Metal(loid)s Toxicity and Bacteria Mediated Bioremediation 171

Mercury (Hg) is found widely in higher aquatic plant species in several forms, including HgS,

Hg²⁺, methyl-Hg and Hg⁰ (Wang and Greger 2004, Kamal et al. 2004, Malar et al. 2015). Higher

quantities of Hg²⁺ are known to be highly phytotoxic to plant cells, causing apparent lesions and

physiological problems, which affects the closing of stomata in leaves and blockage of transportation

of water in plants (Zhou et al. 2007). It is also observed that high amounts of Hg²⁺ have been

found to interfere with the plant mitochondrial function, resulting in the alteration of cell membrane

components, including lipids, and affecting the cellular metabolism (Messer et al. 2005, Cargnelutti

et al. 2006).

Lead impacts negatively on plant shape, growth, photosynthesis and metabolic activities by

interfering with essential enzymes, limiting seed germination (Zulfiqar et al. 2019). Another problem

caused by increased Pb concentrations is oxidative stress. In plants, it promotes the formation of

Reactive Oxygen Species (ROS) (Reddy et al. 2005). Plants which act as accumulators are highly

resistant to heavy metals in their habitat and can tolerate high concentrations. These plants can use

a different mechanism, such as Exclusion, which limits the excessive transportation of metals in

the plant. A constant concentration of different metals can be maintained in the shoots. Secondly,

Inclusion, a mechanism that prompts the plants to absorb the maximum metals from the soil. Thirdly,

Bioaccumulation, which is the build-up of the toxic metal concentrations in the plants. Table 10.2

summarizes the different toxic metals and their impacts on plants.

10.3.3 Effects on Animals

Mutation, carcinogenicity, teratogenicity, poor physical condition and decreased reproduction are

the most common adverse consequences associated with prolonged exposure to heavy metals in.

pets (dogs and cats, in particular). Animal species such as pigs, hens and dairy cows have shown that

elements like Pb and Cd could have subtoxic effects due to their prolonged consumption through

dietary amounts. Both Pb and Cd get accumulated in organs like the liver and kidneys, while lead

(Pb) could also be deposited in the bones of dairy animals. Modest consumption of the livers and

kidneys from Pb-exposed animals could be harmful. Consumption of these organs from Cd-exposed

animals should be avoided. It is quite evident that some toxic metals such as lead, mercury and

selenium with an inappropriate level of some essential trace elements like chromium are observed

in some wild plant species from Northeast India (Kennady et al. 2018). With the help of cutting-edge

technology such as plasma spectroscopy, toxicity behavioural studies can be concluded from bones,

hairs and cellular and surface ultrastructure features. Behavioural investigations revealed numerous

symptoms associated with specific elemental anomalies, such as lack of appetite, constipation,

salivation, photophobia, etc.

Since metals are not degraded by bacteria, and the presence of pollutants in rivers disturbs the

balance of the aquatic environment and causes a massive reduction in marine life (Woo et al. 2009,

Ay et al. 2009). Cellular intoxication, interference in the fish’s metabolic processes, and death at

the cellular level are a few histopathological changes observed due to toxic metals. The continual

flow of water via gills and food sources continuously exposes fish to waterborne and particulate

hazardous metals. The Reactive Oxygen Species (ROS) are often generated by toxic metals and

cause degradation of cellular components, including protein, DNA, enzymes and fatty acids in

the aquatic organisms. These toxic elements interact with the large surface area of fish gills to

produce various abnormalities, including epithelial lifting issues, leucocytic necrosis, etc. (Mehana

et al. 2020).

10.4 Bacteria Mediated Bioremediation

Bacteria use the environment and interactions with other organisms to attain carbon sources and

nutrients for microbial cell maintenance. Bacterial bioremediation involves the use of aerobic and

anaerobic bacteria to reduce environmental pollutants (Nagajyoti et al. 2010). The availability of