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BIOREMEDIATION

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In today’s world, the mam reason for environmental pollution is the nonspecific and disproportionate utilization of biocides (Sanchez-Vizuete et al., 2015). “Remediate” means to solve a problem, and “bio-remediate” means to use biological organisms to solve an environmental problem by eliminating contaminants from the environment. The use of microorganisms as agents for in situ remediations is a well-known subject. Factors like chemical nature and the concentration of pollutants, microbial access to these pollutants, and also physicochemical characteristics of the prevailing environment influence this process (Mrozik, 2010). As reported by El Fantroussi et al. (2005), process efficiency can be improved by biostimulation (addition of excess nutrients) and bioaugmentation (increasing specific microbial load). Bioaugmentation improves bioremediation only if the microbial populations being used possess the following features: fast growth, easy in vitro culturing and also their ability to withstand and survive in extreme environmental conditions with high concentrations of xenobiotic compounds (Singer et al., 2005; Thompson et al., 2005). Studies on the use of mixed cultures of aromatic-degrading microbial strains for the clean-up of the environment has been positive and are being encouraged (Goux et al., 2003; Ghazali et al., 2004).

The property of microorganisms to adhere to surfaces and form biofilm communities is a well-known concept now. Microbes in biofihn communities show differences in both morphological and physiological characteristics. The ability of microbes to form biofilms has been exploited to clean up environmental pollutants as safer alternatives. Microbial biofilms present better bioremediation than their planktonic counterparts. They are better adapted to overcome stress, nutrient deficiency, extremes of pH, temperature, and other harsh environmental conditions as they are protected within a matrix made up of EPS. The survival ability of biofilms under such conditions has been attributed to gene transfer mechanisms (Singh et al., 2006). In vitro studies by Goris et al. (2003) through a bioreactor system, showed the transfer of pCl of Delftia acidovorans, which was tagged with a mini-Tn5 transposon encoding the gene for oxidative deamination of 3-chloroaniline. This tagged plasmid was later transferred to Pseudomonasptitida where 3-chloroaniline was mineralized. Springael et al. (2002) through their bioreactor system studies reported that P putida BN210 contains a transposon called c/c-elernent which carries out degradation of 3-clorobenzoate. Table 7.2 lists the organisms involved in the bioremediation of recalcitrants.

The success of bioremediation using biofilm formers largely depends upon the understanding of microbial interaction within biofilms and also the recalcitrants in the surrounding enviromnent. Microorganisms that form biofilms on hydrocarbon surfaces secrete polymers. These are best suited for recalcitrant treatment of slow degr ading compounds because of their ability to immobilize compound by processes like biosorption, bioac- cumulation, and biomineralization (Singh et al., 2006). Table 7.3 shows different bioremediation methods for mineralization of heavy metals. Every microbial gr oup is versatile and capable of degr ading a wide spectrum of substr ates. Thus bioremediation is not universally restricted to any particular microbial population.

CONCLUSION

Microbial soil communities play a fundamental role in mineralization of soil organic matter, a key process for plant nutrition, growth, and production in agricultural ecosystems. A number of microbial processes take place in the rhizosphere and rhizoplane region which serves as “hotspot” for all biological and physico-chemical activity, transfers, and biomass production. The knowledge of rhizosphere processes is, however, still scanty, especially regarding the interactions between physico-chemical processes and interactions occurring between plant and soil microbes and also between soil microorganisms. Thus there is lot of diversity and abundance of soil-bome microbes that may be strongly influenced by abiotic and biotic factors. Microbes exhibit a natural tendency to interact with surfaces and form complex structures called biofilms in a natural ecosystem. These structured microbial cell communities can be composed of either a single or multiple species. The key to the formation of mixed-species biofilm is cellcell signaling. These mechanisms of signaling occur during plant microbial interactions and also during inter microbial interactions.

TABLE 7.2 Pollutants Studied In Vitro Along With Organisms Involved in the Bioremediation Process

Pollutants

Organism(s)

Bioreactor System

Chlorophenols

4-Chlorophenol

Bacterial consortium from rhizosphere of Phragmites australis

Granular activated-carbon biofilm reactor

2,4-Dichlorophenol

Pseudomonas putida

Rotating perforated tube biofilm reactor

2,4,6-Trichlorophenol

Pseudomonas sp., Rhodococcus sp.

Fluidized bed biofilm reactor

2,4,6-Tetrachlorophenol

Pseudomonas sp., Rhodococcus sp.

Fluidized bed biofilm reactor

Herbicides

2-(2-methyl-4-chlorophenoxy) propionic acid (MCPP)

Mixed culture of herbicidedegrading bacteria

Granular activated-carbon biofilm reactor

2,4 Dichlorophenoxy acetic acid

Mixed culture of herbicidedegrading bacteria

Granular activated-carbon biofilm reactor

Source: Modified from Singh et al., 2006.

TABLE 7.3 Methods of Heavy Metal Bioremediation

Heavy metals

Method of bioremediation in a bioreactor system

Zn, Cd, Ni

Biosoiption

Cu, Zn, Ni, Co

Biosoiption, bioprecipitation

Co

Immobilization

Cd, Cu, Zn, Ni

Adsoiption

Cd, Zn, Cu, Pb. Y, Co, Ni. Pd, Ge

Bioprecipitation

Adapted and modified from Singh et al., 2006.

A diverse array of bacteria associates with root, vasculature, stem, and leaf of plants to foim a biofilm. The interaction effect of plants with these bacteria can have many responses such as symbiosis, beneficial, pathogenic, or biocontrol effect. Biofilm formation of soil microbiota also depends on components of soil like clay, minerals, and metal oxides. However, the underlying mechanism and the effect of soil on its formation of biofihn are poorly understood. Apart from the associated microbes and plants, biofihns also affect the compounds within then vicinity and decide then fate. The role of biofihn communities in the soil can also be extended to then use in the process of bioremediation. The complex architecture of biofihn communities accompanied by the diverse interplay of intercellular genetic exchange, QS signals, and metabolites help in the massive diffusion of nutrients to surviving microbes. Effective bioremediation and recalcitrant treatment would require a better understanding of soil biofihns to develop better bio-degradative processes to improve in situ remediations of the environment, hi spite of all this knowledge about biofihn, the research is still very challenging and draws attention to develop better techniques to understand and exploit the advantages offered by the microbes residing as biofihn communities.

KEYWORDS

  • • acyl-homoserine lactones • exopolysaccharide
  • Aspergillus niger • nitric oxide
  • Azospirillum brasilense • polymicrobial biofilm
  • Bacillus subtilis • Pseudomonas fluorescens
  • • biocontrol effect • quorum sensing
  • • biofilm • Rhizobium
  • • bioremediation • rhizosphere
  • Erwinia amylovora • symbiotic effect
 
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