Toxic substances

The study of ecological effects of toxic substances in the environment is designated as ecotoxicology (Walker et al. 2001). This multidisciplinary science is a meeting place of environmental chemists, toxicologists, and ecologists. Chemists determine the concentration of substances in the environment and study their distribution over environmental compartments and chemical ligands; toxicologists analyse uptake kinetics, biotransformation, and metabolic effects of toxicants; ecologists study the effects of toxic insults at the population, community, and ecosystem levels. Ecotoxicology traditionally has a strong link with environmental policy. Through this link, scientific support is provided for decisions about issues like standard setting, remediation of contaminated sites, and pesticide registration. In industrialized countries new substances are produced continuously, but

Functional significance

Bind to cell envelope of Gram-positive bacteria Stimulate cell growth required for wound healing Forming a complement by binding to invader surface

Extracellular signalling molecules of the Toll pathway Inhibition of trypsin-like serine proteases Possibly similar to serpins, but not previously implicated in immune response

Proteolytical activation of phenoloxidase from its precursor, conversion of dopamine to melanin Possible role in blood clotting

Protein toxic to fungal metabolism

Small antimicrobial protein, precise function unknown

Activation of stress signalling pathways Regulation of antifungal peptide production Regulation of antibacterial peptide production General stress response (see Section 6.2)

Sequestration of extracellular iron their application in society is regulated by safety requirements concerning human and environmental health. Most legislatory systems require that new substances must be tested for their possible adverse effects on ecological receptors before they are admitted to the market.

A well-known principle in toxicology is that toxicity is not an absolute property of a substance, but that the effect of a substance depends on the dose given to the organism. The classical poison is effective in very low amounts, but in principle all substances can be toxic if dosed highly enough. This was recognized already by the Austrian alchemist and physician A.P.T.B. von Hohenheim (1493-1541), better known as Paracelsus, who wrote (Koeman 1996):

Alle Ding sind Gifft... allein die Dosis macht das ein Ding kein Gifft is (Everything is a poison... it is only the dose that makes it not a poison).

Following the Paracelsus principle, an important activity of toxicologists is the establishment of dose-effect relationships, which in ecotoxicology usually take the form of a graph in which some aspect of the performance of a tested organism (e.g. growth or reproduction) is plotted as a function of the concentration of a given toxicant in water, soil, or air. From such a graph two important benchmarks are estimated, the exposure concentration at which a 50% effect is observed (EC50) and the highest exposure at which still no effect is seen (NEC, no effect concentration). Ecotoxicologists are usually concerned with effects that show up after chronic (long-term) exposure and, unlike human toxicologists, study end points that are important for the ecological functions of an organism. In the case of animals, many ecological functions are associated with feeding and behaviour—for example, macroinvertebrates grazing to suppress algal blooms, or earthworm burrowing to improve soil structure—that is why end points in ecotoxicology can be different from those in human toxicology.

The application of genomic technology in toxicology is called toxicogenomics (Lovett 2000; Pennie et al. 2000; Burczinsky 2003; Waters and Fostel 2004), and its ecological counterpart as ecotoxicogenomics (Snape et al. 2004). An important aim of toxicogenomics is to characterize the mode of action of toxicants on the basis of expression profiles. When two toxicants induce the same set of genes in a target organ, they are likely to have the same mode of action (Hamadeh et al. 2002). New substances, such as drugs, industrial chemicals, or pesticides, can be screened for their transcription profile and when the profile is compared with a database of earlier-investigated chemicals any similarities may provide an indication of the hazardous properties of the compound. The first commercial microarrays, designed to screen induction of enzymes in the human liver, were developed at the end of the twentieth century. It is likely that such tools will also be developed for environmental applications, but standardized assays are not yet available.

In this section we will address the question of how genomic technology can improve our insight into ecotoxicity of environmental chemicals. Out of the huge number of chemicals that may cause environmental problems we have selected three classes of toxicant: heavy metals, pesticides, and endocrine disrupters. These three groups have very different environmental effects and serve to illustrate the principles of ecotoxicogenomics.

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