Classification, techniques and cellular processes of microbial remediation

In the previous article, the concept of bioremediation was introduced as an answer to the rising instances of environmental pollution. Although the act of bioremediation can be performed by a wide range of organisms including plants, fungi and microorganisms. Microbial remediation has proven to be the most advantageous and efficient process owing to its wide span of metabolic pathways and enzymes. In this article, bioremediation as a process has been classified, on the basis of application and cellular level processes by the microorganisms. 

Microbial remediation can be classified on the basis of the scale of the bioremediation activity. The large scale practical applications also known as in-situ bioremediation and the cellular level processes of the microorganisms are involved. Practical applications of microbial remediation deals with different techniques of undertaking bioremediation at the site of contamination, the cellular level processes of bioremediation involves various metabolic pathways that microbes use in remediating or transforming the contaminant molecules.

Factors influencing microbial remediation

Bioremediation methods involve designing of cost-effective and reliable treatment plans of a contaminated site on the basis of environmental and nutrient or pollutant factors (1). Furthermore, these factors (shown below) influence the cellular processes of the microorganisms involved and since suitable environmental conditions is required for effective microbial growth at the site, bioremediation approaches often require varying levels of environmental manipulation for success (2).

Factors influencing bioremediation processes, (Source: Sharma 2012)

In-Situ microbial bioremediation

In-situ bioremediation has been classified into multiple approaches on the basis of the extent of human intervention and additional nutrient requirement (3). The table below shows a comparison of different bioremediation approaches on the basis of cost, area of contamination and degree of human intervention. Intrinsic bioremediation refers to naturally occurring processes as a result of existing microorganism population at the site of contamination and requires no human intervention (4). Where naturally occurring bioremediation can be time consuming. Acceleration of the metabolic processes and supplementing microbial growth by addition of oxygen by bioventing or biosparging or nutrients by biosimulation at the site of contamination is advantageous (4).

The process of biosorption has been defined by Kotrba (2012) as a property of living and dead biomass to bind and concentrate inorganic and organic compounds. It involves two processes:

1. Passive biosorption; property shown by dead biomass and fragments of cells or tissues.
2. Active biosorption; active sorption or uptake of compounds by live cells (5,6).

This type of bioremediation process requires addition of dead biomass or specific strains of microbes at the site of contamination in order to encourage sorption of the contaminant. For the purpose of transforming the pollutant to a less toxic form, bioaugmentation technique is applied. Furthermore, it involves introduction of pollutant degrading microbes in the contaminated environment for supplementing the indigenous population as well as speeding up the degradation process (7).

Types of in-situ bioremediation strategies

Cellular microbial bioremediation

At the cellular level, the process of microbial remediation can occur in multiple ways based on the chemical nature of the pollutant. The microorganisms undertake metabolically dependent or independent pathways of remediating the contaminant compound. The physico-chemical process of biosorption has been categorized into different processes like (8):

1. Native biosorption,
2. Complexation (Enzymatic or Extracellular)
3. Precipitation

Native biosorption is the process of passive adsorption of charged compounds onto the microbial surface as a result of electrostatic interaction. It occurs passively without any microbial energy loss. Complexation and precipitation both occur in an actively dependent manner. Precipitation occurs when there is change in speciation of the pollutant as a result of its interaction with a precipitant or by changing levels of environmental pH. It leads to the formation of an insoluble pollutant compound that precipitates on or around the microbial surface (9). The most common examples of precipitants released by microbes are phosphates, carbonates and sulfides produced and released by bacteria for precipitation of soluble metal ions from their surrounding matrix (4,9).

Complexation of pollutants

Complexation of pollutants can be enzyme dependent on microbial surface or through extracellular complexation. Microbes often secrete extracellular compounds such as extracellular polymeric substances (EPS), slime or capsule. It interacts with certain pollutants and immobilizes or precipitates them (8). For example extracellular polymeric substances commonly secreted by microbes helps in biofilm formation. It has the ability to entrap metal ions as well as particulate metal precipitates leading to their immobilization and separation from the environment (10).

Furthermore, microbial remediation by cell surface enzymatic complexation helps in breaking down of the pollutant or direct transformation of the pollutant in the environment without entering the cell (11). This metabolically dependent enzymatic bioremediation happens in the cell by transformation to a less toxic compound or bioassimilation (12).

Consequently, microbial remediation techniques can be selected and applied on the basis of:

1. Overall costs.
2. The extent of human intervention possible.
3. The pollutant and the indigenous microbial community at the site of contamination.

Introduction of specific strains of microorganisms or genetically modified organisms (GMOs) at the site of contamination will require thorough pilot testing as well as review of unpredictable changes in the local ecosystem.

References

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2. Sharma S. Bioremediation: Features, Strategies and applications. Asian J Pharm Life Sci. 2012;2(2):202–13.
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10. Li W, Yu H. Insight into the roles of microbial extracellular polymer substances in metal biosorption. Bioresour Technol [Internet]. 2014 [cited 2016 Oct 18];160:15–23. Available from: http://www.sciencedirect.com/science/article/pii/S0960852413018002
11. Cunha A, Almeida A, Coelho F, Gomes NCM, Oliveira V, Santos AL. Bacterial extracellular enzymatic activity in globally changing aquatic ecosystems. In: Mendez-Vilas A, editor. Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology [Internet]. Formatex Research Center; 2010 [cited 2016 Oct 18]. p. 124–35. Available from: https://www.researchgate.net/profile/Adelaide_Almeida/publication/268291343_Bacterial_Extracellular_Enzymatic_Activity_in_Globally_Changing_Aquatic_Ecosystems/links/54bce4100cf253b50e2d822f.pdf
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