1.8.3 Polycyclic Aromatic Hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are originated from anthropogenic activities, highly organic pollutants and more carcinogens and mutagens. These compounds have influence on the microbial population. PAH assists as energy for
microorganisms, and also it converts ineffective and not as much toxic compounds
by the highly effective and expensive process of biodegradation (Anwar et al. 2016).
PAH contaminations are difficult to degrade and persist in water for longer periods;
hence, it effects the microbial population (Gałązka et al. 2018).
Among the microbes, bacteria is more effective in the degradation of PAHs in
aquatic environments (Johnsen et al. 2005). The bacterial activity is less in soil- or
sediment-based PAHs (Yuan et al. 2001), and in that particular cases, sludge-based
degradation bacteria has to be introduced (Hwang et al. 2003). Dissolution and
vaporization process make the degradable bacteria live than sorption process (Kim
et al. 2007).
Uyttebroek et al. (2007) revealed that PAH degradation is mainly based on the
soil age and its nutrient concentration. Wang et al. (2009) identified that nonspecific
enzymes are not able to degrade the PAHs and remain in the soil for long time. Teng
et al. (2010) studied that dihydriol is an oxygenated intermediate compound that
helps for the degradation of anthracene by the presence of Nocardia, Beijerinckia,
Sphingomonas, Rhodococcus and Paracoccus. Other than bacteria, many aerobic
and anaerobic fungi species are also involved in the degradation of PAHs (Aydin
et al. 2017).
Kadri et al. (2016) observed that many fungal species like Phanerochaete
chrysosporium and Pleurotus ostreatus are able to produce lignolytic enzymes,
namely laccase, Mn peroxidase and lignin peroxidase that degraded the PAH
compounds. Jin et al. (2016) reported that plant growth–promoting rhizobacteria
have the ability to degrade pyrene and other aromatic contaminants.
1.8.4 Microbial Detoxification of Heavy Metals
The entire ecosystem has been contaminated by heavy metals. They are toxic not
only to humans but also to the microorganisms in the soil. Among the microorganisms, mycorrhizal fungi are the only ones which provide a direct link between soil
and root of the crops (Gomathy et al. 2018a, b). Pollutants including heavy metals
are detoxified by microbes in the presence or absence of plant system. Heavy metals
are either beneficial or harmful to microbes (Ayangbenro and Babalola 2017). Some
of the heavy metals like manganese, Fe, nickel, Mg, copper, chromium, cobalt and
Zn are beneficial to microbes during the enzymatic reactions, redox reactions and
stabilization of biomolecules (Bruins et al. 2000). Certain heavy metals like mercury,
lead, antimony, gold, cadmium and silver are not involved in any biological functions and toxic to microbes at high concentrations (Bruins et al. 2000).
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M. Gomathy et al.
Polycyclic aromatic hydrocarbons (PAHs) are originated from anthropogenic activities, highly organic pollutants and more carcinogens and mutagens. These compounds have influence on the microbial population. PAH assists as energy for
microorganisms, and also it converts ineffective and not as much toxic compounds
by the highly effective and expensive process of biodegradation (Anwar et al. 2016).
PAH contaminations are difficult to degrade and persist in water for longer periods;
hence, it effects the microbial population (Gałązka et al. 2018).
Among the microbes, bacteria is more effective in the degradation of PAHs in
aquatic environments (Johnsen et al. 2005). The bacterial activity is less in soil- or
sediment-based PAHs (Yuan et al. 2001), and in that particular cases, sludge-based
degradation bacteria has to be introduced (Hwang et al. 2003). Dissolution and
vaporization process make the degradable bacteria live than sorption process (Kim
et al. 2007).
Uyttebroek et al. (2007) revealed that PAH degradation is mainly based on the
soil age and its nutrient concentration. Wang et al. (2009) identified that nonspecific
enzymes are not able to degrade the PAHs and remain in the soil for long time. Teng
et al. (2010) studied that dihydriol is an oxygenated intermediate compound that
helps for the degradation of anthracene by the presence of Nocardia, Beijerinckia,
Sphingomonas, Rhodococcus and Paracoccus. Other than bacteria, many aerobic
and anaerobic fungi species are also involved in the degradation of PAHs (Aydin
et al. 2017).
Kadri et al. (2016) observed that many fungal species like Phanerochaete
chrysosporium and Pleurotus ostreatus are able to produce lignolytic enzymes,
namely laccase, Mn peroxidase and lignin peroxidase that degraded the PAH
compounds. Jin et al. (2016) reported that plant growth–promoting rhizobacteria
have the ability to degrade pyrene and other aromatic contaminants.
1.8.4 Microbial Detoxification of Heavy Metals
The entire ecosystem has been contaminated by heavy metals. They are toxic not
only to humans but also to the microorganisms in the soil. Among the microorganisms, mycorrhizal fungi are the only ones which provide a direct link between soil
and root of the crops (Gomathy et al. 2018a, b). Pollutants including heavy metals
are detoxified by microbes in the presence or absence of plant system. Heavy metals
are either beneficial or harmful to microbes (Ayangbenro and Babalola 2017). Some
of the heavy metals like manganese, Fe, nickel, Mg, copper, chromium, cobalt and
Zn are beneficial to microbes during the enzymatic reactions, redox reactions and
stabilization of biomolecules (Bruins et al. 2000). Certain heavy metals like mercury,
lead, antimony, gold, cadmium and silver are not involved in any biological functions and toxic to microbes at high concentrations (Bruins et al. 2000).
14
M. Gomathy et al.