16

Biology and Biotechnology of Environmental Stress Tolerance in Plants, Volume 3

as the most common inorganic pollutants (Jaganathan et al., 2018; Awadhesh

et al., 2020; Chourasia et al., 2022; Mangal et al., 2022). Metals, especially

at high concentrations, can harm the microbial population by interfering with

metabolic processes such as cell division suppression, protein denaturation

and cell membrane rupture. Tolerant microorganisms demonstrate the ability

to live in the presence of high heavy metal(loid) concentrations by utilizing

survival strategies such as biotransformation, extrusion, enzyme usage,

generation of exopolysaccharide (EPS), and metallothionein synthesis

(Chinnusamy, Zhu, & Sunkar, 2010; Lal et al., 2020; Devi et al., 2021;

Awadhesh et al., 2021).

Microbe transformation of metal(loid)s could be broadly categorized into

two types, redox conversions (oxidation and reduction) to less toxic state

and conversions from inorganic to organic form and vice versa. Oxidation of

metal(loid)s could help gain energy and on the contrary reduction can occur

through dissimilatory reduction where microorganisms utilize metal(loid)s

as terminal electron acceptor for anaerobic respiration. The reduction process

mainly could be methylation and demethylation. Microbial methylation

is quite important because of volatile products formed during the process

leading complete phase change of inorganic contaminants from solid (soil)

to gas (atmosphere) resulting in complete amelioration. A pictorial represen­

tation of a microbe cell imparting several mechanisms of tolerance to high

heavy metal(loid) (HM⁺) concentration is presented in Figure 1.1.

FIGURE 1.1 Mechanism of microbial detoxification of heavy metals.