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Feature

Pollutants Leave a Mark

Biomarkers can tell us what's loose out there.

Charles Eason and Silke Rumpf

Tracing the physiological impacts of pollutants and pesticides at different levels of the food chain is a sophisticated tool in environmental monitoring. Scientists at Manaaki Whenua-Landcare Research, in collaboration with international researchers, are developing diagnostic tests, or biomarkers, that can detect biochemical and morphological (ie, relating to form, shape or structure) changes in organisms living in a contaminated environment.

Biomarkers detect sub-lethal damage to apparently healthy organisms, serving as early warnings of potentially serious impacts of pollutants on wildlife populations and communities. These same markers are also valuable for monitoring the recovery of contaminated sites. At this stage, we are identifying changes induced by contaminants in species ranging from invertebrates to birds and fish, both introduced and native species.

Rapid developments in medical and agricultural technology in the 20th century were initially heralded with enthusiasm as human and animal health improved with the introduction of drugs and pesticides. DDT, for example, was a major advance as it gave successful control of malaria and typhus. However, by the early 1960s the harmful effects of DDT and other pesticides were becoming evident.

The lone voice of Rachel Carson, warning of impending environmental disaster caused by agricultural and industrial pollution, was joined by those of many scientists and environmental groups over the following 25-30 years. Their concerns had been fuelled by conspicuous industrial and agrochemical pollution of land, water, and air, ranging from local events to contamination on a regional or even global scale.

The Alaskan oil spill, the nuclear catastrophe of Chernobyl, and the toxic effects of DDT on wildlife will not be forgotten. Acid rain, caused by air pollution in European industrial countries, has not only damaged forests and lakes in those countries but has also harmed the environment far beyond their boundaries. Less obvious and therefore less well publicised examples of soil, surface-water, and ground water contamination can be found on every continent.

Such contamination or its impact on the environment is now being studied by practitioners of a new discipline -- ecotoxicology -- that requires the collaboration of scientists trained in ecology, biology, toxicology and chemistry. The philosophical basis for toxicology, as expounded by 15th-century German physician Paracelsus, applies in new fields as it does in the more traditional areas of toxicological research:

All things are poison for there is nothing without poisonous qualities. It is only the dose which makes a thing a poison.

Present ecotoxicology focuses on the entry, distribution, and metabolism of "foreign" chemicals (xenobiotics) into ecosystems and assesses any resultant lethal and sub-lethal effects on both individual organisms and populations.

Exposure to xenobiotics, which may be man-made or naturally occurring, can lead to adaptive responses. For example, fluoroacetate is one of many plant compounds produced as defence mechanisms -- or "natural" pesticide --  against browsing mammals and insects. Where fluoroacetate occurs naturally in plants in Australia, native mammals, birds, and invertebrates have developed the ability to detoxify the compound. Some species have extremely high levels of tolerance to this poison. Possums introduced to New Zealand came from areas outside the range of fluoroacetate-bearing plants, and are very susceptible to fluoroacetate in the form of the pesticide 1080.

Ecotoxicology encompasses the impacts of artificial and naturally occurring toxins. Although people perceive a harmony between natural toxins and the environment, they are concerned that the use of man-made toxins, such as pesticides, is not part of the cycle of nature and is thus uncontrolled. Over the last three decades the incidence of over-use of such toxins has increased, resulting in water contamination, soil pollution and pest-resistance.

Biomarkers

Biomarkers, which provide detectable biological responses within living organisms to xenobiotics, can be used to monitor environmental health. The degree of biomarker change and its significance will obviously depend on the amount or severity of contamination.

In the ecotoxicology context, a biomarker is a change at the molecular or cellular level in an organism exposed to toxic chemicals. When biomarkers are interpreted together with more general features of the organism's biology, a cause-effect relationship can be established. This approach is well established for monitoring human and animal health or for diagnosing diseases.

General health indicators in clinical or veterinary medicine include appearance, temperature, blood pressure, and pulse, which on their own will not define the cause of ill-health. Blood chemistry and tissue biopsy are used for more precise diagnosis. Organ-specific enzymes released into the blood or urine have been used for many years to provide evidence of disease or organ damage by drugs and toxins. For example, the enzyme c-glutamyl aminotransferase (c-GT) is a particularly sensitive indicator of alcohol-induced liver damage. High c-GT levels in the blood may help diagnose alcohol abuse.

In the same way, biomarkers in species ranging from microbes and insects to birds and mammals can help ecotoxicologists identify unwanted chemical effects in the environment. A well documented example is the effects of organochlorine insecticide residues on egg-shell thickness, which resulted in the decline in populations of birds of prey in North America and Europe. Once the use of organochlorine was restricted, bird numbers recovered. The tremendous potential of such biomarkers for monitoring environmental health has only recently captured the attention of the international scientific community.

Evidence and Effects

Chemical analyses are an essential part of any evaluation of environmental contamination because they demonstrate the presence or absence of a particular toxin. However, they provide no information on whether there is an unwanted effect on wildlife. Biomarker responses can provide evidence of exposure to toxins at levels that exceed normal detoxification and repair capacity. These changes can then link toxin exposure to ecologically relevant effects at a population or community level. Assessing levels of contamination and the biological response to contaminants in individual organisms allows a link to be made between the two.

Biomarkers provide sensitive, precise indicators of the nature, magnitude, and cause of adverse effects at an early stage of pollution and therefore are warning signs for possible future damage to ecosystems. Biochemical or morphological changes in an organism can outlive the presence of a contaminant and they can illustrate the cumulative toxic effects of several different contaminants.

Biochemical markers can be divided into non-specific markers, induced by a wide range of contaminants, and specific markers, induced by individual chemicals. Biochemical assays or morphological changes that are specific for certain pesticides can be used to diagnose a problem without the need for chemical analysis to back up the results. Long before the term "biomarker" came into popular use, the inhibition of cholinesterase in animals was recognised as a characteristic indicator of poisoning by organophosphate or carbamate insecticides. This is particularly useful as blood samples can be taken without killing the animal involved.

Biochemical changes that can be induced by a range of pollutants include increased concentrations of mixed-function oxidases (MFO). These enzymes are present in organisms at all levels of the food chain. They are thought to have evolved in animals to meet the challenge posed by naturally occurring xenobiotics in the environment and food. Many xenobiotic substances are enzyme (MFO) inducers, including a wide range of pesticides.

We have designed a programme to develop sensitive biomarkers for environmental monitoring, and for the early prediction and prevention of ecosystem degradation. They range from molecular (involving DNA probes) to specific and non-specific enzyme changes, and indicators of cellular and sub-cellular damage. Once we have developed appropriate biomarkers we will try to correlate changes observed at this level with changes in fertility, behaviour, and population dynamics. Such new technologies will help protect the environment for humans and other animals.

Charles Eason is a scientist with Landcare Research.
Silke Rumpf is a scientist with Landcare Research.