In the rhizosphere, microbial communities accelerate biodegradation processes
and improve co-metabolism to degrade organic pollutants and pesticides by (1) facilitating selective enrichment of biodegrading microorganisms for xenobiotics degradation in root-free soils (Nichols et al. 1997), (2) enhancing metabolism of microbial
growth by secreting natural substrates which depends on the quantity of xenobiotics
(Haby and Crowley 1996), or (3) enriching natural compounds that provoke the
co-metabolism of xenobiotics in specific microorganisms that exhibit genes or
plasmids with degradation functions (Gupta et al. 2016). Thus, the rhizosphere
microorganisms inhibit or tolerate the level of organic contaminants, mainly with
the help of microorganisms linked to metabolic degradation, partially or completely
detoxifying, leading to a decrease in the quality and quantity of contaminants in the
soils (Furukawa et al. 2004; Balcom et al. 2016).
2.4.1 Pesticide Degradation by Bacteria
The biodegradability of microorganisms depends on the physical, chemical, and
microbiological properties of the soil and the chemical properties of the pollutant
(Banat et al. 2000; Van Hamme et al. 2003). Pesticide degradability decreases as
molecular weight and degree of branching increases in pesticide structure. During
degradation processes, bacteria and fungi produce intra- or extracellular enzymes
such as hydrolases, peroxidases, oxygenases, and other enzymes for the degradation
of toxic pesticide molecules (Li et al. 2007; Ortiz-Hernández et al. 2011).
Profenophos, a well-known organophosphate pesticide, is widely used to control
lepidopteron pests of cotton, tobacco, and vegetable crops and is degraded through
hydrolysis by Pseudomonas aeruginosa (Malghani et al. 2009a). Similarly,
P. putida utilized and degraded another organophosphate pesticide, cadusafos,
which is used to control nematode and insect pests (Abo-Amer 2012). Organophosphate pesticide, chlorpyrifos was utilized by soil bacterium Providencia stuartii
under in vitro conditions up to a concentration of more than 700 mg L
À1 (Rani
et al. 2008). Malathion is a broad-spectrum organophosphate used in agricultural
soils. Acinetobacter johnsonii MA19 was isolated from malathion-contaminated soil
samples using enrichment culture method. The degradation rates were significantly
improved by the use of sodium succinate and sodium acetate as additional carbon
sources for the degradation of malathion (Shan et al. 2009).
Kafilzadeh et al. (2015) isolated Klebsiella, Acinetobacter, Alcaligenes,
Flavobacterium, and Bacillus form sediments and water samples, which could
degrade endosulfan effectively. Jayabarath et al. (2010) selected 319 actinomycetes
from saline soils in Sangli district (Maharashtra) for carbofuran tolerance, while
Streptomyces alanosinicus, S. atratus, Streptoverticillium album, Nocardia farcinia,
N. amarae, and Micromonospora chalcea could degrade carbofuran pesticide.
Elgueta et al. (2016) used white rot fungi for degradation of atrazine and reported
that growth and consumption of atrazine by fungi reduced the half-life of atrazine to
6 days. Kabra et al. (2014) reported the degradation ability of green microalga
36
A. Sehrawat et al.
and improve co-metabolism to degrade organic pollutants and pesticides by (1) facilitating selective enrichment of biodegrading microorganisms for xenobiotics degradation in root-free soils (Nichols et al. 1997), (2) enhancing metabolism of microbial
growth by secreting natural substrates which depends on the quantity of xenobiotics
(Haby and Crowley 1996), or (3) enriching natural compounds that provoke the
co-metabolism of xenobiotics in specific microorganisms that exhibit genes or
plasmids with degradation functions (Gupta et al. 2016). Thus, the rhizosphere
microorganisms inhibit or tolerate the level of organic contaminants, mainly with
the help of microorganisms linked to metabolic degradation, partially or completely
detoxifying, leading to a decrease in the quality and quantity of contaminants in the
soils (Furukawa et al. 2004; Balcom et al. 2016).
2.4.1 Pesticide Degradation by Bacteria
The biodegradability of microorganisms depends on the physical, chemical, and
microbiological properties of the soil and the chemical properties of the pollutant
(Banat et al. 2000; Van Hamme et al. 2003). Pesticide degradability decreases as
molecular weight and degree of branching increases in pesticide structure. During
degradation processes, bacteria and fungi produce intra- or extracellular enzymes
such as hydrolases, peroxidases, oxygenases, and other enzymes for the degradation
of toxic pesticide molecules (Li et al. 2007; Ortiz-Hernández et al. 2011).
Profenophos, a well-known organophosphate pesticide, is widely used to control
lepidopteron pests of cotton, tobacco, and vegetable crops and is degraded through
hydrolysis by Pseudomonas aeruginosa (Malghani et al. 2009a). Similarly,
P. putida utilized and degraded another organophosphate pesticide, cadusafos,
which is used to control nematode and insect pests (Abo-Amer 2012). Organophosphate pesticide, chlorpyrifos was utilized by soil bacterium Providencia stuartii
under in vitro conditions up to a concentration of more than 700 mg L
À1 (Rani
et al. 2008). Malathion is a broad-spectrum organophosphate used in agricultural
soils. Acinetobacter johnsonii MA19 was isolated from malathion-contaminated soil
samples using enrichment culture method. The degradation rates were significantly
improved by the use of sodium succinate and sodium acetate as additional carbon
sources for the degradation of malathion (Shan et al. 2009).
Kafilzadeh et al. (2015) isolated Klebsiella, Acinetobacter, Alcaligenes,
Flavobacterium, and Bacillus form sediments and water samples, which could
degrade endosulfan effectively. Jayabarath et al. (2010) selected 319 actinomycetes
from saline soils in Sangli district (Maharashtra) for carbofuran tolerance, while
Streptomyces alanosinicus, S. atratus, Streptoverticillium album, Nocardia farcinia,
N. amarae, and Micromonospora chalcea could degrade carbofuran pesticide.
Elgueta et al. (2016) used white rot fungi for degradation of atrazine and reported
that growth and consumption of atrazine by fungi reduced the half-life of atrazine to
6 days. Kabra et al. (2014) reported the degradation ability of green microalga
36
A. Sehrawat et al.