The Past, Present, and Future Issues on Bioremediation of Pesticides Essay

Bioremediation of Pesticides in Soil – Past, Present and the Future

Bioremediation of pesticides is expensive in terrestrial sites, where large volumes of soil are commonly excavated and transported to a remote site, only to be dumped again complete with pesticides. Bioremediation in situ has many potential advantages but is difficult to manage, and few products are available. These could theoretically be metabolized by a range of microorganisms that may be genetically modified to degrade multiple pesticides that they currently cannot. But what else do the microorganisms need in the soil to maximize the degradation of the pesticides? What does the future hold? How can biotechnology help to remove residual pesticides from soil and protect food?

The history of discovery of microbial pesticide degradation

The history of microbial pesticide degradation can be traced back to the discovery and use of DDT because DDT became the first synthetic pesticide to be used to control pests. DDT was discovered in 1874 by Zeidler, but its potential to control pests was discovered in 1939.1 The chemical was rapidly applied for widespread use in agricultural applications, as well as, in public health programs to control vector-borne diseases such as malaria. DDT had been found to present devastating impacts on the environment; hence, there were efforts to ban its use in the U.S. and other developing countries. The ecological problems associated with DDT depend on its molecular structure and its microbial degradation that is well understood. DDT and other hydrocarbons are not easily degraded into other substances hence; they remain in the environment after being applied to crops as pesticides.2

Table 1: Persistence of pesticides in the environment.

However, with the rapid growth of industrialization, there has been a wide use of a variety of chemicals. This has resulted in the wide use of man-made chemicals that has contributed to a remarkable use of new technologies such as recycling, land-filling, incineration, and refilling. These methods proved to be extremely expensive; hence scientists discovered the ability to remove pollutants from the environment through the use of microbes, through a process known as bioremediation.3 It was discovered that microbes have the potential to transform and degrade xenobiotics. Moreover, microbial capability to degrade pesticides was discovered after their introduction to the market with the initial research conducted on the biodegradation of 2, 4-D.4

The relative usefulness of different organisms and the pesticides they degrade

The degradation of pesticides applied in the soil by microorganisms is the best mechanism to prevent the accumulation of pesticides in the environment. For instance, methyl parathion is a widely used organophosphorus pesticide. A fenpropathrin-degrading bacterium, fenpropathrin, and methyl parathion are pesticides that can be degraded by Sphingobium sp.5 According to field experiments, scientists have discovered that genetically engineered microbes have the potential to degrade many pollutants in the soil.6 There are also several bacteria capable of biodegrading carbofuran.

These include; Pseudomonas, Flavobacterium, Achromobacterium, Sphingomonas, and Arthrobacter. These bacteria have been characterized and classified, to, understand their capability of degrading carbofuran and removing them from the environment. Carbofuran is a pesticide that belongs to the N-methylcarbamate class and is widely used in agriculture. The pesticide is highly toxic to mammals and is classified as highly hazardous. The Sphingomonas sp. first degraded the carbofuran to a carbofuran phenol, and the result was degraded to 2-hydroxy-3- ) 3-methyl propane-2-ol).7 Carbendazin, which is a fungicide, is degraded by a microbial consortium with the degradation capacity of the consortium enhanced by the immobilization on Luffa cylindrica (sponge).

This is a beneficial material for the bioremediation of polluted water with pesticides, particularly in paddy fields.8 In addition, Pseudomonas isolates degrade carbamate carbendazin into 2-aminobenzimidazole. However, a limited number of xenobiotics are metabolized by a single strain, but when coupled with consortia of microorganisms, the degradation becomes complete.9

Environmental Limitations on Bioremediation of Pesticides

The effect of microbial bioremediation of pesticides can face certain limitations. Various factors determine the activities of microorganisms in the environment. For instance, the process of natural bioremediation requires microorganisms to be in their natural environment.10 In addition, microbial bioremediation requires a natural organism to thrive in the contaminated environment to act on the pesticides. There are harsh environmental conditions that may affect the activities of microorganisms.

For instance, saline conditions or a contaminant that may alter the properties of the soil can kill the microorganisms. Moreover, genetically engineered organisms may also be affected by the same problems. Other factors include temperature differences and PH, which have detrimental impacts on the functioning of the microorganisms. Pesticides have a complex structure; hence, they are sometimes not readily biodegraded.11 The ability of microbes to degrade pesticides may be affected by water and oxygen scarcity due to the aerobic nature of bioremediation.12

Ways in which some of the microorganisms have been modified

Micro-organisms can be modified in various ways. For instance, bio-stimulation involves the addition of nutrients and other electron donors or receptors or oxygen to increase the number and activity of the naturally occurring microbes available for remediation. Bacteria can be genetically manipulated to improve their bioremediation capabilities.13 Bacteria have many advantages since they can metabolize best at PH ranges of 6.5 to 7.5. Moreover, the microorganism substrates can be improved by the adaptation of mutagenesis to obtain more favorable characteristics.

Microbial products that can be used during bioremediation

During the process of bioremediation, the contaminant is used as a source of carbon that is desirable for growth. The microorganisms obtain energy by breaking the chemical bonds and removing the electrons away from the contaminants through redox reactions. The energy gained by the microbes during electron transfer is used to yield more cells. Microbes generally use oxygen in a process known as aerobic respiration. The main products that are associated with aerobic respiration include an increase in the microbe population, water, and carbon dioxide. These substances are extremely valuable during the process of bioremediation.

Ways to increase the range and sustainability of bioremediation in future

Micro-organisms are particularly beneficial in the biodegradation of pesticides. Their ability to minimize the concentration of xenobiotics depends on their long-term adaptation to the environment. To increase the range and sustainability of the capability of microbes to degrade contaminants, bioremediation should be used as a technology for the removal of contaminants from the actual location of contamination.

It is important to have knowledge and understanding of the genetics of the appropriate microbe and biochemistry of microorganisms to improve the processes and achieve bioremediation with increased precision, and certainty in the functioning of the microbes. Moreover, it is necessary to improve gene coding for enzymes that have been identified for various pesticides to improve and provide new inputs to understand the capability of the microbes to biodegrade the pesticides and develop an advanced strain to achieve the desired result bioremediation within the shortest time possible. In situ biostimulation requires venting where oxygen and nutrients are injected into the soil.

These substances should be distributed evenly in polluted soil. In situ bioremediation is cheap than conventional remediation since it does not incur other overhead costs, such as transportation. However, due to limitations associated with in situ bioremediation, the best alternative is ex-situ bioremediation where the contaminated soil is transported and treated in another site.14 The ex-situ bioremediation includes approaches such as bioreactor where water and other nutrients are mixed with the contaminated soil to enhance the activity of the microbes. It is a method that best suits clay soils and is a remarkably quick process.

Thus, sustainability can be achieved if the challenges facing bioremediation of pesticides are overcome. This can make the process cheap and non-intrusive as a method to render toxic substances in the soil less harmful over time. More research should be conducted to improve the understanding of microbial behavior, and how they interact with pesticides. Furthermore, through genetic engineering, the sustainability of the microbes that have significant properties for biodegradation can be improved.15

Conclusion

Biodegradation is a process that occurs naturally, and it is a strategy by the microorganisms to enhance their survival when they degrade the xenobiotics. These microorganisms operate in their natural environment. However, certain modifications can make the microbes act at a faster rate in the shortest time possible. The advantages of the process of bioremediation as a destruction technology of contaminates lie in the shorter time it takes to clean up the toxic chemical substances in the environment. The implementation of the process requires training local people to understand the needed aspects of the technology, as well as, a long-term monitoring through which the achievements of the process can be determined.

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