Biological accumulation, often referred to as bioaccumulation, describes the gradual build-up of substances, such as pesticides or other chemicals, within a living organism over time. This process occurs when an organism absorbs a substance at a faster rate than it can excrete or metabolize it. The continuous collection of these substances can lead to increasing concentrations within the organism’s tissues, potentially reaching levels that may impact its health.
Mechanisms of Accumulation
Substances can enter an organism through various pathways, including ingestion, dermal absorption, and respiration. Ingesting contaminated food or water is a common route, as organisms consume substances present in their diet. Aquatic organisms, for instance, can absorb chemicals directly from the water through their gills or skin.
Once inside the body, these substances are often stored in different tissues, particularly fatty tissues, because they may not be easily broken down or removed. The biological half-life of a substance, which is the time it takes for half of the substance to be eliminated from the body, also plays a role. A longer half-life means the substance persists in the organism for an extended period, increasing the likelihood of accumulation.
Common Accumulating Substances
Certain characteristics make substances more prone to biological accumulation within organisms. Persistence is a primary factor, meaning these substances do not easily break down in the environment through natural processes. Their stability allows them to remain in ecosystems for extended periods.
Many accumulating substances are also lipid-soluble, meaning they dissolve well in fats. This property allows them to readily cross cell membranes and be stored in the fatty tissues of organisms. Once stored, they are not easily metabolized or excreted.
Common examples of such pollutants include heavy metals like mercury, lead, and cadmium. Persistent organic pollutants (POPs), such as polychlorinated biphenyls (PCBs) and DDT (dichloro-diphenyl-trichloroethane), are also known to bioaccumulate. These substances often originate from industrial processes, agricultural runoff, and the combustion of various materials.
From Individual to Ecosystem: Biomagnification
While biological accumulation refers to the build-up of a substance within a single organism, biomagnification describes the increasing concentration of these substances at successively higher levels in a food chain. This distinction highlights the broader ecological impact as pollutants move through an ecosystem.
Biomagnification begins when a substance accumulated by a lower-trophic-level organism, such as plankton, is consumed by a predator. As the predator eats many contaminated prey, the substance’s concentration increases in its own tissues. This effect intensifies at each subsequent trophic level, leading to higher concentrations in apex predators.
An example is the biomagnification of DDT in aquatic food chains. DDT from agricultural runoff enters water bodies and is absorbed by plankton. Small fish consuming contaminated plankton accumulate higher levels of DDT, and larger fish eating these small fish experience an even greater concentration. Birds of prey like eagles and ospreys, at the top of this food chain, accumulate high levels of DDT, leading to detrimental effects such as eggshell thinning and population decline.
Another example involves mercury in aquatic ecosystems. Mercury released into water bodies can be converted into methylmercury, a toxic form, by bacteria. Fish absorb this methylmercury through their gills and diet. Larger predatory fish, such as shark, swordfish, and certain tuna species, accumulate higher levels of mercury due to consuming smaller contaminated fish. This poses a concern for human consumption, especially for vulnerable populations like pregnant women, due to the potential for neurological damage.