**2. Heavy metals**

Heavy metals are usually defined as metals having density more than 5 g/cm<sup>3</sup> [15]. They are classified as essential and non-essential metals. The metals which are need for normal cellular growth are essential metals e.g. zinc, nickel, copper, etc. Such metals are required in low concentrations (nM), but at higher concentrations (μM to mM) all heavy metals have detrimental effects to organisms [16, 17]. If the metals have no known biological function, they are called as non-essential metals e.g. e.g., lead, cadmium, mercury [18]. Such metals are toxic at any concentration [8]. The list of essential and non-essential heavy metals is given (**Table 1**). There are 90 naturally occurring elements in periodic table, 21 are non-metals, 16 are light metals and the remaining 53 (with As included) are heavy metals [19]. In periodic table, transition elements are mostly heavy metals. They have incompletely filled 'd' orbitals which allow heavy-metal cations to form complex compounds that may or may not be redox-active. In this way, heavy metals play an important role as 'trace elements' (cobalt, copper, nickel, and zinc) in sophisticated biochemical reactions and are important cofactors for metallo-proteins and enzymes [8]. The toxicity of heavy metal ions starts when their concentration becomes higher


**Table 1.** Essential and non-essential heavy metals.

in the cells, due to which they form complex compounds [15, 18]. Microorganisms acquire resistance to these toxic metals by lateral gene transfer [20]. The interaction of microorganism with metal ions depends on factors like oxidation state of the metal ion, chemical/physical nature of metals, growth phase of microorganism etc. [21].
