In this article we will discuss about:- 1. Introduction to Bioaccumulation of Xenobiotics 2. Process of Bioaccumulation 3. Illustration of Biomagnification of Important Xenobiotics 4. Factors Affecting 5. Elimination 6. Important Cases.
Introduction to Bioaccumulation of Xenobiotics:
Bioaccumulation of xenobiotic compounds first gained public attention in the 1960s with the invention of DDT, DDD and methyl mercury residues in fishes and wildlife.
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Mortalities and reproductive failures in fishes and fish-eating birds were linked to unusually high concentration of DDT or its metabolites in the fatty tissues of these animals.
Since the top-level carnivores, especially birds, had higher residue concentration of these chemicals than the food they consumed, it was logical to postulate that accumulation occurred primarily by transfer through the food chain. Actually, any xenobiotic may accumulate in the biological system only when its rate of uptake exceeds the rate of elimination.
There are three related terms with the process of accumulation of xenobiotic in the organisms:
(i) Bioconcentration
(ii) Bioaccumulation, and
(iii) Bioamplification or Biomagnification.
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It refers to accumulation of xenobiotic chemicals in certain tissues of the organisms at concentrations above those of the immediate environment. The degree of bioconcentration is expressed as bioconcentration factor (BCF), which is represented as the concentration of a chemical in an organism divided by the concentration of the same chemical in the environment or an environmental component (e.g., water).
For instance, for aquatic ecosystem, the BCF may be expressed as –
BCF = (Concentration of chemical in tissue of organisms/Concentration of chemical in water)
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The BCF is mostly used specifically to predict the degree of accumulation by fishes for an organic pollutant in water.
Food is usually the major source of many xenobiotic chemicals for terrestrial species and eventually, if the rate of intake is steady, a steady state is established where one can measure the BCF –
BCF = (Steady concentration of a xenobiotic in an animal/Concentration of the xenobiotic in food)
Organisms having a high surface-to-volume ratio are apt to yield large BCF values. Since chemicals tend to bioconcentrate in fatty acids of tissues, skinny organisms with a lower fat content usually exhibit a low BCF. It is interesting to note that humans tend to have a relatively higher fat content as they age; also females have a higher body fat than males (30 – 36% compared with 18-28%). These information may be valuable while considering the accumulability of a xenobiotic chemical in humans.
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It is a wide term and refers to the accumulation of xenobiotic chemicals both from the medium and through the intake of food. Thus it is the uptake of xenobiotic chemicals dissolved/suspended in the medium and from the ingested food and sediment residues.
The major proportion of the intake of xenobiotic chemicals in case of a predator is by consumption of the preys that have already concentrated a xenobiotic. Walker and Moriarity (1987) suggested the following means for calculating the transfer levels of xenobiotics from a prey to a predator.
Supposing that a predator feeds on only one prey species and that all individuals of that prey species contain the same concentration of a xenobiotic (Q), then –
Q = (f.c)/K01
Where, f = the weight of food consumed per day
c = the concentration of xenobiotic in food
K01 = the rate of constant for excretion of unchanged compound plus metabolism per day.
K01 is closely related to the metabolic capacity of the predator. There exists some marked differences among the various vertebrate species in their capacity to metabolise xenobiotics.
Based on the efficiency and speed of metabolism the following animals, for example, can be ranked as:
Small vertebrates > large vertebrates > omnivores/ herbivores > birds > fishes.
These differences can markedly influence the xenobiotic concentrations along food-chains.
The concentration factor (the concentration of xenobiotic in the predator divided by that in its prey) that reflects the relative magnification of a xenobiotic from the prey to the predator levels can then be expressed as –
(f/K01) . (1/W) = f/(W.K01)
Where, W is the predator’s body weight.
Thus the rate of accumulation and the deleterious effects of a lipophilic xenobiotic in terrestrial species (e.g., humans) would depend mainly on the metabolic fate of the former in the latter.
It is also termed biological amplification. The xenobiotics such as mercurial salts (mercuric chloride), beryllium and lead compounds, aluminium cans, phenolic compounds, pesticides like DDT etc., after getting their way into the environment, either do not degrade or degrade very slowly and are referred as non-biodegradable pollutants.
These not only accumulate but are often biologically magnified or amplified – While energy decreases and becomes more dispersed at each trophic level in the food chain, some non-degradable pollutants become more concentrated with each link in a process that has now come to be known as “biological magnification or biological amplification”.
In simple words, the process of increase in the concentration of xenobiotic along the food chain may be termed as biological magnification or biomagnification. Actually biomagnification is a broader term which refers to the entire process of bioconcentration and bioaccumulation.
Many heavy metals and most organic insecticides concentrate biologically in the food chain. The tendency of a xenobiotic to accumulate in the tissue of the organism is given by bioconcentration factor (BCF) which is represented in L/kg.
BCF may also be expressed as:
BCF = (concentration of the xenobiotic in the organism, mass/vol)/(concentration of the xenobiotic in the environment, mass/vol)
The BCF varies from species to species, and is affected by the metabolism and elimination system in the organisms. It is important to mention that the BCFs of most organic pesticides are much higher than those of inorganic pesticides. Further, it may be noted that BCFs are higher in aquatic ecosystems than in terrestrial ones.
The BCFs of certain organic and inorganic xenobiotics in fishes are:
Chromium = 16 l/kg
Arsenic = 14 l/kg
Lead = 49 l/kg
Chlordane = 14,000 l/kg
DDT = 54,000 l/kg
Process of Bioaccumulation:
In principle form, the bioaccumulation takes place when the rate of uptake of xenobiotic exceeds the rate of elimination.
The process of uptake of xenobiotics from water, sediment and food have been worked out more in the aquatic systems, hence the general process of bioaccumulation applicable for the aquatic systems is being described below:
1. Uptake of Chemicals from Water:
Many workers have reported direct uptake of xenobiotics from aquatic media by various organisms inhabiting in water (such as algae, annelids, insects, molluscs and fishes).
The description of mechanism of uptake process of xenobiotics from water is concentration-based on following three transport processes:
(i) Diffusion
(ii) Special transport, and
(iii) Adsorption.
Many xenobiotics enter the body of organisms by the process of diffusion. It is a physical process and occurs across permeable or semipermeable barriers against a concentration gradient (Δc). It does not require expenditure of energy.
Gills of the fishes are much vulnerable against xenobiotic insults. The lipid bilayers of semipermeable membranes of gills and lining of mouth and G.I. tract of fishes permit the rapid passage of lipophilic xenobiotics.
Weakly acidic and basic substances pass across biological membranes in unionised form. The water and other tiny ions up to moleculer weight of 100 pass through proteinous pores located in the biological membrane. Considerable proportion of xenobiotics have been found to passively diffuse across general body surface of various hexapods. The plasma proteins are known to facilitate the transport of xenobiotics into the blood.
Special transport of xenobiotics across biological membranes include both active and facilitated transport. The active transport takes place against concentration gradient and requires energy to move the xenobiotics.
On the other hand, facilitated transport, does not require energy input and it does not concentrate xenobiotics against any concentration gradient. Metals have been found to be accumulated by both these transport processes. Low water salinity enhances the rate of uptake of many chemical substances.
(iii) Adsorption:
It is also a physical process – a surface phenomenon. The adsorption of xenobiotics to the various adsorbents is a result of van der Walls’ dipole-dipole interactions or coulombic forces between them. Xenobiotics are often absorbed either on the body surface of aquatic insects or directly to the gill surface of other aquatic organisms by covalent, electrostatic and molecular forces. It is particularly important as initial step in the accumulation process.
2. Bioavailability of Xenobiotic in Water:
The uptake of the xenobiotic chemicals by the living forms largely depends upon their availability in the dissolved form in the aquatic media. It becomes obvious that the factors affecting the concentration of xenobiotics may also affect their uptake by the animals.
The notable processes impairing bioavailability of xenobiotics in aquatic environment are adsorption to suspended solids, sediments, humic acids and other macromolecules, formation of colloidal suspensions, chelation, complexation and ionisation. Suspended particulates and adsorbents such as humic acids reduce the uptake of lipophilic xenobiotics by reducing their concentration in water.
3. Uptake of Xenobiotics from Food:
Various xenobiotic chemicals absorbed through the gill surface and the integument are also readily absorbed by the G.I. tract by similar mechanisms of diffusion and transport. The lipophilic chemical compounds present in food are efficiently absorbed owing to long-term contact between the food and the membranes. Weak acids and bases are generally absorbed in unionised form. The pH of the stomach favours the diffusion of weak acids whereas the pH of the intestine favours the absorption of neutral or weakly basic xenobiotics. Uptake of metals from food depends on their forms viz., free or bound.
Illustration of Biomagnification of Important Xenobiotics:
1. Pesticides (DDT as an Example):
There are some established instances revealing the phenomenon of biomagnification of pesticides in developed countries, and Paul Miller won the Nobel Prize in 1948 for its (DDT) synthesis. However though its use is still widespread in developing countries, its use in the U.S.A. is banned since 1973.
Salient example of biological magnification of DDT in an aquatic system is outlined below, where concentration of DDT continuously increased from water to birds:
Wordwell et al (1967) studied DDT accumulation in a typical food chain of aquatic ecosystem.
The results are shown in Table 22.1:
The end result of DDT use is that entire population of predatory birds such as the fish hawk (osprey) and of detritus feeders as fiddler crabs are wiped out. Birds are more vulnerable as DDT interferes with their egg shell formation by causing a breakdown in steroid hormones which results in fragile eggs that break before the young can hatch.
Besides pesticides, there are also some metals like mercury, lead, copper etc. that show a similar behaviour of biomagnification in food chain. Both inorganic and organic forms of mercury are found in industrial effluents, polluting chiefly the water bodies. Mercury pollution due to methyl mercury is a global problem.
Methyl mercury was responsible for the Minamata epidemic in Japan and Sweden.
Mercury is a very stable heavy metal having a half-life of 18.2 years — a very long period of time that provides ample opportunity of its getting picked up by living organisms.
Inorganic mercury released into the environment is converted into more toxic methyl mercury compounds by the action of certain anaerobic bacteria; this transformation occurs in the sediments and bottom muds of waterways.
Under anaerobic conditions, Clostridium cochlearium, and, under aerobic conditions, Pseudomonas sp. and a fungus neurospora crass convert Kg to more toxic monomethyl mercurials. Alkaline conditions favour the conversion of monomethyl mercury to volatile dimethyl mercury which could subsequently contaminate the media; under mild acidic conditions, dimethyl mercury is converted back to non-volatile methyl mercury, which may then be concentrated by aquatic biota and magnified through the food-chain.
Because of its affinity to tissue components of living organism, Hg rapidly binds with tissues and stays there for a long time. It accumulates in tissues like liver and muscles. It is soluble in both polar and apolar fractions.
The biomagnifications of Hg takes place along the following food-chain:
The amount of Hg accumulated by fishes does not harm it. But man eating such contaminated fishes regularly accumulates more Hg in his tissues which, when it reaches the threshold level, starts producing toxicity. Only methyl mercury form of mercury is persistent and accumulates in food-chain.
Mercury concentration in blood and brain of the affected fetus has been reported to be about 20% higher than mother. Methyl mercury penetrates through placenta. Hg poisoning is caused due to inactivation of several sulfhydryl enzymes by replacement of hydrogen atoms in sulfhydryl groups.
Similar to pesticides and metals, the radioactive substances such as P-32, Strontium-90, Iodine-131, Caesium-137 etc. tend to accumulate in body tissues and lead to biomagnification. As an example, P-32 was found to be concentrated as much as 5,000 times above levels in the water in white fish downstream from the Hanford Atomic Power Plant on the Columbia River.
Blue gills and crappies in the same river water were found to contain 20,000-30,000 times the concentration of P-32, while filamentous algae were found to have up to 100,000 times the water level of P-32. The concentration of P-32 increases from river water to egg yolk of ducks and geese (about two million times more than in water).
Fe-59, Zn-65 and I-131 are the other radioactive isotopes that can be concentrated by the living organisms. These are the sources of radiation exposure to the general public when certain foods are eaten. Fishes may contain P-32, oysters and clams may contain Zn-65 and milk may contain I-131.
Concentration factors for Cs-137 is 250 in muscle and for Sr-90, it is 500 in bones of water birds as compared to concentration of these radio nuclei in water.
Concentration factors for Sr-90 in various components of an aquatic food web near an atomic power plant are shown in Fig. 22.2:
Factors Affecting Biomagnification:
Biomagnification or bioamplification of an environmental xenobiotic chemical depends on the following factors:
i. Physical properties of xenobiotics.
ii. Duration of the persistence of the xenobiotic molecules in the environment.
iii. Stability of the xenobiotic chemicals in the environment.
iv. Availability of the primary pickers in the environment.
v. Affinity of the toxicants to various tissue components of the living organisms.
vi. Availability of the next trophic level organism.
vii. Number of the living organisms picking the xenobiotic molecules at different trophic levels in any ecosystem.
viii. Amount of food intake by organisms.
ix. Texture of the aquatic environment.
x. Body size and lipid content of the tissues of the organisms. In principal form — more the lipid content and the size of the organism, more is the accumulation of xenobiotics — and, consequently, more is the chance of biomagnification or bioamplification.
xi. Rate of elimination of xenobiotics by the living organism.
Elimination of Xenobiotics:
Elimination of xenobiotics in the vertebrates may occur through various routes viz., transport across integument or respiratory surfaces, excretion from kidney and via biliary route.
(i) Elimination of Xenobiotics through Gills:
Elimination of nonpolar xenobiotics in fishes takes place through gills. Various xenobiotics such as weak electrolytes, weak bases, insecticides like DDT and pentachlorophenol are principally eliminated through gills of fishes.
(ii) Elimination of Xenobiotics by Liver and Gall Bldder:
Liver is the principal organ for biocatalytic conversion of lipophilic xenobiotics into their hydrophilic forms. The biotransformed toxicants are then transported from liver to the gall bladder. The gall bladder discharges these biotransformation products of xenobiotics with the bile into the small intestine and, ultimately, these are eliminated through feces. Various xenobiotics like Hg, Pb, As etc. are generally transported into the blood and excreted in the bile.
(iii) Elimination of Xenobiotics through Kidney:
Kidney usually eliminates hydrophilic xenobiotics of low molecular weight — either by glomerular filtration or by diffusion or by secretory processes of tubules. Xenobiotics bound to plasma proteins are retained by the glomeruli and, hence, are not eliminated unless they become free.
Important Cases of Biomagnification of Xenobiotics:
Mercury pollution, especially due to methyl mercury, is a global problem. Mercury, a byproduct of the production of vinyl chloride, is used in many chemical industries. It is also a byproduct of some incinerators, power plants and laboratories. In Japan, illness and even death occurred in the 1950s among fishermen who consumed fishes, crabs and shell-fish contaminated with methyl mercury from the industrial city of Minamata in Japan.
The main unit of this city, Shin-Bihon Chisso Hiryo Co., began to produce vinyl chloride and acetaldehyde by the catalytic conversion of acetylene, since 1949. During this process, some of the HgCl2 catalyst was unknowingly converted to methyl mercury. This factory kept ejecting its effluent wastes into the Minamata Bay of Kyushu in Japan during 1953-1961, and at Nigata, also in Japan, in 1965. The people who took the fishes, crabs, and shell-fish suffered from Minamata disease. In Japan, in 1952, due to this disease 17 people died and 23 were disabled permanently.
Further, in Sweden also, many rivers and lakes became contaminated due to widespread use of mercury compounds as fungicides and algaecides in paper and pulp industries, and in agriculture. Chloral alkali plants were the chief source of mercury containing effluents. Methyl mercury is stable, persistent and accumulates in food-chain. It is soluble in lipids and, thus, after being taken by animals, it accumulates in fatty tissues.
Fish may accumulate the methyl mercury ions directly. There may be nearly 3,000 times more mercury in fish than in water. Swedish fish-eaters have been reported to have high mercury content in their blood.
The symptoms of Minamata include numbness, visual disturbances, dysphasia, ataxia, mental deterioration, convulsions and, finally, death. Mercury readily penetrated the CNS of children born in Minamata causing teratogenic effects. In drosophilla, methyl mercury (0.25 ppm) treatment brought chromosomal disjunction in gametes.
It is a case of Cd (cadmium) biomagnification. It was first reported in Japan. People at Toyana, Prefecture, Japan, consumed Cd-contaminated fishes during 1940-1960 and fell victim to this disease. Cadmium binds with tissue protein in living organisms and accumulates.
It is a case of biomagnification of PCBs. PCBs are commonly used in electrical capacitors, paints, transformers, heat-transfer fluids and varnishes. Polychlorinated biphenyl (PCBs) are lipophilic and highly stable xenobiotics. These accumulate in the fatty tissues of the living organisms.
PCB magnification has been reported in bald eagles and human beings in the following manners:
PCBs → Crustaceans Fishes → Human
Or
PCBs → Crustaceans → Small birds → Eagle
Or
PCBs → Crustaceans → Eagle
It reveals that both man and eagle — being on the top of the food-chain — accumulate maximum amount of PCBs. PCBs are excreted with breast milk. High concentration of PCBs impairs reproduction and proves fatal for bald eagles.
Nausea, vomiting, weakness, anorexia, fatigue, numbness of extremities and abnormalities in pigmentation are the principal symptoms due to PCB intoxication. In children, PCB produces abnormal tooth formation.