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Possibilities, practical hindrance and future direction

Bioiemediation is a wonderful process of treating pesticide contaminants by utilizing the natural survival activities of organisms. In this process, the desired organisms utilize the toxic pesticide contaminant as a source of carbon and energy for their growth and formation of new cells/ reproduction. Bioiemediation involves both phytoremediation and microbial remediation. Though both methods are effective, however microbial bioiemediation is more effective, highly applicable, and economic as compared to phytoremediation. The concept of bioiemediation is relatively new, and the first application was for removing oil spills from the coastal area of Santa Barbara, California, USA in I960. Thereafter, the science of bioiemediation has made tremendous improvements and presently bioiemediation can be applied for a wide range of contaminants including pesticides, heavy metals, polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), synthetic dyes, pharmaceuticals, etc. In this context, it is important to draw attention to the fact that biodegradation and bioiemediation are different. Though both covers the formation of non-toxic components/ metabolites from toxic contaminants, biodegradation is a naturally occurring phenomenon, whereas bioiemediation is a technology. Microbial bioiemediation holds immense potential to treat pesticide contamination. As discussed earlier, soil acts as a major sink of pesticide contamination, and such contamination can occur in the surface layer, vadose zone, and groundwater. Phytoremediation is possible for surface soil, but it is difficult for treating vadose zone and groundwater. Microbes offer the best possible way to treat pesticides in all three zones, i.e., aerobic and anaerobic layers of soil. However, certain factors like (i) availability of pesticides (considering the concentration and toxic nature) to the microbes (ii) physiological conditions of microbes including growth, metabolism, enzymatic activity, population dynamics, etc. (iii) survival of desired microbes in the soil environment after facing competition, predation and antagonism from native microbes (iv) environmental factors like moisture content, temperature, pH. nutrients, aeration status, etc. affect the performance of microbes on the available forms of toxicant/contaminants. Bioiemediation techniques offer several advantages over the traditional methods of remediation like:

  • • Bioiemediation involves natural microbes which immobilise/metabolise/convert toxic pesticide residue into non-toxic molecules or complete degradation to carbon dioxide and water, whereas conventional treatment methods offer primary removal of toxic pesticide residue from one environmental part and have a serious concern of secondaiy disposals like landfill and incineration. This secondaiy disposal may lead to indirect contamination of other environmental parts.
  • • Bioiemediation offers a scope of large scale application without displacement oftlie contaminated media whereby the natural activity of the medium is maintained, whereas traditional methods require physical transport to bring the contaminated medium to the treatment plant. Further, the quantity/scale of treatment for traditional methods is small as compared to the bioiemediation methods. Hence, traditional methods require a transport cost also.
  • • Bioiemediation is easy and less effort is required for on-field applications. Labour involvement is also less in bioiemediation as compared to conventional methods. Bringing the contaminated medium to the treatment plant and reallocation of the medium after treatment makes a traditional/ conventional treatment method cost and labour intensive.
  • • The energy requirement (either chemical/thermal) for traditional methods is high, whereas bioiemediation techniques do not require any external energy. Microbes degrade contaminants and in each step, it releases carbon and energy which promote natural growth and population of the desired microbes. Hence, the bioiemediation technique is a sustainable process.

• There is no requirement for hazardous chemicals in the bioremediation technique. Sometimes

fertilizers are used as nutrient supplements for the microbes.

• Microbial bioremediation techniques allow treating areas like vadose zone which is difficult by

traditional treatment methods.

The process of bioremediation involves either degradation or transformation or a combination of both pathways. Several bioremediation techniques utilize microbes like bioreactors, landfarming, soil washing with microbes, and bioaugmentation. Biostimulation, bioattenuation, solid-phase bioremediation, biopiles, bioventing, etc. have been developed and applied in the laboratory-scale study, and some have been tried for field studies. However, under field conditions success of a microbial bioremediation technique depends on (i) detailed knowledge of all naturally occurring biological processes at the site/field (ii) detailed scientific information on the biodegradation of contaminant/pesticide generated in the laboratory, and (iii) on-site monitoring of biodegradation process. The success of a microbial bioremediation technique depends primarily on the nature of the microbe. In recent years, thorough investigations have been carried out to understand the potential microbes and then molecular genetics. Several studies for microbial degradation of contaminants like DDTs, endosulfans, carbaryl, monocrotophos, atrazine, lindane, etc. (Lai et al. 2006, Kumar et al. 2007, Barraga et al. 2008) have been earned out at genetic levels. Findings indicated that Ese and Esd genes for endosulfans (Kumar et al. 2007), linA genes (linAl and linA2) for lindane (Lai et al. 2006), atzAgeue for atrazine (Neumann et al. 2004), etc. have a significant role in determining the biodegradation potentiality of desired microbial species. The biodegrading microbes utilise the contaminant/pesticide residue as source of energy and carbon under certain environmental conditions and detailed information is generated in lab studies. Hence, for a successful field application of a microbial bioremediation technique, desired conditions are needed to be created. In many cases, it has been observed that a potential microbe (under laboratory condition) gives a poor performance in the field because of a higher level of contaminant concentration which may be toxic, resulting in low survival of degrading microbes under field condition. Therefore, to promote the growth of the contaminant degrading microbes, a detailed study of the site/field is required. Moreover, these degrading microbes do not always strive for toxicants/contaminants. Easy availability of other sources for carbon and energy may result in a lower affinity for the contaminant/pesticide residues, resulting in poor field performance of a potential microbe. These limitations can be overcome by either applying fertilizers to supply nutrients or bioventing to supply oxygen or a combination of both is required to stimulate microbial growth. The introduction of a genetically modified native or non-native microbe for bioremediation is a promising aspect but the future effects of the genetically modified microbe on native soil population are not fully understood. Further, microbial bioremediation of air contamiuants/pollutants is limited and inefficient, considering the volumes of polluted air generated from industries.

Microbial bioremediation of pesticides is a technique of the future. It is a ‘green’ technique as compared to the traditional methods of remediation. However, more research in this field is required to overcome the limitations discussed earlier. Frontier research in a better understanding of microbes at an ecological, biological, and genetic level will provide a basis for the selection and utilization of microbial population for bioremediations. Omics-based technologies and data information have opened a new horizon for a better understanding of bioremediation. Further, researches in innovative engineering technologies to supply the desired stimulant like nutrients or oxygen to the specific microbial population is also an underexplored area. New techniques to promote contaminant’s availability to the degrading microbes are another new area of research.

 
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