**4. Trophic transfer of heavy metals**

As HMs in the atmosphere is permanent, they accumulate in living organisms and are passed in the food chains from one trophic stage to another. The degree to which heavy metals are deposited in biota depends on their accumulation rate and their removal rate from the body.

Heavy metals, i.e. water, sediments and soil, may enter the body of an organism directly from the abiotic system, or may enter the body of the organism from its food/prey. For example, heavy metals may reach the body of the fish directly from water or sediments through the gills/skin of the fish or from the food/prey of the fish through its food canal. Over successive trophic levels in a food chain, the concentration of a heavy metal can increase or decrease. The retention of heavy metals in an organism's body depends on a variety of factors, such as the speciation of the metal concerned and the physiological mechanisms established by the organism for heavy metal control, homeostasis and detoxification. Because of their lipophilicity, methylated forms of heavy metals like Hg are accumulated to a greater extent in biota and also biomagnified in food chains. In metal-rich environments, such plants have the potential to survive and are called metallophytes. Special mechanisms for

**Figure 5.** *Trophic transfer of heavy metals in the environment.*

dealing with higher heavy metal concentrations in soil have been established by these special plants and are classified into three groups, i.e. excluders, markers, and hyper accumulators [57]. To define the trophic transfer of heavy metals, certain words are used (**Figure 5**).
