Bioconcentration is the accumulation of a chemical in or on an organism when the source of chemical is solely water.
Bioconcentration is a term that was created for use in the field of
aquatic toxicology
Aquatic toxicology is the study of the effects of manufactured chemicals and other anthropogenic and natural materials and activities on aquatic organisms at various levels of organization, from subcellular through individual organisms to communit ...
.
Bioconcentration can also be defined as the process by which a chemical concentration in an aquatic organism exceeds that in water as a result of exposure to a waterborne chemical.
There are several ways in which to measure and assess
bioaccumulation and bioconcentration. These include:
octanol-water partition coefficient
The ''n''-octanol-water partition coefficient, ''K''ow is a partition coefficient for the two-phase system consisting of ''n''-octanol and water. ''K''ow is also frequently referred to by the symbol P, especially in the English literature. It is a ...
s (K
OW), bioconcentration factors (BCF), bioaccumulation factors (BAF) and biota-sediment accumulation factor (BSAF). Each of these can be calculated using either
empirical data
Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
or measurements as well as from
mathematical models
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used in the natural sciences (such as physics, ...
.
One of these mathematical models is a
fugacity
In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas whic ...
-based BCF model developed by
Don Mackay
Donald Scrimgeour Mackay (born 19 March 1940) is a Scottish former professional football player and manager.
Managerial career
After a spell coaching the Bristol City youth team between 1974 and 1978, where he was part of the backroom staff th ...
.
Bioconcentration factor can also be expressed as the ratio of the concentration of a
chemical
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...
in an
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
to the
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number concentration'', ...
of the chemical in the surrounding
environment
Environment most often refers to:
__NOTOC__
* Natural environment, all living and non-living things occurring naturally
* Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
. The BCF is a measure of the extent of chemical sharing between an organism and the surrounding environment.
In surface water, the BCF is the ratio of a chemical's concentration in an organism to the chemical's aqueous concentration. BCF is often expressed in units of liter per kilogram (ratio of mg of chemical per kg of organism to mg of chemical per liter of water).
BCF can simply be an observed ratio, or it can be the prediction of a partitioning model.
A partitioning model is based on assumptions that chemicals partition between water and aquatic organisms as well as the idea that chemical equilibrium exists between the organisms and the aquatic environment in which it is found
Calculation
Bioconcentration can be described by a bioconcentration factor (BCF), which is the ratio of the chemical concentration in an organism or biota to the concentration in water:
Bioconcentration factors can also be related to the octanol-water partition coefficient, K
ow. The octanol-water
partition coefficient
In the physical sciences, a partition coefficient (''P'') or distribution coefficient (''D'') is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. This ratio is therefore a comparison of the solub ...
(K
ow) is correlated with the potential for a chemical to
bioaccumulate
Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...
in organisms; the BCF can be predicted from log K
ow, via computer programs based on
structure activity relationship (SAR)
or through the
linear equation:
Where:
at equilibrium
Fugacity capacity
Fugacity and BCF relate to each other in the following equation:
where Z
Fish is equal to the
Fugacity capacity
The fugacity capacity constant (Z) is used to help describe the concentration of a chemical in a system (usually in mol/m3Pa). Hemond and Hechner-Levy (2000) describe how to utilize the fugacity capacity to calculate the concentration of a chemical ...
of a chemical in the fish, P
Fish is equal to the density of the fish (mass/length
3), BCF is the partition coefficient between the fish and the water (length
3/mass) and H is equal to the
Henry's law
In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. It was formulate ...
constant (Length
2/Time
2)
Regression equations for estimations in fish
Uses
Regulatory uses
Through the use of the
PBT Profiler and using criteria set forth by the
United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
under the
Toxic Substances Control Act
The Toxic Substances Control Act (TSCA) is a United States law, passed by the 94th United States Congress in 1976 and administered by the United States Environmental Protection Agency (EPA), that regulates chemicals not regulated by other U. ...
(TSCA), a substance is considered to be not bioaccumulative if it has a BCF less than 1000, bioaccumulative if it has a BCF from 1000 to 5000
and very bioaccumulative if it has a BCF greater than 5,000.
The thresholds under
REACH are a BCF of > 2000 L/kg bzw. for the B and 5000 L/kg for vB criteria.
Applications
A bioconcentration factor greater than 1 is indicative of a
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
or
lipophilic
Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such non-polar solvents are themselves lipo ...
chemical. It is an indicator of how probable a chemical is to
bioaccumulate
Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated ...
.
These chemicals have high lipid affinities and will concentrate in tissues with high lipid content instead of in an aqueous environment like the
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
. Models are used to predict chemical partitioning in the environment which in turn allows the prediction of the biological fate of lipophilic chemicals.
Equilibrium partitioning models
Based on an assumed steady state scenario, the fate of a chemical in a system is modeled giving predicted endpoint phases and concentrations.
It needs to be considered that reaching steady state may need a substantial amount of time as estimated using the following equation (in hours).
[Hawker D.W. and Connell D.W. (1988), Influence of partition coefficient of lipophilic compounds on bioconcentration kinetics with fish. Wat. Res. 22: 701–707, doi: 10.1016/0043-1354(88)90181-9.]
For a substance with a log(K
OW) of 4, it thus takes approximately five days to reach effective steady state. For a log(K
OW) of 6, the equilibrium time increases to nine months.
Fugacity models
Fugacity is another predictive criterion for equilibrium among phases that has units of pressure. It is equivalent to partial pressure for most environmental purposes. It is the absconding propensity of a material.
BCF can be determined from output parameters of a fugacity model and thus used to predict the fraction of chemical immediately interacting with and possibly having an effect on an organism.
Food web models
If organism-specific
fugacity
In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas whic ...
values are available, it is possible to create a food web model which takes
trophic webs into consideration.
This is especially pertinent for
conservative chemicals
Conservatism is a Philosophy of culture, cultural, Social philosophy, social, and political philosophy that seeks to promote and to preserve traditional institutions, practices, and values. The central tenets of conservatism may vary in r ...
that are not easily metabolized into degradation products.
Biomagnification
Biomagnification, also known as bioamplification or biological magnification, is any concentration of a toxin, such as pesticides, in the tissues of tolerant organisms at successively higher levels in a food chain. This increase can occur as a ...
of conservative chemicals such as toxic metals can be harmful to
apex predators
An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own.
Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the highest trophic lev ...
like
orca whales,
osprey
The osprey (''Pandion haliaetus''), , also called sea hawk, river hawk, and fish hawk, is a diurnal, fish-eating bird of prey with a cosmopolitan range. It is a large raptor reaching more than in length and across the wings. It is brown o ...
, and
bald eagles
The bald eagle (''Haliaeetus leucocephalus'') is a bird of prey found in North America. A sea eagle, it has two known subspecies and forms a species pair with the white-tailed eagle (''Haliaeetus albicilla''), which occupies the same niche as ...
.
Applications to toxicology
Predictions
Bioconcentration factors facilitate predicting contamination levels in an organism based on chemical concentration in surrounding water.
BCF in this setting only applies to aquatic organisms. Air breathing organisms do not take up chemicals in the same manner as other aquatic organisms. Fish, for example uptake chemicals via
ingestion
Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in a substance through the mouth into the gastrointestinal tract, such as through eating or drinking. In single-celled organisms ingest ...
and osmotic gradients in
gill lamellae
Lamellae on a gecko's foot.
In surface anatomy, a lamella is a thin plate-like structure, often one amongst many lamellae very close to one another, with open space between. Aside from respiratory organs, they appear in other biological roles ...
.
When working with
benthic macroinvertebrates
Benthos (), also known as benthon, is the community (ecology), community of organisms that live on, in, or near the bottom of a seabed, sea, river, lake, or stream, also known as the benthic zone. , both water and
benthic
The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...
sediments may contain chemical that affects the organism. Biota-sediment accumulation factor (BSAF) and
biomagnification factor
Biomagnification, also known as bioamplification or biological magnification, is any concentration of a toxin, such as pesticides, in the tissue (biology), tissues of tolerant organisms at successively higher levels in a food chain. This increas ...
(BMF) also influence toxicity in aquatic environments.
BCF does not explicitly take metabolism into consideration so it needs to be added to models at other points through uptake, elimination or degradation equations for a selected organism.
Body burden
Chemicals with high BCF values are more lipophilic, and at equilibrium organisms will have greater concentrations of chemical than other phases in the system. Body burden is the total amount of chemical in the body of an organism,
and body burdens will be greater when dealing with a lipophilic chemical.
Biological factors
In determining the degree at which bioconcentration occurs biological factors have to be kept in mind. The rate at which an organism is exposed through respiratory surfaces and contact with dermal surfaces of the organism, competes against the rate of excretion from an organism. The rate of excretion is a loss of chemical from the respiratory surface, growth dilution, fecal excretion, and
metabolic biotransformation
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
.
Growth dilution is not an actual process of excretion but due to the mass of the organism increasing while the contaminant concentration remains constant dilution occurs.
The interaction between inputs and outputs is shown here:
The variables are defined as:
C
Bis the concentration in the organism (g*kg
−1).
t represents a unit of time (d
−1).
k
1 is the rate constant for chemical uptake from water at the respiratory surface (L*kg
−1*d
−1).
C
WD is the chemical concentration dissolved in water (g*L
−1).
k
2,k
E,k
G,k
B are rate constants that represent excretion from the organism from the respiratory surface, fecal excretion, metabolic transformation, and growth dilution (d
−1).
Static variables influence BCF as well. Because organisms are modeled as bags of fat, lipid to water ratio is a factor that needs to be considered.
Size also plays a role as the surface to volume ratio influence the rate of uptake from the surrounding water.
The species of concern is a primary factor in influencing BCF values due to it determining all of the biological factors that alter a BCF.
Environmental parameters
Temperature
Temperature may affect metabolic transformation, and bioenergetics. An example of this is the movement of the organism may change as well as rates of excretion.
If a contaminant is ionic, the change in pH that is influenced by a change in temperature may also influence the bioavailability
Water quality
The natural particle content as well as organic carbon content in water can affect the bioavailability. The contaminant can bind to the particles in the water, making uptake more difficult, as well as become ingested by the organism. This ingestion could consist of contaminated particles which would cause the source of contamination to be from more than just water.
References
External links
PBT ProfilerRuth "The Hammer" SofieldPersistent Organic PollutantsUnited States Environmental Protection Agency
{{DEFAULTSORT:Bioconcentration Factor
Toxicology
Environmental science
Aquatic ecology
Chemistry
Environmental toxicology