounds from industrial smokestacks and automobile emissions that return to the ground in rainfall. Deliberate discharge of compounds into water is another source of chemical pollutants.

Once a toxic pollutant is in the water or soil, it can easily enter the food chain. For example, in the water, pollutants adsorb or stick to small particles, including a tiny living organism called phytoplankton. Because there is so little pollutant stuck to each phytoplankton, the pollutant does not cause much damage at this level of the food web. However, a small animal such as a zooplankton might then consume the particle. One zooplankton that has eaten ten phytoplanktons would have ten times the pollutant level as the phytoplankton. As the zooplankton may be slow to metabolize or excrete the pollutant, the pollutant may build up or bioaccumulate within the organism. A small fish might then eat ten zooplankton. The fish would have 100 times the level of toxic pollutant as the phytoplankton. This multiplication would continue throughout the food web until high levels of contaminants have biomagnified in the top predator. While the amount of pollutant might have been small enough not to cause any damage in the lowest levels of the food web, the biomagnified amount might cause serious damage to organisms higher in the food web. This phenomenon is known as biomagnification.

Mercury contamination is a good example of the bioaccumulation process. Typically, mercury (or a chemical version called methylmercury) is taken up by bacteria and phytoplankton. Small fish eat the bacteria and phytoplankton and accumulate the mercury. The small fish are in turn eaten by larger fish, which can become food for humans and animals. The result can be the build up (biomagnification) of large concentrations of mercury in human and animal tissue.

One of the classic examples of bioaccumulation that resulted in biomagnification occurred with an insecticide called dichlorodiphenyltrichloroethane (DDT). DDT is an insecticide that was sprayed in the United States prior to 1972 to help control mosquitoes and other insects. Rain washed the DDT into creeks, where it eventually found its way into lakes and the ocean. The toxic pollutant bioaccumulated within each organism and then biomagnified through the food web to very high levels in predatory birds such as bald eagles, osprey, peregrine falcons and brown pelicans that ate the fish. Levels of DDT were high enough that the birds' eggshells became abnormally thin. As a result, the adult birds broke the shells of their unhatched offspring and the baby birds died. The population of these birds plummeted. DDT was finally banned in the United States in 1972, and since that time there have been dramatic increases in the populations of many predatory birds.

The bioaccumulation and biomagnification of toxic contaminants also can put human health at risk. When humans eat organisms that are relatively high in the food web, we can get high doses of some harmful chemicals. For example, marine fish such as swordfish, shark, and tuna often have bioaccumulated levels of mercury, and bluefish and striped bass sometimes have high concentrations of polychlorinated biphenyls (PCBs). The federal government and some states have issued advisories against eating too much of certain types of fish because of bioaccumulated and biomagnified levels of toxic pollutants.

Advances are being made in efforts to lessen the bioaccumulation of toxic compounds. Legislation banning the disposal of certain compounds in water helps to reduce the level of toxic compounds in the environment that are capable of being accumulated in the food chain. As well, microorganisms are being genetically engineered so as to be capable of using a toxic material such as mercury as a food source. Such bacteria can directly remove the compound from the environment.