What is Bioaccumulation?
Bioaccumulation is the gradual accumulation of substances or chemicals in an organism over time. It happens when the organism absorbs substances or chemicals at a rate that is faster than the rate at which the substance or chemical is eliminated or lost by catabolism and excretion. Pollutants like heavy metals enter a food chain and accumulate in biological tissues by aquatic organisms, from sources like food, water, and particles of suspended sediment.
Bioaccumulation can also be defined as an increased concentration of a substance or chemical in a biological organism over time. Once living organisms take up substances like metals and store them faster than excreted or metabolized, accumulation occurs. Even if environmental levels of the toxin are not very high, the longer the biological half-life of a toxic substance is, the greater the risk of chronic poisoning.
Bioaccumulation occurs because the chemical is taken up faster than it can be used. Also, it occurs because these chemicals cannot be broken down for the organism to use. Meaning the chemical cannot be metabolized. Some substances that are harmful to health can accumulate in living tissues.
How does Bioaccumulation work
Toxins enter a food chain through various means. Plants take in toxins directly from the soil. For a substance to bioaccumulate, it needs to be:
- Biologically active
- Mobile and able to be taken up by the organism
When organisms that are herbivores eat contaminated plants, the toxins accumulate in their fatty tissues. Then, if a carnivorous organism eats several toxin-laden herbivorous organisms, the toxins become even more concentrated in its body. This process however continues up the food chain.
Toxicity in organisms induced by metals is associated with biomagnification and bioaccumulation. However, the storage or uptake of the metals faster than the excretion and metabolism rate results in the accumulation of the metal. Thus, the existence of various harmful substances and chemicals in the environment can be assessed with proper knowledge of bioaccumulation. In addition to helping with chemical control and usage.
Organisms take up chemicals by breathing, swallowing, or absorption through their skin. Once the chemical concentration within an organism is higher compared to the organism’s environment, it is said to be bioconcentration. Also, biomagnification is another process that relates to bioaccumulation. It involves the concentration of the chemical or substance increasing as it moves from one trophic level to another. Nevertheless, bioaccumulation is a process necessary for an organism to develop and grow but the accumulation of harmful substances can also occur.
An uptake is the entrance of a chemical into an organism. The chemical can be taken up by the organism through breathing it in, absorbing it through the skin, or swallowing it. For bioaccumulation to occur, there is an uptake of the chemical. The chemical is taken without regard to subsequent storage, metabolism, and excretion.
Bioavailability is the availability of a compound at a given time to cross the cellular membrane of an organism from the habitat of the organism.
For instance, acid and alkaline mine waters usually contain dissolved metals and metal-oxide particulates at high concentrations. Wetlands acidification can elevate the concentrations of metals. Hence increasing the potential bioavailability in freshwater biota and aquatic plants. This can however influence the uptake of metals in both rooted and submerged plants.
Bioconcentration or biological concentration is the bioaccumulation process by which the chemical concentration in an organism becomes higher than the concentration of the chemical in the environment around the organism. The process is the same for anthropogenic and natural chemicals. Though usually, the term bioconcentration refers to chemicals that are foreign to the organism.
In the field of aquatic toxicology, bioconcentration can also be defined as the process by which a chemical concentration in an aquatic organism due to exposure to waterborne chemicals exceeds that in water. Biological concentration for fish and other aquatic animals after chemical uptake through the gills or the skin is usually the most important bioaccumulation process. However, there are various ways to assess and measure bioaccumulation and bioconcentration. These ways are:
- Octanol-water partition coefficients (KOW)
- Bioconcentration factors (BCF)
- Bioaccumulation factors (BAF)
- Biota-sediment accumulation factor (BSAF)
Each of these assessments can be calculated using either empirical data or measurements from mathematical models. The fugacity-based BCF model is one of these mathematical models developed by Don Mackay.
Bioconcentration factor (BCF)
The bioconcentration factor (BCF) is calculated and expressed as the ratio of the chemical concentration in an organism to the chemical concentration in the surrounding environment. It is the ratio of the chemical concentration in a biota or an organism to the concentration in water. The bioconcentration factor (BCF) is simply a measure of the extent of chemical sharing between an organism and its surrounding environment.
The biological concentration factor in surface water is the ratio of the concentration of a chemical in an organism to the chemical’s aqueous concentration. Bioconcentration factor (BCF) is usually expressed in units of a liter per kilogram. That is the ratio of mg of chemical per kg of the organism to mg of chemical per liter of water.
The biological concentration factor (BCF) can be an observed ratio or the prediction of a partitioning model. A partitioning model is grounded on assumptions that chemicals partition between aquatic organisms and water. It is also based on the idea that chemical equilibrium exists between the aquatic environment in which the organism is found and the organism itself.
A bioconcentration factor indicates a chemical is hydrophobic or lipophilic when the BCF is greater than 1. The BCF serves as an indicator for the likelihood of a chemical to bioaccumulate. These chemicals have high lipid affinities. Hence they will concentrate in tissues with high lipid content rather than in an aqueous environment like the cytosol. However, to predict chemical partitioning in the environment, models are used. This allows the prediction of the biological fate of lipophilic chemicals.
Bioaccumulation Factor (BAF)
Bioaccumulation factor (BAF) describes the tendency of a chemical to be more concentrated in an organism than in the organism’s surrounding environment. Thus, the bioaccumulation factor (BAF) is calculated by dividing the chemical concentration in the organism by the chemical concentration in where the organism lives (soil, water, or sediment).
Bioaccumulation factors assume that exposure through all potential routes has occurred. Potential routes such as direct uptake from the environmental medium and ingestion of contaminated foods. However, bioconcentration factors and bioaccumulation factors are similar. Nevertheless, Bioconcentration factors (BCFs) are applied typically to aquatic organisms and refer to water exposures. However, bioconcentration factors (BCFs) have been used in mesic soil environments considering the pore-water transfer of chemicals to invertebrates.
Both bioconcentration and bioaccumulation show that the organism with the chemical in its environment is at a steady state. They assume net uptake and do not account for the ability or rate at which the chemical is metabolized by the organism. Moreso, the bioaccumulation factor (BAFs) or bioconcentration factor (BCFs) are often site-specific and species-specific. Hence, failing to consider species-specific toxicokinetics may result in misleading conclusions.
For assessing bioaccumulation, the bioaccumulation of a chemical is often reported by BAF. Describing the increase of contaminants like persistent organic pollutants (POPs) from water to biota due to uptake from all exposure routes.
BAF= POP biota/POP water
POP biota= the contaminant concentration in the organism corrected for the animal or plant’s lipid or organic carbon content
POP water= the dissolved contaminant concentrations in water
However, bioaccumulation factor BAFs include uptake from all exposure routes. Whereas bioconcentration factor (BCF) describes only the exposure from the abiotic environment. The BCF also describes uptake due to the equilibrium partitioning of contaminants between the organic phase in the biota and the surrounding environment.
Contaminants with bioaccumulation factor BAF or bioconcentration factor BCF that are higher than 5000 (wet weight basis) are categorized as bioaccumulative by several chemical management programs. When empirical data are not available, BAF can be estimated by empirical or mechanistic models.
Several studies have shown a relationship between BCF and BAF. Studies show, as measured by the octanol-water partitioning coefficient (KOW) a chemical’s relative solubility in lipids compared to that in water. As octanol and lipids are assumed to have similar properties. Empirical studies show a high degree of variability in BAFs, even though there is a theoretic 1:1 relationship between BAF or BCF and KOW on a logarithmic scale.
Nevertheless, for biomagnifying chemicals, BAF is usually higher than BCF. This is because dietary exposure and uptake are included in BAF. Furthermore, biomagnification factors (BMF) can describe dietary enrichment. The BMF quantifies the factorial difference in concentration of contaminant between a predator and its diet.
Examples of Bioaccumulation
- Mercury contamination through industrial emission or rain
- Bioaccumulation is a defense mechanism by some organisms
- Accumulation of some compounds to toxic levels
- Pollution of DDT
- Feeding on organisms that produce toxins naturally
- Long term exposure to noxious chemicals
Mercury contamination through industrial emission or rain
A good example of the bioaccumulation process is mercury contamination. Usually, mercury or the chemical version, methylmercury is taken up by phytoplankton and bacteria. Also through industrial emissions and rain, methylmercury gets into freshwater habitats. When small fish feed on the phytoplankton and bacteria, they accumulate mercury.
As the food chain progresses, the small fish are eaten by larger fish. These larger fish can become food for other animals and humans. As the concentration of mercury or methylmercury increases up the food web, it can reach dangerous levels for both the fish and the animals or humans that feed on fish. Feeding on these larger fish from time to time can result in the build-up of large concentrations of mercury in the tissue of animals and humans.
Fishes like sharks, swordfish, and tuna usually have bioaccumulated levels of mercury. Likewise, striped bass and bluefish sometimes have high concentrations of polychlorinated biphenyls (PCBs). This is why the federal government and some states have issued advisories against eating too much of certain fish types. Seabirds like the Atlantic puffin and Coastal fish like smooth toadfish are often monitored for heavy metal bioaccumulation.
Another example of mercury poisoning can be seen in the process of stiffening the felt used in making hats. More than a hundred years ago, this process in hat making involved mercury. The mercury forms methylmercury which is lipid-soluble and tends to accumulate in the brain. Hence resulting in mercury poisoning.
Bioaccumulation is a defense mechanism by some organisms
An organism may consume toxic animal prey or plants as a means of defense. Hence exhibiting bioaccumulation as a defense mechanism. By consuming toxic animal prey or toxic plants, the organism may accumulate the toxin. This organism then presents a deterrent to its potential predator.
A typical example is the tobacco hornworm which consumes tobacco plants. The hornworm eventually gets nicotine concentration to a toxic level in its body. Hence, poisoning of initial small consumers can be passed to affect other consumers along the food chain over time.
Accumulation of some compounds to toxic levels
Some compounds are not usually toxic but can be accumulated to toxic levels in organisms. A typical example is vitamin A. Vitamin A becomes concentrated in the livers of carnivorous animals like polar bears. The polar bear accumulates extremely large amounts of vitamin A in its livers since it is a pure carnivore that feeds on other carnivores like seals.
It was believed by the native peoples of the arctic that carnivore’s livers should not be eaten. However, arctic explorers from eating the livers of bears have suffered hypervitaminosis A. Moreso, there is one example of similar poisoning of antarctic explorers from eating canine livers, where the exploration companion of Sir Douglas Mawson died from eating the liver of one of their dogs.
Pollution of DDT
One typical example of bioaccumulation that led to biomagnification happened with an insecticide called dichlorodiphenyltrichloroethane (DDT). Before 1972 in the United States, DDT is an insecticide that was sprayed to help control mosquitoes and insects. However, the DDT was washed into creeks by rain. Eventually, it was watered down into lakes and the ocean.
The DDT bioaccumulated within each organism. Then over time, it biomagnified through the food web to very high levels. Especially in predatory birds that ate fish like osprey, bald eagles, brown pelicans, and peregrine falcons. The DDT levels were so high that the eggshells of the birds became abnormally thin. Due to the thinness of the eggshell, the adult birds broke the shells of their unhatched offspring and the offspring died. As a result of this, the population of these birds declined greatly. So, in 1972, DDT was finally banned in the United States and since then there have been significant increases in many predatory birds population.
Feeding on organisms that produce toxins naturally
Toxins that are naturally produced can also bioaccumulate. The marine algal blooms (red tides) for example can cause local filter-feeding organisms to become toxic. Filter-feeding organisms like oysters and mussels will get toxic as a result of red tide. Also, fishes in the coral reef can accumulate a toxin called ciguatoxin from feeding on reef algae. These coral reef fish can be responsible for the poisoning known as ciguatera.
Long term exposure to noxious chemicals
Also when humans are exposed to noxious chemicals for a long time, bioaccumulation can occur. Humans can be exposed to noxious chemicals in their workplaces or homes. The accumulation of a toxin at a low rate can even produce a detrimental level of the compound in the body over time. Especially, if the compound accumulates in the fatty tissues.
Lead is an example of a poisonous heavy metal that bioaccumulates in fat. This is why lead-based paints have been banned from sales in the market. However, some older homes may still contain lead-painted surfaces. Furthermore, other fat-soluble (lipid-soluble) poisons are DDT and tetraethyllead compounds (the lead in leaded petrol). These chemical compounds are stored in the body’s fat tissues. Hence, these compounds are released when the fatty tissues are used for energy, causing acute poisoning.
Bioaccumulation vs Biomagnification
Biomagnification and bioaccumulation are 2 terms that are commonly used for metal toxicity. Bioaccumulation refers to how pollutants or substances enter a food chain and relates to the accumulation of these substances in the biological tissues of the organisms (mostly aquatic organisms). It happens when the organism absorbs substances or chemicals at a rate that is faster than the rate at which the substance or chemical is eliminated or lost by catabolism and excretion.
Bioaccumulation can also be defined as an increased concentration of a substance or chemical in a biological organism over time. Once living organisms take up substances like metals and store them faster than excreted or metabolized, accumulation occurs. Even if environmental levels of the toxin are not very high, the longer the biological half-life of a toxic substance is, the greater the risk of chronic poisoning. However, understanding bioaccumulation processes is crucial in protecting human beings and other organisms from the adverse effects of toxic chemicals and metal exposure.
On the other hand, biomagnification refers to the inclination of substances or pollutants to get concentrated as they move from one trophic level to the next. This process is also known as biological magnification or bioamplification and occurs when a substance or pollutant increasingly builds up as it moves up through a food chain.
Biological amplification however is the concentration of a substance or toxin in the organism’s tissues at successively higher levels in a food chain. The increase of the toxin at successively higher levels could be as a result of persistence, food chain energetics, and a low or non-existent rate of internal degradation or excretion of the substance.
Bioaccumulation of heavy metals
Heavy metals bioaccumulate in organisms because most of them are water-insoluble and cannot be broken down by environmental processes. Some human activities contribute to the bioaccumulation of heavy metals in organisms. Anthropogenic activities such as gold mining, metal mining, industrial waste, and electronic waste can contribute heavy metals to the environment. Hence causing health issues to animals and humans. Heavy metals are toxic. Examples of heavy metals are lead, nickel, cobalt, cadmium, chromium, mercury, and tin. Also, some essential nutrients can become toxic in high doses, such as zinc, iron, and copper.
Heavy metals like cobalt, cadmium, mercury, lead, and nickel interfere with blood cells formation. Moreso some heavy metals adversely affect the kidneys, liver, nervous system, and circulatory system. Some can even cause cancer or reproductive problems. Bioaccumulation of heavy metal is a dangerous trend and needs to be curtailed. Scientists even use some plant species to draw toxins and heavy metals from contaminated soil. This process still is dangerous and unreliable because other organisms might feed on the plants. Thus bringing the toxins into the food chain to be bioaccumulated.
Bioaccumulation of toxins
Chemical toxins and pollutants that are bioaccumulated come from various sources. Pesticides, for example, are contaminants that bioaccumulate in organisms. Freshly sprayed pesticides can be watered down into creeks by rain. As a result, they eventually find their way to the ocean, rivers, and estuaries.
Chemical compounds from industrial smokestacks and automobile emissions are another major source of toxic contaminants. These compounds return to the ground in rainfall. Also, the deliberate discharge or dumping of chemical compounds into water is another source of toxic contamination.
Toxins are persistent, stable, and don’t break down over time. Unfortunately, chemicals like dioxins, PCBs, and DDT have become prevalent in our modern industrial world today, finding their way into living organisms. Since organisms are interconnected in every ecosystem through food webs and food chains. Once toxins get into an organism, they can linger and bioaccumulate. Hence bioaccumulated toxins can spread through the interconnecting food web to the whole ecosystem.
- The bioaccumulation and biomagnification of toxic substances and heavy metal can put human health at risk. Since humans eat relatively high organisms in the food web, they can get high doses of some harmful chemicals.
- Some animal species can be affected by the bioaccumulation of harmful substances. Hence distorting the ecosystem by reducing the population of predators that control prey populations and can also lead to the loss or extinction of some species.
- Bioaccumulation of some toxins can cause mutations in the physiological and morphology of animals and humans. For example, is the thinning of eggshells of predatory birds caused by bioaccumulation of DDT.
Bioaccumulation prevention and control
- Legislation is being made, banning the disposal of certain compounds in water to help reduce the level of toxic compounds.
- Microorganisms are being genetically engineered and modified to use toxic substances such as mercury as a food source. That way such genetically modified bacteria can remove the toxic compound directly from the environment.
- The federal government and some states have issued advisories against eating too much of certain fish types that have been monitored to be bioaccumulating toxic substances.
- Proper industrial waste disposal and treatment should be utilized and the use of pesticides should be reduced.