**3.2 Biological receptors in benthic organisms**

Chemicals present in the soil interact with the soil constituents in such a way that over a period, the absorbed contents are not easily available for uptake by benthic organisms. For example, soil pH modifies metal solubility by controlling metal dissolution and precipitation and influences the ionization of pH-dependent ion-exchange sites on organic matter and metal oxide clay minerals. The biological receptors present in the soil-dwelling organisms readily absorb the available fraction of metals and store them either for detoxification or accumulated them in the form of toxicological accumulation. The non-sequestered portion that is not modified by the soil constituents remains as bioavailable fractions for the organism's uptake. Metallotheniens and chlorogosomes are examples of biological receptors in earthworms. Cadmium exposure can induce the production of cysteine-rich metalloproteins called metallotheniens and can be stored in a distinct subtype of sulfur-rich granule termed as cadmosomes [12]. Metallotheniens are sulfur-rich proteins with a low molecular weight that bind metals. Chlorogosomes are phosphate-rich structures with significant cation exchange capacities. The organic matrix of chlorogosomes is a highly complex mixture of carbohydrates, amino acids and lipids as well as redox pigments such as riboflavin, thiamine, carotene and metalloporphyrins [13].
