1.1 Heavy Metals
3
metals occur in a variety of forms and species which affects their toxicity in the
environment. Heavy metals present a serious threat to the marine environment due to
their potential toxic effects (Pastorok and Bilyard 1985; Negri and Heyward 2001).
Once released into the aquatic environment, trace elements are transported and
distributed between the aqueous phase and sediment through the process of adsorption and desorption (Mondal et al. 2017). The trace elements can occur in several
forms such as a free ion; as an ion bound to different ligands (formation of complex
compounds); as the precipitated molecule of a compound, suspended in the liquid
phase or adsorbed on the surface of suspended or colloidal matter (e.g. Manahan
2003). Ionic species are more bioavailable and toxic compared to those bound to
particles and organic compounds (Kroon et al. 2020). A significant quantity of free
metal ions forms complexes via adsorption, hydrolysis, and co-precipitation and gets
deposited in the sediment while only a small portion of ions remain dissolved in the
water column (Bastami et al. 2015).
Heavy metals have greater detrimental environmental effects when present in a
water body that has a low pH because changes in metal speciation result in a shift
to more toxic ionic metal species (Bielmyer-Fraser et al. 2018). Furthermore, heavy
metal contamination may have devastating effects on the ecological balance of the
recipient environment and a diversity of aquatic organisms (Ashraf 2005; Vosyliene
and Jankaite 2006; Farombi et al. 2007).
1.2 Toxicity of Heavy Metals
All heavy metals can be toxic if present above threshold concentrations. Toxicity
of the metals is the capability of a metal to cause undesirable effects on organisms
(Ayangbenro and Babalola 2017). Ansari et al. (2004) reported that metal toxicity
depends on factors like chemical speciation of metals in the aquatic environment,
presence of other toxicants, environmental conditions including temperature, pH,
salinity and dissolved oxygen; condition of the organism tested; kinetics of toxic
reactions and adaptation of the organism to the absorption of metals. Furthermore,
factors such as the type of metals, their biological roles, and the type of organisms
exposed to it also affect metal toxicity (Akan et al. 2010). In addition to this, the
toxicity of the metal is also dependent on the amount accessible to organisms, the
absorbed dosage, the route and duration of exposure (Mani and Kumar 2014).
Elevated levels of heavy metals in the marine environment could lead to sub-lethal
effects in the marine organism including changes in physiology, tissues, biochemistry,
behavior and reproduction. Chiarelli and Roccheri (2014) reported that often metals
penetrate the cells via transport mechanisms normally used for other purposes and
are irreversibly accumulated in cells where they interact with cellular components
and molecular targets. Kim et al. (2014) and Tamás et al. (2014) mentioned that the
toxicity of the metals has been associated with blockage of oxidative phosphorylation,
glutathione depletion and inhibition of antioxidant enzymatic activity, production
3
metals occur in a variety of forms and species which affects their toxicity in the
environment. Heavy metals present a serious threat to the marine environment due to
their potential toxic effects (Pastorok and Bilyard 1985; Negri and Heyward 2001).
Once released into the aquatic environment, trace elements are transported and
distributed between the aqueous phase and sediment through the process of adsorption and desorption (Mondal et al. 2017). The trace elements can occur in several
forms such as a free ion; as an ion bound to different ligands (formation of complex
compounds); as the precipitated molecule of a compound, suspended in the liquid
phase or adsorbed on the surface of suspended or colloidal matter (e.g. Manahan
2003). Ionic species are more bioavailable and toxic compared to those bound to
particles and organic compounds (Kroon et al. 2020). A significant quantity of free
metal ions forms complexes via adsorption, hydrolysis, and co-precipitation and gets
deposited in the sediment while only a small portion of ions remain dissolved in the
water column (Bastami et al. 2015).
Heavy metals have greater detrimental environmental effects when present in a
water body that has a low pH because changes in metal speciation result in a shift
to more toxic ionic metal species (Bielmyer-Fraser et al. 2018). Furthermore, heavy
metal contamination may have devastating effects on the ecological balance of the
recipient environment and a diversity of aquatic organisms (Ashraf 2005; Vosyliene
and Jankaite 2006; Farombi et al. 2007).
1.2 Toxicity of Heavy Metals
All heavy metals can be toxic if present above threshold concentrations. Toxicity
of the metals is the capability of a metal to cause undesirable effects on organisms
(Ayangbenro and Babalola 2017). Ansari et al. (2004) reported that metal toxicity
depends on factors like chemical speciation of metals in the aquatic environment,
presence of other toxicants, environmental conditions including temperature, pH,
salinity and dissolved oxygen; condition of the organism tested; kinetics of toxic
reactions and adaptation of the organism to the absorption of metals. Furthermore,
factors such as the type of metals, their biological roles, and the type of organisms
exposed to it also affect metal toxicity (Akan et al. 2010). In addition to this, the
toxicity of the metal is also dependent on the amount accessible to organisms, the
absorbed dosage, the route and duration of exposure (Mani and Kumar 2014).
Elevated levels of heavy metals in the marine environment could lead to sub-lethal
effects in the marine organism including changes in physiology, tissues, biochemistry,
behavior and reproduction. Chiarelli and Roccheri (2014) reported that often metals
penetrate the cells via transport mechanisms normally used for other purposes and
are irreversibly accumulated in cells where they interact with cellular components
and molecular targets. Kim et al. (2014) and Tamás et al. (2014) mentioned that the
toxicity of the metals has been associated with blockage of oxidative phosphorylation,
glutathione depletion and inhibition of antioxidant enzymatic activity, production