Bioconcentration and biomagnification of radionuclides of biochemically-significant elements.
Anti-nuclear-energy activists often talk about the potential for biomagnification of radioactive nuclides in the environment as the consequence of any, even extremely dilute, releases of such radionuclides into the environment.
Consider the following claims, typical of such views, for example:
“Tritium is also more dangerous when it becomes organically bound in molecules of food. As such it is incorporated into molecules, including DNA within bodily cells. Chronic exposure to contaminated food causes 10% of the tritium to become organically bound within the body where it has a biological half-life of 21 to 550 days meaning that it can reside in the body from one to twenty-five years.”
“When tritium is released to the environment, it is taken up by plants and trees, partially incorporating into the ecosystem. Trees constantly transpire water vapor into the air; it has been found that higher concentrations of tritium occur at night at breathing height in a forest that has incorporated tritium from a nearby reactor.”
(Extracts from Nuclear Power is Not the Answer)
So, does bioconcentration of tritium in the food chain occur?
Bioconcentration or biomagnification of tritium in the form of tritiated water in biological systems does not occur at all, although it can occur with some other radionuclides. This is due, in part, to the relatively small biological half-life for water in biological systems, and the large concentration of light water in the environment, which results in massive isotopic dilution of tritium entering the ecosphere. Tritium is simply hydrogen, and your body cannot tell one hydrogen atom from the next – whilst large volumes of water are constantly being taken into the body, and passed out of the body, the total amount of hydrogen within ones body remains essentially constant, as is also the case for the various other chemical elements which make up the human body.
Biomagnification is the buildup of certain chemical pollutants in the bodies of organisms at higher trophic levels of food webs – the bioaccumulation of a substance up the food chain by transfer of residues of the substance in smaller organisms that are food for larger organisms in the food web. It generally refers to the sequence of processes that results in higher concentrations in organisms at a higher trophic level – further “up the food chain”. Organisms at lower trophic levels accumulate small amounts. Organisms at the next higher level eat many of these lower-level organisms and hence accumulate larger amounts. These processes result in an organism having higher concentrations of a substance than is present in the organisms prey or food. Biomagnification can result in higher concentrations of the substance than would be expected if water were the only exposure mechanism. Accumulation of a substance only through contact with water is known as bioconcentration.
Bioaccumulation is a general term for the accumulation of such substances in an organism or part of an organism. The process of bioaccumulation involves the biological sequestering of substances that enter the organism through respiration, food intake and/or other routes of absorbtion of the substance. Such sequestering results in the organism having a higher concentration of the substance than the concentration in the organisms surrounding environment. The level at which a given substance is bioaccumulated depends on the rate of uptake, the route of uptake, how quickly the substance is eliminated from the organism, transformation of the substance by metabolic processes, the lipid (fat) content of the organism, the hydrophobicity of the substance, environmental factors, and other biological and physical factors. As a general rule the more hydrophobic a substance is the more likely it is to bioaccumulate in organisms, such as fish. Increasing hydrophobicity (lipophilicity) leads to an increasing propensity towards bioaccumulation.
A related term is bioconcentration. Bioconcentration is a process that results in an organism having a higher concentration of a substance than is in its surrounding environmental media, such as stream water. Bioconcentration differs from bioaccumulation because it refers only to the uptake of substances into the organism from water alone. Bioaccumlation is the more general term because it includes all means of uptake into the organism.
Biomagnification, or bioamplification, occurs within a trophic level, and is the increase in concentration of a substance in an organisms tissues due to uptake from food and sediments in an aquatic milieu, wheras bioconcentration is defined as occurring when uptake from the environment is greater than the rate of excretion. Where bioaccumulation refers to how pollutants enter a food chain; biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next, up the “food chain.” Whilst bioaccumulation refers to an increase in concentration of a pollutant from the environment to the first organism in a food chain, biomagnification refers to an increase in concentration of a pollutant from one link in the food chain to another.
We are traditionally concerned about these phenomena because together they mean that even small concentrations of toxic substances in the environment can find their way into organisms in high enough dosages to cause problems. In order for biomagnification to occur, the pollutant must be long lived, fat-soluble, mobile, and biologically active – i.e. toxic. If a pollutant is short-lived, it will be broken down before it can become dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms. If the pollutant is soluble in water it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time. Lipid soluble (lipophilic) substances cannot be excreted in urine, an aqueous solution, and so accumulate in fatty tissues of an organism if the organism lacks enzymes to degrade them. When eaten by another organism, fats are absorbed in the gut, carrying the substance, which then accumulates in the fats of the predator. Since at each trophic level of the food chain there is an energy loss, a predator must consume lots of prey, and therefore consumes significantly larger amounts of any biomagnifying lipophilic substance consumed by the prey organism.
There are two main groups of toxic substances that that are subject to biomagnification – toxic metals and persistent halogenated organic compounds. Both are lipophilic and not easily degraded. Novel organic substances are not easily degraded because organisms lack previous exposure and have thus not evolved specific detoxification and excretion mechanisms, as there has been no selection pressure from them. These substances are consequently known as persistent organic pollutants, and include the synthetic organic chlorine compounds which are today well-known for their potential for biomagnification and environmental harm, such as the insecticide DDT.
Heavy metals are chemically stable because they are chemical elements, and therefore cannot be destroyed or converted into a non-toxic form. (Except for the case of a radioactive metal, which will change into a differerent chemical element when it undergoes radioactive decay.) Organisms, particularly those subject to naturally high levels of exposure to metals, have mechanisms to sequester and excrete metals. Problems arise when organisms are exposed to higher concentrations than usual, which they cannot excrete rapidly enough to prevent damage. These metals are transferred in an organic form.
A classic example of a toxic heavy metal is mercury, which forms organic species such as methylmercury, which is lipid soluble, and can easily biomagnify in environmental systems. Other toxic transition metals – the so-called “heavy metals” – can be subject to biomagnification to some degree, too – for example, the toxic metal cadmium. Since biochemical behavior is independent of what the isotopic composition of the metal is, a radionuclide, such as the low-yield fission product , perhaps, is subject to a potential for biomagnification just like any other cadmium. For example, though mercury is only present in small amounts in seawater, it is absorbed by algae, generally as methylmercury. It is efficiently absorbed, but only very slowly excreted by organisms. Bioaccumulation and biomagnification result in buildup in the adipose tissue of successive trophic levels: zooplankton, small nekton, larger fish etc. Anything which eats these fish also consumes the higher level of mercury the fish have accumulated. This process explains why predatory fish such as swordfish and sharks or birds like osprey and eagles have higher concentrations of mercury in their tissue than could be accounted for by direct exposure alone. For example, herring contains mercury at approximately 0.01 ppm and shark contains mercury at greater than 1 ppm.
Now, let’s talk about radionuclides. radioactive nuclides such as, say, hydrogen-3, carbon-14, iodine-131 or strontium-90, for example, can certainly be uptaken by living organisms – including, but not limited to, humans. Tritium (hydrogen-3) is almost always present in the environment in the form of water – and, of course, every living thing uptakes water from its environment, so, if there is present in the water – keep in mind that all water has some naturally occurring in it – then will be absorbed by the organism. Now, remember – it is just hydrogen.
The biochemistry of cells does not care in the slightest what nuclide a particular atom of hydrogen or a particular element is – it is only concerned with the chemistry of the material. Therefore, the will be used by the cells of the organism, and incorporated into tissues and biomolecules along with every other hydrogen atom that the organism has uptaken. Since an atom of tritium is just another hydrogen atom, of course it is exchanged into and incorporated into some hydrogen-containing biomolecules – in other words, just about any organic molecule found in a biological system. However, water is constantly being excreted from living systems as well as constantly being uptaken – a cell of Escherichia coli contains 70% water by mass, a human body 60-70%, plant tissue up to 90% and the body of an adult jellyfish is made up of 94 to 98% water. Now, the proportion of water in an organism such as these is constant – except for a small difference due to the growth of the organism, the rate of water uptake into an organism is equal to the rate of water excretion, and whilst hydrogen – which could be , or , it doesn’t matter – is constantly being moved between water molecules and more complex biomolecules, and it is constantly being excreted – both in the form of water and in the form of more complex biomolecules.
However, some tritium taken up into the body could be incorporated into a biomolecule in the tissues of the body, and it could remain there for some time – but as more stable hydrogen is constantly being exchanged through the body in large quantities, the concentration of tritium within the body from any given intake will decay exponentially, just as the metabolism and excretion of, say, a drug taken into the body follows an exponential decay law – hence, we speak of the biological half-life.
The overall amount of hydrogen per unit mass of a living organism is essentially constant. If there is a constant environmental source of a radionuclide such as tritium – such as the natural cosmogenic formation of tritium – then the overall amount of tritium per unit mass of a living organism is essentially constant.
Exactly the same argument applies to radionuclides of other biologically active elements – like, say, carbon-14. is constantly uptaken from the atmosphere by plants in the form of , and incorporated into the organic biomolecules within the tissues of the plant as the plant grows – and when an animal eats the plant, the -containing biomolecules are metabolised by the animal. As with hydrogen nuclides, the is constantly being turned over between living systems and the ecosphere, as organic compounds are excreted by the animals – for example in the exhalation of . Again, the overall concentration of within the tissues of a living system is held constant – this is how carbon-14 dating works! If the concentration of within a living organism wasn’t constant, then obviously -dating of once-living materials would be impossible.
The same arguments apply to, say, strontium, or iodine – whilst a radionuclide such as strontium-90 certainly can be uptaken into the human (or animal) body, and can be used in osteogenesis and incorporated into bone because of the chemical similarity between and , the overall concentration of calcium – calcium and strontium combined, for that matter – within the body is maintained at a certain static level. When iodine – some of which could be contaminated with, say, radioactive iodine-131 – is uptaken by the body, iodide ions are used by the thyroid gland in the biosynthesis of the iodine-containing thyroid hormones, thyroxine and triiodothyronine. But, of course, the concentration of thyroxine and triiodothyronine in the blood is kept around a fairly homeostatic value – whilst some iodine is essential for the biosynthesis of these hormones and is thus essential for health, it does not accumulate arbitrarily in the body, either in the form of the iodine-containing hormones or as free iodide ion.
Obviously we know that such chemical elements – radionuclides or not – are uptaken extensively by biological systems; but that is not the same thing as biomagnification – do they bioaccumulate or biomagnify?
Clearly, they do not.
It has in fact been determined empirically in Perch Lake in Canada that there is a progressive decline in the concentration of cobalt-60 and strontium-90 as they are transferred to higher trophic levels. In other words, predators, such as carnivorous fish, have lower concentrations of cobalt-60 and strontium-90 per unit body weight than do forage fish, insects and plants, as a result of biological discrimination against uptake of these elements of limited biochemical usefulness. It is now generally agreed that the same principle is valid for most other radionuclides, with a few exceptions, such as tritium, soluble potassium-40 (which occurs naturally, not as a result of anthropogenic nuclear technology), and caesium-137, which is a chemical analog of potassium, which are incorporated without biodiscrimination into all living organisms. Most radionuclides are not subject to biomagnification up the food chain, thus differing them from organic pollutants such as DDT. This is of significant importance in the assessment of radioactive releases into the environment.
Editor’s note: As you may note, I’ve just discovered WordPress’s built-in LaTeX engine. Unfortunately, it looks a little bit awkward in the above post, as you can see, with the TeX-formatted text much bigger than the surrounding text. I’ll have to see if there’s a way around that in future.