1.Biochemical biomarkers Based on the ability of the pollutant to generate a response at the gene level, inducing or increasing specific enzymes involved with detoxification of contaminants. The detoxification of xenobiotics often involves one of three detoxification strategies: - cytochrome P450, - conjugation with glutathione and -chelation by proteins (Metallothioneines).

*EROD, ethoxyresorufin-o-dethylase. ^AHH, aryl hydrocarbon hydroxylase. *GSH, glutathione.polyaromatic hydrocarbon.

2.Immunochemical biomarkers The specific reaction between antigens and antibodies can be used to determine the presence of xenobiotics in environmental samples. Antibodies against PCBs (polychlorinated biphenyls), PCDDs (polychlorinated dibenzo-p-dioxins), and PCDFs (polychlorinated dibenzofurans) have been developed and used in an ELISA system to determine PCBs in samples (Hahn, 2002).

3.Genetic biomarkers Ames test The Ames test was developed to test substances for their ability to produce mutations in bacteria. The test consists of the treatment of a Salmonella typhimurium histidine auxotroph (His - ). The test compound is added to the Salmonella typhimurium His - in an extract of rat liver. If the compound is a mutagen then mutations will cause revertants- colonies will form on a medium lacking histidine. The number of colonies will give a measure of the mutagenic potential.

4.Molecular biology biomarkers Another development using genetic manipulation of biological material for the estimation of toxicity has been the generation of a transgenic strain of the nematode Caenorhabditis elegans. The lacZ gene from E. coli is fused to the hsp16 gene. When the nematode is stressed the enzyme is induced in the worm. The enzyme can be detected by the addition of a substrate such as o-nitrophenyl-p-galactopyranoside (ONPG), which will produce a blue colour when cleaved by the enzyme.