In this article we will discuss about:- 1. Definition of Ecotoxicology 2. Ecotoxicological Episodes 3. Scientific Approach 4. Entry, Movement and Fate of Pollutants in Ecosystems.
Definition of Ecotoxicology:
Ecotoxicology can be defined as “the study of harmful effects of chemicals upon ecosystems”. This implies that ecotoxicology is concerned not only with the detections of chemicals per se, but with biological effects of toxic chemicals that contaminate or have contaminated the environment. These biological effects may be anything from a molecular effect in a species to the effects on the biosphere as a whole.
Public concern about global and principal aspects of environmental pollution has shed new light on the role of ecotoxicology in the society. It is interdisciplinary in nature and includes experimental and theoretical methods from related sciences like biology, chemistry, toxicology, geology, geography and even physics.
Ecotoxicology perhaps started with the green wave in 1960s so as to reveal the effects of toxic substances in the environment. The first ecotoxicological model emerged in late 1970s and the first conference on the development of ecological models was held in 1980 in Copenhagen.
Subsequently, application of ecotoxicological models in environmental management of toxic substances quickly became a powerful tool. The range of available ecotoxicological models is very wide with respect to chemical compounds, environmental problems and the ecological component and processes involved. Toxicological processes are a concern relevant for all structural and functional levels within and around ecosystems.
Some toxicological processes occurring naturally are described below:
A. At the Species Level (All Biotic):
Plants may use toxic substances in competing with each other. Plants can use toxic substances against some animals. Animals can use toxic substances to hunt their prey. Fish and amphibians use poisonous secretions for defense. Some algal blooms in oceans and wetlands can produce poisonous chemicals.
i. Some forest systems can alter the soil surface in such a manner that many other species cannot germinate.
ii. Eutrophication of water bodies can kill many species by oxygen depletion.
iii. Salinization of lowlands in semi-deserts can render such places uninhabitable for plants and animals.
Ecotoxicological Episodes:
i. DDT:
Perhaps most widely discussed case of biological damage as a result of anthropogenic chemicals is the relationship between application of p, p’ -DDT (dichloro diphenyl trichloroethane; systematic name: 1,1, I-trichloro- 2, 2-di -(4-chlorophenyl)-ethane) as insecticide and a dramatic population decline of the peregrine -falcon (Falco peregrinus) in the 1950s and 1960s. In USA DDT was first synthesized in 1874 by the German Chemist Othmar Zeidler, and its insecticidal activity was detected in 1939 by Paul Muller, who was honoured for this finding with the Nobel prize for medicine in 1946.
From 1940 to 1970, DDT was used in high volumes throughout the world and proved its merits in the control of malaria and typhus. It had reached annual levels of around 100,000 t in 1963. One major cause for the observed population decline of the falcons was their reduced breeding success, which could be traced back to a significant eggshell thinning caused by DDT and its metabolite DDE in the bird.
Uptake of DDT occurred with contaminated prey species, illustrating that persistent insecticides may well reach and affect non-target animals at higher trophic levels through bioaccumulation along the food chain.
The relatively high persistence of DDT is associated with its lipophilicity, which in turn enables the compound to penetrate insect cuticles much better than animal skins and thus makes it appear safe and target-specific for infield applications, DDT is neurotoxic and interferes with the axonic membrane of the nervous system, and DDT as well as DDE were shown to impair the thyroid function.
Analysis of falcon eggs revealed substantial concentrations of DDE, and correlations between eggshell thickness and DDE burdens suggest that this metabolite is the active agent. There is evidence that the eggshell thinning is caused by inhibition of the enzyme Ca2+– ATPase, but the molecular level of the toxic mechanism has not been fully unraveled.
In 1972, DDT production and application were banned in the USA and Germany, and nowadays DDT is still being used in developing countries in estimated annual volumes of 40,000 t. Due to its high stability under environmental conditions, substantial levels of DDT and DDE can be found in environmental compartments all over the world, and recent investigations suggest DDE to act as a potent antagonist of the androgen receptor.
The DDT story with the peregrine falcon thus illustrates how complex and unexpected the ecotoxicological effect of xenobiotics may be, and how much information on various levels of biological organization and about the toxic agent itself is necessary to detect and understand such events. It also shows how difficult it is to develop ecotoxicological knowledge for a predictive assessment of potential environmental damage due to xenobiotics.
Another well-known example of ecotoxicological effects with a clear chemical cause at the system level is given by the acidification of lakes by acid rain. Major sources of acid rain are anthropogenic emission of sulfur oxides and nitrogen oxides, which undergo hydrolyzation and oxidation in the troposphere and return to the ground as H2SO3, H2SO4, and HNO3, respectively, dissolved in raindrops. Uncontaminated rain already has a pH of ca. as a result of dissolution of background, airborne CO2, and acid rain may reach pH values of 4 and below.
In Scandinavian lakes and those of the Great Lakes area of Canada and the USA, acidic deposition has led to death of aquatic organisms and corresponding reductions in species diversity, disruption of normal food-chain relations, and shifts to greater abundance of acidophilic species.
A common geogenic feature of these lakes is their relatively low buffer capacity, which makes them particularly susceptible to a drop in pH due to the uptake of airborne acids. The resultant ecotoxicological effects could be traced back to (at least) two components: acidic water is directly toxic for biota perturbating the osmoregulation caused by disruption of the trans-epithelial electrochemical gradient, and acidification of lake water leads to mobilization of heavy metals with subsequent uptake and more specific toxic effects in aquatic organisms.
A typical example is the generation of toxic Al3+ by acidic hydrolysis of Al(OH)3, a natural component of sediments and soils. In contrast to the DDT example outlined above, the primary action of the chemical contaminant (the acid) is the alteration of an important physicochemical characteristic (the pH) of the abiotic environmental compartment, and the associated ecotoxicological effect results from a superposition of chemical stress by the primary agent as well as by other mobilized toxicants.
Much work has been done to identify ecological consequences in the acidified lakes. In the northwestern of Ontario in Canada, the effect of pH on aquatic communities has been studied in long-term ecosystem-level experiments with poorly buffered small lakes. Artificial acidification was achieved by gradual addition of sulfuric acid to decrease the lake pH to 5.0 over a period of eight years.
Small species with high reproduction rates and wide dispersal powers, such as phytoplankton, turned out to be most sensitive at the early stage of acidification, and other early indicators of acidic stress were provided by morphological abnormalities in benthic invertebrates. On the other hand, global ecosystem functions, such as primary production, nutrient cycling and respiration, remained essentially unaffected by acidification of the lake water.
A third and somewhat different example of a causative relationship between chemical pollution and environmental damage in the field is the depletion of stratospheric ozone by anthropogenically emitted chlorofluorocarbons (CFCs). Manufacture of CFCs started in the 1930s, and their physicochemical property profile and lack of toxicity made them highly suitable as refrigerants.
Annual worldwide production reached levels of around 1.2 million tons in the 1980s, and it can be assumed that most anthropogenic CFCs migrate upwards into the stratosphere within 10 years of their initial emission. First warnings about a potential depletion of the ozone layer came in the 1970s, and since the late 1980s there is convincing evidence that stratospheric ozone loss beyond diurnal and seasonal variation patterns has indeed occurred.
The mechanism of ozone removal by CFCs starts with photolytic generation of chlorine radicals, which then participate in the catalytic degradation cycle of stratospheric ozone. Typical residence times for the relevant CFCs of around 50 to 100 years indicate that even after a complete stop of CFC release into the atmosphere, reduction of the ozone layer would continue for years.
One important function of stratospheric ozone is its absorption of UV-B radiation (290-320 nm wavelength), through which it protects humans from skin cancer, and animals and crops from general UV damage. It follows that increased transmission of the UV -B component of sunlight due to depletion of the ozone layer is likely to affect a variety of species, including microorganisms.
Current discussion focus on potential deleterious effects on krill in the Antarctic with subsequent implications for associated food chains. Another matter of concern is the potential sensitivity of crop plants to increased UV-B radiation, which may also make the plant more susceptible to other natural and anthropogenic stressors. Finally, depletion of stratospheric ozone might lead to climatic changes with a corresponding impact on the global ecosystem.
These scenarios demonstrate that although CFCs do not act directly on biological systems, their interference with the ozone layer may well have dramatic indirect effects on the population, community and ecosystem level. Because the initial stressor would be electromagnetic radiation and thus non-chemical, the CFC problem would not fall within the focus of classical ecotoxicology. However, the potentially hazardous consequences for biological systems could still be traced back to interferences with natural ecosystems.
During the 1960s and 70s, the Minamata Bay mercury pollution disaster received global media attention, opening the world’s eyes to the negative health effects of methyl mercury. Between 1932 and 1968, the Japanese Chisso Corporation discharged about 27 tonnes of methyl mercury with its wastewater into the bay.
The pollution caused severe damage to the central nervous system of the people who ate large quantities of contaminated fish and shellfish from the bay. In addition, congenital Minamata disease occurred as many infants were born with a condition resem-bling cerebral palsy caused by methyl mercury poisoning of the foetus during pregnancy. The disease, which was officially recognized on 1 May 1956, caused many people to lose their lives or suffer from physical deformities.
After the cause of the disease was finally confirmed, a number of measures were implemented, ranging from regulation of the factory effluent, voluntary restrictions on harvesting of fish and shellfish from the bay, installation of dividing nets to enclose the mouth of the bay and prevent the spread of contaminated fish, and dredging of mercury-containing sediments.
It was only in October 1997 that the dividing nets that had closed off the bay for 23 years were removed. After several studies confirming that mercury levels in fish were below regulatory levels and had remained so for three years, Minamata Bay was reopened as a general fishing zone.
Till 1992, 2,252 people were diagnosed with” Minamatc Disease”, with 1,043 deaths reported.
Scientific Approach to Ecotoxicology:
Ecotoxicology is shared by several disciplines of science including chemistry, biochemistry, microbiology, biotechnology, ecology, toxicology, anthropology, wildlife and forestry etc. Currently, there is growing concern internationally about human health.
Scope of ecotoxicology in relation to a few scientific disciplines is discussed below:
i. Chemistry:
Chemical approach to ecotoxicology is often analytical. Quick screening of a toxicological effect is essential. For example Ames test for testing mutagenicity and related carcinogenicity in selected bacteria can be used for polyaromatic hydrocarbon, benzo (a) pyrene. Such mutagenicity due to DNA damage may manifest as cancer in mammalian organs. Ames test is widely used in biological monitoring practices today.
The main principle of biochemistry that can be applied to ecotoxicology is the modern concept of enzyme regulation by metabolites of xenobiotics. Many xenobiotics can interfere with the finely balanced biochemical reactions of living cells by disturbing the complex web of molecular interactions necessary for life.
Additional problems are DNA damage and disruption of cell membrane by reactive oxygen species generated by reactions between chemicals and atmospheric molecular oxygen.
Microbial communities are responsible for the transformation and recycling of organic and inorganic molecules in the environment. This activity results into the maintenance of nutrient cycles and soil fertility. Toxic substances too in the environment are detoxified by microorganisms. Other symbiotic interactions between microorganisms and plants that are of crucial importance for plant growth and nutrition are dinitrogen fixing microorganisms which have close associations with roots of many plant species.
Furthermore, microorganisms associated with the gut of animals provide essential vitamins that animals cannot synthesize by themselves. On the negative side microorganisms are responsible for causing a wide variety of diseases in plants and animals. The interdependence between microorganisms and higher life forms means that toxins that effects either group of organisms can have important consequences for the other.
Biotechnology is an applied science that aims to harmers different life forms for the benefit of man. Agriculture, antibiotic production, and bioremediation are examples of such applications. With the advance of biological sciences comes the responsibility for biotechnologists to ensure that the integrity of the environment is maintained by ensuring that the biological functions that allow life on this planet to thrive are not impaired.
Interestingly, recombinant technology has also opened the way to construct organisms that can be used to monitor the environmental impact of toxicants. The creation of super bug by Indian born scientist Anand Chakrobroty that selectively feeds on hydrocarbons has been used to clean oil spills in the sea. The development of Bt cotton is another step taken to protect cotton plants from pests. Several other examples of bioremediation support a relationship between biotechnology and ecotoxicology.
Important principles of toxicology that relate to ecotoxicology include the concepts of extent of exposure, persistence, and distribution of chemicals in the environment. Subsequently predictions can be made on the toxicity of such chemicals to individual organisms or populations.
The starting point of such analysis is often the chemical structure of each toxin, which will allow some prediction of the behavior of the chemical in the environment to be made. One of the best examples of this kind is the environmental impact assessment of the insecticide DDT. DDT is almost completely insoluble in water but readily soluble in fat.
Furthermore, DDT and its degradation product DDE are highly persistent. Its low solubility means that it is easily dispersed in aquatic environments (in the case of DDT it is found at low concentrations all over the globe) and only accumulates in places with a high fat content (i.e., living organisms).
Once it has entered living organisms, it persists and accumulates in the fat tissues of organisms that are higher up the food chain resulting in toxicity. For this reason populations of both fish eating birds and bird eating raptors such as peregrines and hawks were badly affected by DDT, even in cases where they inhabited pristine environments.
Another aspect of toxicology is risk analysis. This quantitative topic is most easily studied when death rates within a population can be quoted. Whereas the rates can be determined experimentally for animals, plants, and microorganisms, the determination of the effects on the human population is inevitably more difficult to determine, and one of the few situations where effects are well known is the mutagenic effects of radio-nucleotides, resulting from contamination caused by accidents at nuclear power installations or nuclear warfare.
Entry, Movement and Fate of Pollutants in Ecosystems:
Organic and metallic (including radionuclide) toxicants that enter the environment are regarded as pollutants of air, soil, and water. Their movement and trans-port is through air and water depending on their volatility and solubility, respectively. Soil and the detritus of sediments provide important solid supports for adsorption to regulate the movement and flow of the pollutants through terrestrial ecosystems, as well as influencing the localization and persistence of the pollutants in the environment.
In this respect clay and humus particles that are electrostatically charged have an important role to play. Some organisms will take up the pollutant and concentrate them in their cells, a phenomenon known as bioaccumulation or biomagnification. This is particularly serious where the toxicant enters the food chain leading to progressive accumulation of toxic molecules higher up the chain, including humans.
This is often the case with molecules that are relatively biologically inert such as heavy metals, PCBs, and organochlorine insecticides. Accumulation of such molecules in different species can lead to toxicity ex-pressed as reduced growth, reduced fecundity, changes in behavior, susceptibility to diseases, or increased mortality.
However, on the positive side, where there is storage of the pollutant in the cell, as is normally the situation with (heavy) metals, it is possible that the pollutant might be harvested and therefore removed from the environment. This especially applies to plants that hyper accumulate metals in their tissues. The application of this process for the cleanup of contaminated land is known of phytoremediation.
Also, many pollutants are biologically degradable and can therefore be detoxified. Both plants and mammals have a range of enzymes that are involved in detoxification of molecules that are potentially harmful to them (cytochromes P-450, for example), whereas the almost limitless metabolic capacity of a wide variety of microorganisms allows degradation of pollutants, especially hydrocarbons, into nontoxic molecules such as water and carbon dioxide.
The stimulation of microbial degradation and metabolism of pollutants is known as bioremediation and is currently used to clean up contaminated land while microbial degradation of pollutants present in sewage and water is used in a variety of water purification systems. The fate of pollutants in the environment is therefore not only dependent on the molecular characteristics of the pollutant but also on a range of biotic and abiotic factors.