**3. Bioaugmentation**

such capability for root colonizing, pollutant degrading bacteria utilize the growing root system and hence this acts as an injection system to spread the bacteria through soil [45].Plant root performs certain specialized roles, including the ability to synthesize, accumulate and secrete a diverse array of nutrient compound consequently no require‐ ment of exogenous carbon source, roots may regulate the soil microbial community in their immediate vicinity, cope with herbivores, encourage beneficial symbioses, change the chemical and physical properties of the soil and inhibit the growth of competing plant species [46]. PGP bacteria may facilitate plant growth either directly or indirectly [47]. Though the rhizoremediation process takes place naturally, but it can be modified by premeditated exploitation of the well-equipped rhizospheric microorganisms whereas it can

Incorporation of plant and PGPR having the pollutant degrading activity may be performed. Similarly, Kuiper *et al.* [48] described the pair of a grass species with a naphthalene degrading microbe which protected the grass seed from the toxic effects of naphthalene and the growing

Previously, several researchers have also used this symbiotic relationship of plant and microbes for degradation of hazardous and xenobiotic compounds like PCBs, PAHs and TCE [49]. Mechanical injection of contaminated sites with pollutant degrading bacteria has been used to clean polluted sites in an inexpensive and less labor intensive way than the removal

Polycyclic aromatic hydrocarbons (PAHs) are of particular concern because of their toxic, mutagenic and carcinogenic properties [51]. There is thus a chief interest in studying micro‐ organisms present in contaminated environments as a means for bioremediation. The fate of PAHs and other organic contaminants in the environment is associated with both abiotic and biotic processes, including volatilization, photooxidation, chemical oxidation, bioaccumula‐ tion and microbial transformation. Microbial activity has been deemed the most influential and significant cause of PAHs removal [3, 12]. PAHs may also be degraded by some micro‐ organisms in the soil [52]. The term bioremediation refers to the use of living organisms to degrade environmental pollutants [53]. Bioremediation is generally considered to include natural attenuation, biostimulation or bioaugmentation, the deliberate addition of natural or engineered microorganisms to accelerate the desired catalytic capabilities. According to the Environmental Protection Agency in the United States [54], natural attenuation processes may reduce contaminant mass (through destructive processes such as biodegradation and chemical transformations), reduce contaminant concentrations (through simple dilution or dispersion). Eventually, even the contaminants bound to the soil particles gets biodegraded by the bacterial

be proficient by using suitable plant microbe pairs.

334 Applied Bioremediation - Active and Passive Approaches

and/or combustion of polluted soils [50].

species present in the environment.

**2. Bioremediation**

roots exploited with the naphthalene degrading bacteria into soil.

Bioaugmentation is the introduction of microorganisms with specific catabolic abilities into the contaminated environment in order to supplement the indigenous population and to speed up or enable the degradation of pollutants [48, 55]

Bioaugmentation has proven successful for remediation of PAHs in sediments with poor or lacking intrinsic degradation potential [17], while other studies demonstrated that bioaug‐ mentation did not enhance biodegradation significantly compared to natural attenuation [56]. One of the main problems in applying bioaugmentation is to ensure the survival and activity of the introduced organisms in the environment [55]. Bioaugmentation can be inhibited by a variety of factors including pH and redox, the presence of toxic contaminants, concentration and bioavailability of contaminants or the absence of key co-substrates [48]. However, the key factor for the success of bioaugmentation process is the selection of the appropriate bacterial strain. When selecting the strain for augmentation purposes, the kind of microbial communi‐ ties present in the source habitat should be considered [57]. Bioaugmentation strategies may prove successful especially in the remediation of manmade contaminants, where specialized bacteria with the appropriate catabolic pathways may not be present in the contaminated habitat [55].
