Bioaccumulation

Bioaccumulation refers to the gradual build-up of chemical substances such as pesticides, metals or other environmental contaminants within the tissues of an organism. This process occurs when the rate at which a substance is absorbed exceeds the rate at which it is metabolised or excreted. Substances with long biological half-lives pose a particularly high risk, as even low environmental concentrations may lead to chronic toxicity over time. Understanding bioaccumulation is essential for assessing ecological risk, regulating chemical usage and evaluating environmental contamination.

Processes and Mechanisms

Organisms absorb chemicals through respiration, ingestion or dermal contact. When internal concentrations exceed those of the surrounding environment, the process is termed bioconcentration. In many cases, contaminants may become increasingly concentrated at successive trophic levels—known as biomagnification. While some accumulation of natural nutrients is necessary for growth and development, harmful compounds may similarly accumulate with severe physiological effects.
Chemical characteristics, including lipid solubility and resistance to metabolic breakdown, strongly influence bioaccumulation potential. Biotransformation processes within organisms may either reduce toxicity or, in some cases, convert contaminants into more harmful forms. Metals illustrate this principle clearly: storage or uptake faster than excretion results in rising concentrations and associated toxicity.

Terrestrial Examples

Historical occupational poisoning associated with mercury gave rise to the expression “mad as a hatter”. Mercury compounds used in felt manufacture formed lipid-soluble species such as methylmercury, which accumulate in the brain and trigger neurological damage. Other lipophilic toxins, including tetraethyllead from leaded petrol and the pesticide DDT, accumulate in adipose tissue and may be released rapidly during fat breakdown.
Radioisotopes can also accumulate. Strontium-90, a component of nuclear fallout, mimics calcium and becomes incorporated into bone, delivering long-term radiation exposure.
Certain organisms exploit bioaccumulation for defence. For example, the tobacco hornworm (Manduca sexta) sequesters nicotine from its plant diet, deterring predators. Some carnivores accumulate high levels of vitamin A in liver tissue; polar bear livers, for example, can contain toxic concentrations capable of causing hypervitaminosis A in humans. Several Arctic explorers suffered poisoning after consuming the livers of dogs or wild carnivores.

Aquatic Examples

Bioaccumulation is especially significant in aquatic systems, where contaminants in water and sediments readily enter food webs. Coastal fish such as smooth toadfish, and seabirds like Atlantic puffins, are routinely used as bioindicators of heavy-metal contamination. Methylmercury, originating from industrial emissions and atmospheric deposition, accumulates in freshwater organisms and biomagnifies through food chains, posing risks to both wildlife and humans.
Commonly tested species in laboratory assessment include common carp, rainbow trout and bluegill sunfish. Fish absorb contaminants via gill membranes or through diet; lipid-soluble pollutants tend to accumulate most rapidly.
Toxins produced naturally within marine ecosystems can also bioaccumulate. Red-tide algal blooms generate harmful compounds taken up by filter-feeding organisms such as mussels and oysters. Reef-associated fish may accumulate ciguatoxin from algae, causing ciguatera poisoning in humans.
In nutrient-rich water bodies, biodilution may occur, where pollutant concentrations decrease with rising trophic level due to the high biomass of algae and bacteria diluting contaminant burdens. Environmental changes such as ocean acidification can, however, increase metal bioavailability and enhance uptake by aquatic plants and animals.

Turtles as Model Species

Turtles serve as valuable model organisms for studying bioaccumulation because they inhabit stable home ranges and readily reflect local environmental contaminant profiles. Both marine and freshwater turtles accumulate synthetic organic pollutants such as PFAS, trace elements and heavy metals including mercury, cadmium and selenium. These substances enter waterways through industrial discharge and atmospheric deposition, subsequently being taken up by aquatic plants and sediments that form part of turtle diets.
Once contaminants enter the bloodstream and tissues, concentrations can increase to levels that disrupt metabolic, endocrine and reproductive functions. Marine turtles are particularly accessible for sampling due to their use of shoreline habitats, enabling the collection of blood, tissue and egg samples.

Developmental Effects in Turtles

Toxic substances can be transferred from adult turtles to their eggs, compromising embryonic development. In the Australian short-neck turtle (Emydura macquarii), PFAS compounds accumulated by females were passed to eggs, altering metabolic processes, fat storage and gut microbiomes in hatchlings. Similarly, high concentrations of heavy metals have been shown to reduce hatching success in species such as the Amazon River turtle. Metal toxicity can reduce fat reserves and disrupt water regulation within eggs, lowering survival rates.

Ecological and Environmental Significance

Bioaccumulation plays a central role in ecological risk assessment and environmental management. By analysing contaminant levels in organisms, researchers can identify pollution sources, evaluate ecosystem health and inform regulatory decisions. Bioaccumulation is also closely linked with biomagnification, a key concept in understanding the movement of pollutants through ecosystems and their ultimate impact on top predators, including humans.