What is Bioaccumulation?
Bioaccumulation is the net accumulation of a chemical substance within an organism when the rate of uptake from the environment and food is greater than the rate at which the substance is metabolized, transformed, or eliminated.
In simple terms, an organism may continuously take up a contaminant from food, water, sediment, soil, or air. If the organism cannot eliminate the substance efficiently, its concentration can gradually increase within the body.
How Does Bioaccumulation Occur?
Bioaccumulation can occur when contaminants enter an organism through different exposure routes. Important routes include:
- Ingestion: Consumption of contaminated food, water, soil, or sediment.
- Respiratory uptake: Absorption of contaminants from air or water through respiratory surfaces such as lungs or gills.
- Dermal uptake: Absorption of certain chemicals through the skin or other external surfaces.
Bioaccumulation is particularly important for substances that are persistent and difficult for organisms to metabolize or eliminate. Many lipophilic substances can accumulate in fatty tissues, whereas some metals and other contaminants may accumulate through different biological mechanisms.
Bioaccumulation vs. Biomagnification
Bioaccumulation and biomagnification are related concepts, but they describe different processes.
| Aspect | Bioaccumulation | Biomagnification |
|---|---|---|
| Definition | Net accumulation of a substance within an individual organism. | Increase in the concentration of a substance at higher trophic levels of a food chain or food web. |
| Scale | Occurs within a single organism. | Occurs across trophic levels. |
| Main factor | Uptake exceeds elimination or metabolic transformation. | Contaminant is transferred through feeding relationships and becomes more concentrated at higher trophic levels. |
| Time/position | Can increase during the lifetime of an organism. | Associated with movement through successive trophic levels. |
| Example | A fish gradually accumulates methylmercury from its diet and surrounding water. | Methylmercury concentration can become higher in large predatory fish than in the organisms they consume. |
Examples of Bioaccumulating Substances
Many different contaminants can bioaccumulate under suitable environmental and biological conditions. Important examples include persistent organic pollutants and certain metals.
- Mercury, especially methylmercury: Methylmercury can accumulate in aquatic organisms and reach relatively high concentrations in predatory fish.
- DDT and its metabolite DDE: DDT is a persistent organochlorine pesticide. DDE is particularly associated with eggshell thinning in several species of birds of prey.
- Polychlorinated biphenyls (PCBs): Persistent industrial chemicals that can accumulate in organisms and aquatic food webs.
- Per- and polyfluoroalkyl substances (PFAS): A large group of persistent fluorinated substances. Certain PFAS can persist in organisms and the environment.
- Dioxins and furans: Persistent environmental contaminants that can accumulate in organisms, particularly those associated with fatty tissues.
- Some organochlorine pesticides: Certain persistent organochlorine compounds can accumulate in organisms and food webs.
- Certain heavy metals: Metals such as cadmium (Cd), lead (Pb), and mercury (Hg) can accumulate in organisms, although their biological behavior differs from that of persistent organic pollutants.
- Microplastics: Microplastics can be ingested by aquatic and terrestrial organisms. Some particles may be retained for periods of time, while others can be eliminated. Their potential for bioaccumulation and transfer through food webs depends on particle properties and biological and environmental conditions.
Real-World Examples
1. Minamata Disease
Minamata disease was associated with industrial mercury pollution in Minamata, Japan. Mercury released into the environment was converted by microorganisms into methylmercury, which entered aquatic food webs.
Methylmercury accumulated in aquatic organisms and became highly concentrated in some predatory fish and shellfish. People who consumed contaminated seafood were exposed to high levels of methylmercury, resulting in severe neurological effects. Prenatal exposure could also cause serious developmental neurological impairment.
2. DDT and Birds of Prey
DDT was widely used as an insecticide during the twentieth century. Because DDT and its metabolites are persistent and lipid-soluble, they can accumulate in organisms and move through food webs.
The metabolite DDE became particularly important because exposure was associated with eggshell thinning in several species of birds of prey. Reduced eggshell strength contributed to reduced reproductive success and population declines in affected bird populations.
DDT residues → insects → small birds → predatory birds
Why Is Bioaccumulation Important?
Bioaccumulation is important because some contaminants can remain in an organism for long periods and may reach concentrations that can affect biological functions. The effects depend on the chemical, concentration, duration of exposure, organism, and susceptibility.
Aquatic food webs provide important examples. A contaminant present at a relatively low concentration in water may be taken up by aquatic organisms and transferred through feeding relationships. Persistent contaminants may then accumulate in larger organisms, particularly when elimination is slow.
Bioaccumulation in Humans
Humans can also accumulate certain environmental contaminants through food, drinking water, air, and other exposure routes. Dietary exposure is particularly important for contaminants associated with aquatic and terrestrial food chains.
For example, some species of large predatory fish can contain higher concentrations of methylmercury because they consume many contaminated organisms over their lifetime. This is one reason why health authorities provide advice about the types and amounts of fish that different groups should consume.
Fish can be an important part of a healthy diet, but advice about fish consumption should take into account the species, size, preparation, local contamination levels, and the individual's age or pregnancy status. Follow current recommendations from the relevant health or food-safety authority for your region, particularly regarding fish species with higher mercury concentrations.
Factors Affecting Bioaccumulation
| Factor | Effect on Bioaccumulation |
|---|---|
| Persistence | Persistent substances are more likely to remain in the environment and organisms for longer periods. |
| Lipophilicity | Lipophilic substances may preferentially partition into lipid-rich tissues and may be eliminated slowly. |
| Exposure concentration | Greater exposure can increase the amount of a substance taken up by an organism. |
| Exposure duration | Long-term exposure can allow a contaminant to accumulate when uptake exceeds elimination. |
| Metabolism and elimination | Rapid metabolism or elimination can reduce accumulation, whereas slow elimination can promote it. |
| Age and feeding behavior | Age, diet, habitat, and trophic position can influence exposure and contaminant concentration. |
Summary
Bioaccumulation is the net buildup of a chemical substance within an individual organism when uptake exceeds elimination. It can occur through ingestion, respiratory uptake, dermal exposure, or other routes. Persistent contaminants such as methylmercury, PCBs, certain PFAS, DDT residues, and some metals can accumulate under suitable conditions.
Biomagnification is different: it describes the increase in contaminant concentration at higher trophic levels of a food chain or food web. Both processes are important for understanding how persistent environmental contaminants move through ecosystems and may affect wildlife and humans.