What is Bioaccumulation?
Bioaccumulation is a biological process by which organisms, often at the lower end of the food chain, accumulate certain substances or chemicals at a rate greater than they are metabolically able to excrete. This process typically involves the accumulation of substances like heavy metals, pesticides, and pollutants in living organisms. The term "bioaccumulation" itself is a fusion of "bio," which pertains to life, and "accumulation," which signifies the buildup of substances over time.
Mechanism of Bioaccumulation
Bioaccumulation is driven by several interconnected mechanisms that allow substances to move through ecosystems and accumulate in various organisms.
1. Contaminant Entry
The first step in the bioaccumulation process is the introduction of a contaminant into an ecosystem. This can occur through various means, including:
Runoff: Chemicals from agricultural fields can wash into nearby water bodies.
Industrial Discharge: Factories may release pollutants directly into the environment.
Atmospheric Deposition: Some contaminants settle from the atmosphere onto land or water surfaces.
2. Uptake by Primary Producers
Once a contaminant is present in an ecosystem, it often comes into contact with primary producers. In aquatic ecosystems, this could be phytoplankton, while in terrestrial ecosystems, it might be plants. These organisms absorb the contaminants from their surroundings, either through water or soil.
3. Transfer Up the Food Chain
As primary producers are consumed by herbivores and herbivores, in turn, are consumed by carnivores, the contaminants enter the food chain. This transfer of substances from one organism to another is a key aspect of bioaccumulation. Each step up the food chain leads to an accumulation of contaminants.
4. Biomagnification
Bioaccumulation doesn't stop with the initial uptake by primary producers. It intensifies as you move up the food chain and affect the biodiversity. This phenomenon is known as biomagnification. Predatory species at the top of the food chain accumulate more significant amounts of these substances due to their consumption of numerous contaminated prey.
Factors Affecting Bioaccumulation
1. Chemical Properties of Contaminants
The nature of the contaminants themselves is a critical factor affecting Bioaccumulation
Persistence: Contaminants that resist degradation over time tend to accumulate more.
Lipophilicity: Substances that are fat-soluble tend to accumulate in organisms with higher fat content.
Toxicity: Highly toxic compounds often accumulate in organisms more readily.
2. Biological Factors
The biology of the organism plays a significant role in bioaccumulation. Different species have varying abilities to metabolize and excrete contaminants.
Metabolism: Species with slower metabolisms may accumulate substances more readily.
Excretion Rate: Organisms with efficient excretion mechanisms can reduce bioaccumulation.
Feeding Habits: Carnivores may accumulate more contaminants compared to herbivores due to the concentration of substances up the food chain.
3. Environmental Conditions
The environment in which an organism resides can greatly influence bioaccumulation.
Temperature: Warmer temperatures can accelerate biochemical processes, affecting both metabolism and the degradation of contaminants.
pH Levels: The acidity or alkalinity of an environment can impact the solubility and mobility of contaminants.
Presence of Other Substances: The coexistence of other substances can either enhance or inhibit bioaccumulation. Some substances may compete with contaminants for uptake.
Examples of Bioaccumulation
1. Mercury in Fish
Mercury
One of the most well-known examples of bioaccumulation involves the bioaccumulation of mercury in aquatic ecosystems. Mercury is released into the environment through various industrial processes, including coal burning. When mercury enters water bodies, it undergoes chemical transformations, eventually turning into methylmercury, a highly toxic form.
Mechanism
Small aquatic organisms, such as plankton and small fish, absorb this methylmercury from the water. As larger predatory fish consume these smaller organisms, they accumulate higher concentrations of methylmercury. This bioaccumulation continues up the food chain, with apex predators like sharks and large tuna accumulating significant levels of methylmercury.
Consequences
The consequences of this bioaccumulation are severe. Consumption of fish with high mercury levels can lead to mercury poisoning in humans, causing neurological damage, developmental issues in children, and various health problems.
2. PCBs in Bald Eagles
Polychlorinated biphenyls (PCBs)
Polychlorinated biphenyls (PCBs) are another example of bioaccumulative substances. PCBs were widely used in electrical equipment and various industrial applications in the past. Despite being banned in many countries, these chemicals persist in the environment due to their resistance to degradation.
Mechanism
Bald eagles, apex predators in their ecosystems, have been heavily impacted by PCB bioaccumulation. These chemicals find their way into water bodies, where they are absorbed by fish. Bald eagles, being fish-eating birds, consume these contaminated fish.
Consequences
Over time, PCBs accumulate in the bodies of bald eagles, leading to reproductive issues such as thin eggshells and reduced hatching success. This bioaccumulation has contributed to the decline of bald eagle populations in some areas.
3. DDT and the Peregrine Falcon
Dichlorodiphenyltrichloroethane (DDT)
DDT is a synthetic pesticide that was widely used in agriculture and for controlling disease-carrying insects. However, it was discovered that DDT had detrimental effects on wildlife and the environment.
Mechanism
When DDT is applied to crops, it can run off into nearby water bodies, where it accumulates in aquatic organisms. The peregrine falcon, a bird of prey, was severely impacted by DDT bioaccumulation. These birds feed on smaller birds that had consumed DDT-contaminated insects.
Consequences
As DDT accumulated in the peregrine falcons' bodies, it led to thinning of their eggshells, causing reproductive failure. Efforts to ban DDT and protect peregrine falcon populations ultimately led to the recovery of these birds, highlighting the importance of addressing bioaccumulation.
Bioaccumulation is a complex process that has significant effects on the ecosystems. Understanding the mechanisms behind bioaccumulation is crucial for addressing environmental concerns and ensuring the health of our planet.
References
Imhoff, J. & Clough, Jonathan & Park, Richard & Stoddard, Andrew & Hayter, Earl. (2004). Evaluation of chemical bioaccumulation models of aquatic ecosystems: final report. US Environmental Protection Agency, Athens, GA.