Ecotoxicological concepts and applications

 The combination of the disciplines ecology and toxicology evolved into another integrative discipline known as ecotoxicology. The science of ecotoxicology is an outgrowth of the link between toxicology, ecology and chemistry. Environmental toxicological studies focus on the nature, properties, effects and detection of toxic substances in the environment and in any environmentally exposed species. The term ecotoxicology was first introduced by Truhaut in 1969 as a natural extension of toxicology. The major difference between toxicology and ecotoxicology is that toxicology deals with the effects of poisons on individual organisms whereas ecotoxicology deals with the effects on population of individuals.

 Aim of ecotoxicology

The aim of ecotoxicology centered on determining the effects of pollutants on the structure and functions of intact ecosystems, communities or assemblages. Ecotoxicologist is one who uses ecological parameters to assess the effects of toxic substances on ecosystems.

Scope of ecotoxicology

- to generate data that will be useful for risk assessment and environmental management.

-to meet legal requirements for regulating the development, manufacture or release of potentially dangerous substances.

- develop empirical or theoretical principles to further understanding of the behavior and effects of chemicals on living systems.

Toxic agent + Environment + host = toxic effects

       Objectives of ecotoxicology

The objective of ecotoxicology is to understand the mechanisms and processes whereby the environmental chemicals exert their effects on ecosystems and their impact on the populations or communities. The purpose of ecotoxicological tests is to predict the response of natural systems using tests in laboratories or model ecosystems. It is therefore an essential tool in the prevention of pollution by supporting environmental policies, laws, standards and control measures.

      

Definitions of ecotoxicology

 Ecotoxicology is a branch of toxicology concerned with the study of toxic effects, caused by natural and synthetic pollutants, to the constituents of ecosystems, animals (including human), vegetables and microbes in an integrated context (Truhaut,1977).

Ecotoxicology is the study of the effects of toxic substances occurring in both natural and man-made environments (Duffus 1980).

Ecotoxicology is the study of the impacts of pollutants upon the structure and function of ecological systems(from molecular to ecosystem) (Landis and Yu1995)

Ecotoxicological tools

Toxicological impacts in the ecosystem can be elucidated through a combination of bioassays using environment/ animal models and long-term field observations. Laboratory bioassays are performed to study feeding, growth, respiration, reproduction, histology, enzyme assays and mortality. Field observations such as tissue concentrations of toxins, species number, species density and population dynamics are crucial. Field experiments like the containment of test organisms at contaminated sites and environmental simulations (microcosms and mesocosms) aid in constructing theoretical models. Modeling throws more insights into our understandings of mechanisms of chemical movements in the environmental compartments.

Toxicological impact on the environment is a four-part process

The release of a chemical into the environment.
The transport of the chemical into biota, with or without chemical/bio-transformation.
The exposure of the chemical to one or more target organisms and
The response of the constituents or whole of the biosphere to the chemical exposure.

The paradigm shift from toxicology to ecotoxicology: Assumptions:

The range of variables that affect population responses is greater than the range that affects individual responses to pollutants.The sublethal effects on individuals may be as important as lethal effects.
 Different individuals of a given species or different populations of the same species may not respond in an identical manner to a pollutant.

Toxic substances have a strong influence both on the ecosystems and on the organisms in the ecosystems. These interactions may be complex and involve a number of parameters or organisms. This may also involve food chains and complex food webs. So there is a great deal of complexity exists because of the great variety of environmental factors and their interactions. The science, techniques and applications of ecotoxicology are evolving rapidly and will continue to contribute more on the understanding of pollution problems.

    Toxicologic principles and laws

Paracelsus (1567) stated “all substances are poisons; there is none which is not a poison. The proper dose separates a poison from a remedy”. Paracelsus is often referred to as the father of toxicology. According to him, the dose makes the poison and the sublethal dose determines the magnitude of the response. There is a threshold concentration for every toxin, which begins to produce effects on organisms; concentrations below this threshold will not have effect.

In order to understand the effects of toxins, Paracelsus believed  the following:

1. It was necessary to use experiments to identify and understand responses to chemicals.
2. There is a difference between toxic and therapeutic properties of chemicals.
3. The dose of a chemical is important in making the distinction between therapeutic and harmful effects and
4. It is possible to identify to some extent, the degree of chemical specificity.

M.J.B.Orfila (1815) described the harmful effects of chemicals on organism. He is often referred to as the founder of toxicology.

  Terminology
A toxic agent is anything that can produce an adverse biological effect. It may be chemical, physical or biological in form.

Chemical agent-e.g. cyanide

Physical agent-e.g. radiation

Biological agent – e.g. snake venom

A toxic substance is simply a material which has toxic properties. It may be a individual toxic chemical or a mixture of chemicals e.g. lead chromate.

It may be organic toxins or inorganic toxins. Organic toxins contain carbon and are man-made large molecules. Inorganic toxins are specific chemicals that are derived from minerals.

Toxic substances may be systemic toxins or organ toxins. A systemic toxin is one that affects the entire body. e.g. potassium cyanide. It affects every cell and organ in the body.

A organ toxin may affect only specific tissues or organs. Lead is a specific organ toxin. It affects 3 target organs namely CNS, kidney and hematopoietic system.

Concept of toxicity – toxicity is usually defined as the inherent property of a chemical to produce adverse biological effects. Toxicity is a function of  concentration (dose) and the duration of exposure. It is characterized in terms of acute or chronic effects and local or systemic effects. Analytical instruments cannot measure toxicity. They can measure the concentration of a chemical in the environment or in organism.

Application of knowledge and techniques of ecotoxicology

1. Determining contaminants leaching from wastes, their critical ecological thresholds and breakpoints.
2. Developing bio-marker –based monitoring systems.
3. Establishing protocols for the protection of natural environment.
4. Developing guidelines for risk assessment of anthropogenic wastes.
5. Contributing remedial ecological restoration measures.