1 Introduction

1.1 Background

Due to growing global industrialisation, numerous pollutants are being released, seriously harming all life forms. There is a risk of cancer, mutagenesis, and other harmful consequences, particularly in humans. Environmentally damaging pollutants including oil hydrocarbons, heavy metals, and pesticides have a negative impact on ecosystem health [1].

The remediation of polluted areas is crucial for both environmental preservation and urban development in order to restore the functions of the contaminated environment. The ability of microorganisms to degrade a wide variety of organic molecules and absorb inorganic chemicals is well documented. Currently, a procedure called “bioremediation” uses bacteria to clean up pollution. There are different types of bioremediation techniques: bioreactors, which are primarily used for water treatment; in situ land treatment for soil and groundwater; and biofiltration of the air.

However, the primary stage in successful bioremediation is the characterisation of the contaminated site. Preliminary evaluations of the environmental conditions, pollutant type, soil composition, removal costs, and treatment duration will determine which in-situ or ex-situ biological remediation strategies are the most suitable and practical. The use of surfactants and biosurfactants in remediation approaches stands out among alternatives because these chemicals can lower interfacial and surface tensions and speed up biodegradation in polluted soils.

It is worth noting that this technology has its own boundaries. Chlorinated hydrocarbons or other aromatics are virtually recalcitrant to biodegradation or are abated very slowly. Based on these considerations, this article will address how most soil contamination occurs within O&G operations, the different ex-situ or in-situ bioremediation techniques accessible, and the current scenario of new technologies related to bioremediation procedures.

Bioremediation is the procedure by which living beings such as algae, vegetation, and bacteria are used to regenerate, reduce or remove contamination from the environment. Hydrocarbons can be broken down by microorganisms into environmentally-safe products or are transformed by other microbial species into the water, CO2 and other inorganic elements. Since practical bioremediation entails the action of microbial enzymes to convert contaminants into non-hazardous substances, operating conditions must be adjusted to allow microbial cells to flourish quickly.

This bioprocess has stood up among the most promising emerging treatments to get rid of oil and derivatives since its first practice in 1989 to remediate the environmental disaster of Exxon Valdez [2].

1.2 The Invisible Workforce

By making nutrients available and producing the essential growth regulators, soil microorganisms actively design the soil to prepare it for plant development. They also aid in the breakdown of xenobiotics and the transformation of organic substances in the soil. In adhering to and desorbing inorganic nutrients from physical surfaces as well as deteriorating organic wastes to incorporate them into the soil, natural microbial communities serve unique functional functions.

The overall contribution of microorganisms and plants contributes to the suitability of the soil for farming and agriculture. It is highlighted that even little human interventions, such as sewage sludge injection, could raise the soil’s resident Proteobacteria and Bacteroidetes population and improve the process of soil formation in bauxite-overloaded disposal regions. Microbes control the numerous biogeochemical cycles, which are a crucial aspect in maintaining soil fertility in addition to the process of nutrient cycling.

Figure 1. Soil microbes. Courtesy of Pacific Northwest National Laboratory. Source: https://www.flickr.com/photos/pnnl/25164629313

The invisible workforce is a number of microorganisms working sequentially to cut down environmentally toxic substances. By exploiting microorganisms to convert contaminants into non-toxic compounds, bioremediation minimises pollution. Both aerobic and anaerobic organisms may be involved, and they frequently employ this breakdown as a source of energy.

For instance, Candida yeasts isolated from hydrocarbon-contaminated soil have demonstrated a high ability to break down toluene but not benzene, revealing the difficulty of the latter contaminant’s degradation [3]. Adsorption has also been considered as a method of removing BTEX in addition to microbial decomposition. An intriguing study comparing the effectiveness of organophilic clay as an adsorbent to traditional clay found that it removed BTEX with very high efficiency (95.6%), indicating that this material could be used as an alternative in environmental applications due to its favourable microstructure, ease of use, and low cost.

Amendments are made to allow for the introduction of bioremediation in situations where the appropriate microorganisms are insufficiently prevalent or entirely missing. The water or soil is injected with certain microbial agents like fungus, archaea, and some aerobic bacteria. When the right environmental conditions are present, a straightforward procedure known as bioaugmentation can improve conditions quickly.

Critical circumstances for bioremediation include some of the following:

  • Host microorganisms that act as a source of food and energy for parasitic organisms.
  • Parasitic microbes for consuming and eliminating their undesirable hosts.
  • Enough oxygen levels to facilitate aerobic biodegradation.
  • A lot of water or moisture in the soil.
  • The core of microbial life’s energy supply is a carbon source.
  • The ideal temperature for the microorganisms to thrive.
  • Some nutrients, such as phosphorus, sulphur, nitrogen, and potassium, help the development of microorganisms.
  • A pH balance between 6.5 and 7.5 creates the ideal balance of acid and alkaline.

2 Soil Pollution

2.1 Overview of Soil Contamination

The term “soil contamination” refers to the buildup of pathogens, chemicals, radioactive waste, salts, and other persistently toxic substances in the soil that are damaging to biological processes [4]. As a result, the balance of ecosystems and human health is impacted by the increased quantities of harmful substances in the soil, mostly caused by heavy metals, pesticides, and petroleum derivatives. When a pollutant enters the soil, it may be absorbed by the earth, removed by wind and runoff, or leached by infiltration water, moving to the lower layers and entering groundwater.

Soils can be adulterated by a variety of causes, such as chemical spillages or the accumulation of heavy metals from industrial emissions. Agricultural soils can become spoiled due to pesticide use or via the heavy metals contained within agrochemicals [5].

Reports on the primary sources of contamination in many different parts of the world were published during the 2018 Global Symposium on Soil Pollution (GSSP), held in Rome, Italy. Earthquakes, hurricanes, and human mistakes are the leading causes of soil pollution from oil spills across the African continent. Sometimes, such as in the case of terrorist attacks, conflicts, sabotage and bunkering, or abusive disposal, petroleum spills are not the result of accidents [6].

Despite the dearth of information on the Central and South American regions, indiscriminate use of pesticides and fertiliser spills from the mining, extraction, and transportation of petroleum products and open-air rubbish dumps are the main drivers of soil contamination. An overview of soil degradation in Europe showed that some countries have detailed lists of contaminated lands and focused primarily on research to remediate them, with mineral oils and heavy metals being the most common pollutants.

Figure 2. FAO’s propaganda on soil pollution. Source: https://www.fao.org/world-soil-day/about-wsd/wsd-2018/campaign-materials/en/

The main impacts of contamination can be summarised as follows:

  • In Agriculture: drops of soil fertility and nitrogen fixation, sludge storage, reduction of agricultural production, increase in soil erosion and nutrient depletion, and imbalance between vegetable and animal life in the soil.
  • In the Environment: consumption and changes in soil composition and microflora and unavailability or low productivity of the acreage for crops intended for food/feed.
  • In the Urban Environment: complications in waste management and issues of public health such as potable water contamination.

Three categories of soil remediation techniques—chemical, physical, and biological—are available, with the latter being applied either inside the polluted area (in situ) or outside of it (ex situ). Techniques for bioremediation use natural mechanisms that can successfully biodegrade various contaminants, even persistent ones; as a result, they may be a practical and efficient strategy to reduce soil contamination.

2.2 Petroleum and Derivatives

Besides other pollutants, oil pollution has become another major concern around the globe. Although it has a major impact on the marine environment, oil pollution of soil and inland water also occurs due to spills during transportation. The severity and toxicity of the oil contamination may depend on the degree of spillage and exposure of other organisms.

Although crude oil and its derivatives are responsible for human life comfort as per raw materials for the production of plastics and fuels, their sustained use can cause environmental problems that affect the atmosphere, soil, oceans, and other water resources. If used as energy transferors, oil products release carbon dioxide (CO2), the gas clichéd for the greenhouse effect, along with other air toxins such as nitrogen oxides (NOx), carbon monoxide (CO), and particulate.

In addition to oil spills, oil exploration and production (E&P) affect the environmental dynamics. Among the most notable instances of hydrocarbon pollution since the early twentieth century, blowout is the largest contributor to the global amount of oil spilled into the medium. Blowout is a rampant flow of crude oil, gas, or “produced” water from an oil well due to some failure in its pressure control system, which is mainly caused by human error [7].

Contamination by oil products can occur even lacking human intervention. Examples are natural oil flows or tar pits, where oil or underground bitumen moves up to the surface, respectively. In soils, petroleum hydrocarbons are present as an elaborate mixture of aliphatic and aromatic compounds, including volatile organic compounds from the BTEX group (benzene, toluene, ethyl benzene, xylenes) and Polycyclic Aromatic Hydrocarbons (PAHs).

These compounds are capable of inducing changes in particle texture, granulometry, shape and porosity in fine-grained soils to such an extent as to significantly reduce both the number of micropores and the overall surface; at the same time, the macropore features remain almost the same. In the case of coarse-grained soils, contamination can form hydrocarbon-coated particles and fill both macro- and micropores. Since physical interactions between particles are dominant in such a soil class, characteristics such as porosity, texture, and cohesion are the most affected.

2.3 Nature and Chemical Composition of Spilled Petroleum

The rate of biodegradation of petroleum hydrocarbons varies depending on the composition and chemical nature of the constituent parts. Crude oil is a liquid petroleum containing thousands of hydrocarbon components. Each component has a unique chemical behaviour that makes it either readily biodegradable, quite difficult to digest or not degradable at all.

Petroleum hydrocarbon molecules can be grouped into four broad categories: saturates (branched, unbranched and cyclic alkanes), aromatics – ringed hydrocarbon molecules such as monocyclic aromatic hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs), resins (polar oil-surface structures dissolved in saturates and aromatics), and asphaltenes (dark-brown amorphous solids colloidally dispersed in saturates and aromatics). In the structural arrangement of the four main hydrocarbon components of crude oil, saturates make up the outermost layer of the oil, whilst asphaltenes constitute the innermost portion of the oil due to their greater molar masses.

According to the Minister of the Environment and the Minister of Health Canada, the susceptibility of crude oil components to microbial degradation are in the following order: alkanes > light aromatics (MAHs) > cycloalkanes > heavy aromatics (PAHs) > asphaltenes. Resins are easily degraded naturally because they are light polar molecules [8].

PAHs contain more than one benzene ring, and those that are made up of two or three cyclic rings, such as naphthalene (two-ringed), phenanthrene (three-ringed) and anthracene (three-ringed) with molecular weights of 128, 178 and 178 g/mol, respectively, are referred to as low molecular weight or light PAHs. PAHs are made up of four rings and above, such as pyrene (four-ringed), chrysenes (four-ringed), fluoranthene (five-ringed), benzo[a]pyrene (five-ringed) and coronenes (seven-ringed) with molar masses of 202, 228, 202, 252 and 300 g/mol, respectively, are referred to as high molar masses or heavy PAHs.

PAHs are common petroleum contaminants in the environment that are considered to be potentially mutagenic and carcinogenic. The Breast Cancer Fund reported that heavy PAHs such as benzo[a]pyrene damage the DNA of living organisms (i.e. they are genotoxic) and are implicated in human breast cancer. This accounts for a large number of studies on the biodegradation of PAHs in order to safeguard the environment and biodiversity from severe long-term ecological and medical damage by oil spills. However, the focus has been on the biodegradation of light PAHs, whilst very little research has been carried out on the biodegradation of heavy PAHs that have been found to be of medical importance.

Asphaltenes are considered to be highly resistant to biodegradation due to their heavy, viscous nature. Asphaltenes are very complex chemical structures made up of sulphur (0.3-10.3%), nitrogen (0.6-3.3%), oxygen (0.3-4.8%) and trace amounts of metals such as iron, nickel and vanadium. In addition, asphaltenes have the highest molar mass of all hydrocarbon compounds in crude oil, with values ranging from 600 to 3 x 105 g/mol and from 1000 to 2×106 g/mol. This chemical complexity has rendered asphaltenes resistant to microbial attack, and unfortunately, few studies have been carried out to enhance the potential of biodegradation of asphaltenes [9].

2.4 Ecotoxicity of Hydrocarbons

Ecologically discussing bioremediation refers to the interaction between three factors: invisible workforce, contaminant, and environment.

The ecotoxicity of petroleum hydrocarbons in soil is an important basis for the management and control of petroleum pollution in soil. Only by exploring the toxicity mechanism of petroleum hydrocarbons based on the real ecotoxicity effect can we carry out remediation treatment and protection work of contaminated soil in the most optimal mode [10]. At present, researchers at home and abroad have carried out ecotoxicity tests based on soil enzyme activity, phytotoxicity, earthworm toxicity and luminescent bacteria toxicity tests of different species and levels, but there are relatively few quantitative studies on the ecotoxicity of petroleum-contaminated yellow cotton soil. Many studies have reported the changes in soil physicochemical properties, microbial community structure, and plant growth caused by oil pollution.

Petroleum derivatives can accumulate at various trophic levels in food chains and disrupt biological or physiological processes in several species, thus causing mutagenesis, impaired reproductive ability, and hemorrhage in the exposed population. Among petroleum by-products, the ecotoxicity of compounds belonging to the BTEX group is the most upsetting, as they are fat soluble, depress the central nervous system and are toxic at very low concentrations.

Figure 3. Workers cleaning Papamoa Beach after oil from the grounded ship Rena reached shore in New Zealand, 2011. Source: https://commons.wikimedia.org/wiki/File:Rena_oil_spill_cleanup.jpg

Plant secondary metabolites are known to change in response to a combination of environmental factors, both biotic and abiotic stressors. Phenolic compounds, for instance, change in response to a wide range of environmental factors, such as chemical pollution, light quality and intensity, mechanical wounding, nutrient limitations or frost, and herbivore or pathogen damage affecting above- or belowground plant tissue.

Phenolic compounds can be exuded by plant roots or leached from leaves. They can alter the uptake of elements; for example, coumarins coming from root secretions mobilise iron from the soil through reduction and chelation, helping to overcome the often limiting availability of this element in well-aerated or alkaline soils.

3 Trends In O&G Bioremediation

3.1 Classification

Oil bioremediation is highly efficient but still takes a few weeks or even months to complete, perhaps due to the diverse and complex processes involved. The use of microorganisms is affected by some factors comprising the type of pollutant, the state of environmental elements, as well as the occurrence of nitrogen and phosphorus sources. Temperature has a strong influence on oil hydrocarbon degradation due to its impact on petroleum physics and chemical composition, microbial metabolism, and the structure of the microbial conglomerate.

Figure 4. Different bioremediation strategies. Source: https://www.mdpi.com/1996-1073/13/18/4664

According to the sort of application, it is possible to categorize bioremediation as an in-situ or ex-situ process (Figure 4). Even though the ex-situ bioremediation methods are usually costlier because of excavation and transport expenditures, they can be applied to remove a more significant number of contaminants under controlled conditions. On the other hand, despite the nonexistence of excavation costs, sometimes the cost of installing the equipment on site, together with the impossibility of seeing and performing an effective control below the surface of a contaminated area, can make in-situ bioremediation techniques unfeasible.

Hence, the remediation cost is not the part that determines the method to be applied to a given contaminated site. Instead, the main factor in deciding which method of bioremediation to use is the type of contaminant.

In the context of bioremediation, the expression “in situ” means that bioremediation takes place at the unclean site without transferring polluted materials [11]. In-situ practices can be classified as intrinsic or projected bioremediation, the latter consisting of a series of techniques.

In the case of “ex-situ” techniques, the polluted material is withdrawn and degraded in select facilities outside the affected site. After excavation, the contaminated soil is moved elsewhere for treatment. The pick of an ex-situ technique is usually made based on the following aspects: extent and depth of contamination, operating costs, location and geological features of the contaminated site, and type of contaminant.

Ex-situ methods allow better control of environmental conditions, increasing the biodegradation rate compared to in-situ alternatives. Furthermore, thanks to the option of homogenising the polluted ground, the operation is naturally more uniform and takes a shorter time.

However, soil excavation leads to an increase in the mobility of pollutants and exposure to them; thus, the site must often be pre-adapted by deploying coating systems in the place in order to prevent the leakage of contaminants. Ex situ treatments can treat a far greater number of contaminants than in situ, but they are more expensive due to excavation and transportation.

The use of various bioremediating agents like fungi, bacteria, nematodes, and algae, and further contribution of modern technology, including nanoscience, are helping to find new ways to genetically  engineer microorganisms for need-based functions.

3.2 Bioreactors

The word ‘bioreactor’ refers to any gear or manufactured facility that supports a bioactive system. Sludge reacting equipment is used to treat hydrocarbon pollutants safely and quickly. Materials are kept in a container where a three-phase mixture is ultimately obtained. The biofilm created stimulates the biodegradation of contaminants and enriches the biomass level.

Remediation via bioreactors may be aerobic or anaerobic. Containers are often made of stainless steel or tough glass and generally have cylindrical shapes and volumes ranging from a few litres to cubic meters. The contaminated input can be supplied to the reactor as a dry substance or suspension.

Treating oil and gas waste in a bioreactor has many advantages over other methods, including satisfactorily controlling process variables (temperature, pH, substrate concentration, mixing intensity, aeration rate and inoculum level) and effectively handling heavy metals, pesticides and volatile organic compounds, including BTEX. Bioreactors are rated as one of the most excellent methods for treating contaminated soil, as the operating conditions can be controlled, thus allowing a proliferation in microbial biodegradation activity.

3.3 Nematodes

Nematode parasites are a sharp indicator of heavy metals in the aquatic ecosystem. They point to environmental pollution and, more interestingly, are involved in cleaning, nutrient mobilisation, nitrification, and enzyme activation in the rhizosphere.

For heavy metal treatment, nematodes are being used in [12]:

  • Nematodes Caenorhabditis elegans, Plectus acuminatus, and Heterocephalobus pauciannulatusare indicators of pollution but also excellent bioremediators of heavy metals in marine and pelagic habitats.
  • Bioaccumulation of heavy toxic metals in guts and muscles of fish can be done in the Ascarissp and Echinocephalus., which are reported as natural bioremediation of heavy metals in Liza vaigiensis.

3.4 Phycoremediation

While wetland-based bioremediation can be highly effective, the rate of operation is relatively slow. An alternative biological solution is a remediation through microalgal cultivation. In this case, microalgae either assimilate the water contaminants via metabollism, or the contaminants adsorb to binding sites inside the cell or to cell walls. Because this method permits the synchronous production of biomass through photosynthetic carbon fixation, microalgal-based systems could improve the economic viability of produced water bioremediation.

It is a fantastic form of remediation in aquatic ecosystems. Microalgae, called by some “wonder organisms,” can efficiently accomplish bioremediation by two mechanisms: bioassimilation and biosorption. They can grow in polluted water as “algal blooms” and assimilate various pollutants. Industrialisation has led to increased emission of pollutants into ecosystems. Metal pollutants can quickly enter the food chain if heavy metal-contaminated soils are used to produce food crops.

  • Algal blooms can grow in contaminated water and assimilate various toxins.
  • After harvesting and lipid/protein extraction, the algal biomass is used as an efficient biosorbent.
  • Algal blooms are excellent for removing pesticides from water bodies.

The obtention of economically viable microalgal cultivation systems is promoted by several factors, like the potential of the microalgae to grow on unclaimed, cheap wastewater their remediation performance related to the dissolved pollutants present, and their capacity to accumulate biomass that can be cost-effectively harvested and transformed into valuable products, for example, biodiesel.

The algae-based industry will likely rely on the development of genetically engineered strains with optimised traits tailored to the specific needs of customised cultivation systems. This suggest that model organisms, for which the science is most advanced, have great potential to generate the insights necessary for developing transformative technologies.

Phaeodactylum tricornutum is a marine microalgal organism known to be the preeminent model species within the prolific group of eukaryotic microalgae called diatoms. Being the prevalent members of phytoplankton communities in rivers, lakes, and oceans, diatoms fix up to 25% of atmospheric CO2 globally each year [13]. They are also deemed as one of the most promising candidates for creating cheap, viable next-generation biofuels; they are superior lipid producers (which they synthesise as an energy store in times of nutrient deficit) and because of the advanced genomic resources that have been developed over the past two decennia.

In this sense, the biotechnological potential of the model diatom Phaeodactylum tricornutum dramatically exceeds that of other species: it has the most extensive knowledge base and is amenable to the most advanced methods for genome editing. This has simplified, for example, the generation of improved lipid-producing strains. Furthermore, P. tricornutum has been successfully cultivated in various wastewater media, sometimes with simultaneous removal of nutrients and enhanced lipid production.

3.5 Biopiles

The process of bioremediation with biopiles involves stacking polluted soil and then aerating it to speed up biodegradation, mostly through increasing microbial activity. This technology’s components include leaching, aeration, and watering. Due to its construction qualities and the advantageous cost-benefit ratio that enable effective bioremediation, provided that proper management of nutrients, temperature, and aeration is guaranteed, it is being used more frequently [14].

Biopiles are appropriately constructed on a waterproof concrete slab to reduce the amount of leachate transferred to the surface. They are covered with a waterproof membrane to prevent pollution emissions, the contaminated ground outside, and the effects of wind and rain.

3.6 Intrinsic bioreemdiation

Intrinsic bioremediation, also known as passive bioremediation or natural attenuation, is a type of natural degradation that relies only on the metabolism of local microbes to eliminate harmful toxins and makes no use of an artificial stage to speed up the biodegradation process.

The lack of outside influences makes this approach the least expensive in-situ bioremediation method. However, for bioremediation to be ongoing and sustained, regular monitoring is required.

The critical requirements to be met to make the application of intrinsic bioremediation effective are the following:

  • optimum environmental conditions (pH, temperature, moisture degree, O2 concentration);
  • available C and N sources to sustain microbial growth;
  • suited population of biodegrading organisms in the polluted site;
  • sufficient time for microbiota to convert pollutants into less harmful products.

Considering that native bacteria are highly effective in degrading pollutants for the reason that they are already adapted to the site conditions and that there is a microbe/hydrocarbon liaison established in the course of evolution, the hydrocarbon-degrading aptitude is common among microbial consortia. Before applying intrinsic bioremediation, though, a risk assessment should be performed to ensure that the time needed to complete bioremediation is shorter than that required by the chemical to reach the closest point of human and animal exposure.

3.7 Bioslurping

Bioslurping process (also known as multi-phase extraction process, vacuum-enhanced extraction, or even sometimes dual-phase vacuum extraction—DPVE) is a technology that employs a high-vacuum system to extract both contaminated groundwater and soil vapour. In practice, liquid and vapour can be withdrawn together under a vacuum operating a down well stinger (called slurping) or separately, pulling the liquid with a submersible pump and the gas with a soil vapour extraction unit. Recently, the stinger approach has been proven to be generally more efficient [15].

Additional benefits of the process are the capability to lower the water table within the contamination plume, causing contaminants in the newly exposed capillary fringe to be attainable by soil vapour extraction (SVE).

Systems must be designed and operated on a site-by-site basis. The technique suits medium permeability horizons with generally volatile or semi-volatile pollutants or free phase products. Separated water and free phase products are split, and the water is treated and either returned to the site or disposed to the sewer. The free phase derivative is collected and disposed of off-site at a recycling facility; abstracted vapours are treated to remove volatile organic compounds.

The technique also facilitates the ingress of air and thus oxygen into the plume, enhancing microbial activity and thus in situ biological treatment. This is often called bio-slurping or bioventing and can help reduce dissolved phase contaminants following product removal.

3.8 Biosparging

Bio-sparging is a technique in which air is forced beneath the groundwater table. This injection enriches the water with oxygen, and the biological breakdown is stimulated. Generally, the injected air quantities tend to be lower than those in conventional sparging due to the volatilisation of contaminants.

The process uses indigenous microorganisms to biodegrade organic constituents in the saturated zone. Such as the area in an aquifer below the water table, where relatively all pores and fractures are saturated with water. In biosparging, air (or oxygen) and nutrients (if needed) are added into the saturated zone to enhance the biological bustle of the indigenous microbiota. Biosparging can be used to slash concentrations of chemical constituents (such as crude oil constituents and crude products) that are dissolved in the water and within the capillary fringe.

3.9 Soil Health Restoration

Soil health can be managed not only by removing the accumulated harmful chemicals but also by adding more minerals to the soil to improve its health. Utilising microorganisms that can both increase the fertility-contributing nutrients in the soil and simultaneously remove or mitigate the effects of harmful xenobiotics will restore the health of the ground.

The current trend debates the prospects of utilising the microbial detoxifying, biotransforming, and bioremediatory role in eliminating various xenobiotics in the soil, with a simultaneous role in upgrading soil fertility. Additionally, the use of bioengineered microbes to assist the process of detoxification or enhance their efficiency is also targeted in this paper to find effective solutions for timely soil health restoration.

Rhizoremediation, the phenomenon of improving soil health using root-associated microbes, comprises the participation of rhizobacteria that transform xenobiotics in their root area and simultaneously produce plant growth-promoting factors to aid the plants. The use of plant growth-promoting rhizobacteria (PGPR) for the treatment of many soil pollutants such as petroleum, heavy metals, particularly mercury, polychlorinated biphenyls, etc., is found to be very effective [16].

3.10 Advanced Bioengineering

Since oil is a complex mixture of hydrocarbons, genetically modified microorganisms are more efficient in remediating these contaminated sites than indigenous strains.

Superbug growth by plasmids containing multiple genes with degrading enzymes may be inoculated in an organism. A tailored Acinetobacter baumannii S30 pJES with the high efficiency to cut down total petroleum hydrocarbon (TPH) was produced with a reporter lux gene that facilitates bioremediation site monitoring. Likewise, Streptomyces coelicolor M145 was engineered to boost the efficiency of n-hexadecane degradation by overexpressing alkB gene encoding for the enzyme alkane monooxygenase.

In another development, Acinetobacter sp. BS3 was generated with insertion of xylE gene encoding for catechol 2,3-dioxygenase enzyme from Pseudomonas putida strain BNF1 accountable for biodegradation of petroleum derivatives, which are aromatic in nature. This custom-made strain expressed enzyme with broad substrate specificity, hence exhibiting a superior efficacy in degrading a diversity of n-alkanes and other aromatic hydrocarbons when compared to its wild strain.

Figure 6. The diagram depicts two approaches to flooding indigenous bacterial populations with catabolic genes of interest. Source: https://www.nature.com/articles/s41598-020-72138-9

Engineered psychrophilic recombinant Antarctican Pseudoalteromonas haloplanktis TAC125 auspiciously expressed toluene-o-xylene monooxygenase (capable of degrading a comprehensive list of aromatics) along with its inherent laccase-like protein to try the remediation of cold and marine xenobiotic loaded effluents. Such solutions will allow the remediation of aromatics even in cold climate regions whenever needed.

Another major hazard is heterocyclic aromatic compounds (HACs) and polycyclic aromatic hydrocarbons (PAHs), which are indispensable raw materials in drug and pesticide manufacturing. These HACs are highly toxic, carcinogenic, and mutagenic to living organisms.

Bacteria belonging to Sphingomonas and Sphingobium genera were found to accomplish biodegradation of such toxic compounds. Strains of these genera were also proficient in degrading an array of hydrocarbon compounds like dioxins, acridine, fluorene, carbazole, m-xylene, HCH, phenanthrene, pentachlorophenol (PCP), etc., which are aromatic in nature.

5 Some Final Words On The Future Of Bioremediation

Despite the fact that bioremediation is not a new approach, our ability to use hidden microbial interactions to our prospective advantage grows as we gain more understanding of these processes. Bioremediation uses less energy and resources than traditional technology, and it prevents the buildup of hazardous waste in the form of poisonous by-products. Even though using traditional methods may sometimes take substantially longer, bioremediation provides financial and technological benefits.

The specific microorganisms necessary for pollutant breakdown are assisted by selecting the limiting component needed to promote their development, and bioremediation may be tailored to the needs of the polluted location in question. The application of synthetic biology methods and procedures for pre-adapting microbes to the pollution in the environment to which they are to be supplied may further increase this specific tailoring [17].

Pollution is a hazard to human health because it causes environmental harm, which affects the sustainability of our world and natural life. Our ability to grow food is further hampered by damaged soils. To ensure that our children and grandchildren have access to clean air, water, and healthy soils, bioremediation can help reduce and eliminate the pollution created.

6 References

[1] https://microbiologysociety.org/blog/bioremediation-the-pollution-solution.html

[2] https://archive.epa.gov/epa/aboutepa/bioremediation-exxon-valdez-oil-spill.html

[3] Hesham, A.E.L.; Alrumman, S.A.; ALQahtani, A.D.S. Degradation of toluene hydrocarbon by isolated yeast strains: Molecular Genetic approaches for identification and characterisation. Russ. J. Genet. 2018, 54, 933–943.

[4] Mareddy, A.R. Environmental Impact Assessment. Theory and Practice; Technology in EIA; ButterworthHeinemann: Oxford, UK, 2017; Chapter 13; ISBN 9780128111390.

[5] FAO. Status of the World’s Soil Resources; Food and Agriculture Organization of the United Nations: Rome, Italy, 2015; ISBN 9789251090046.

[6] FAO. Be the Solution to Soil. Available online: http://www.fao.org/3/Ca1087en/Ca1087en.pdf

[7] Sales da Silva, I. G., Gomes de Almeida, F. C., Padilha da Rocha e Silva, N. M., Casazza, A. A., Converti, A., & Asfora Sarubbo, L. (2020). Soil bioremediation: Overview of technologies and trends. Energies13(18), 4664.

[8] https://www.canada.ca/content/dam/eccc/documents/pdf/pded/base-oils/Draft-screening-assessment-base-oils1.pdf

[9] Macaulay, B. M., & Rees, D. (2014). Bioremediation of oil spills: a review of challenges for research advancement. Annals of environmental Science8, 9-37.

[10] https://www.epa.gov/sites/default/files/2016-01/documents/bmpfin.pdf

[11] Banerjee, A.; Roy, A.; Dutta, S.; Mondal, S. Bioremediation of hydrocarbon—A review. Int. J. Adv. Res. 2016, 4, 1303–1313.

[12] https://www.intechopen.com/chapters/81848

[13] Gillard, J.T.F.; Hernandez, A.L.; Contreras, J.A.; Francis, I.M.; Cabrales, L. Potential for Biomass Production and Remediation by Cultivation of the Marine Model Diatom Phaeodactylum tricornutum in Oil Field Produced Wastewater Media. Water 2021, 13, 2700. https://doi.org/10.3390/w13192700

[14] Jørgensen, K.S.; Puustinen, J.; Suortti, A.M. Bioremediation of petroleum hydrocarbon-contaminated soil by composting in biopiles. Environ. Pollut. 2000, 107, 245–254.

[15] James G. Speight, 8 – Remediation technologies, Editor(s): James G. Speight, Natural Water Remediation, Butterworth-Heinemann, 2020, Pages 263-303, ISBN 9780128038109, https://doi.org/10.1016/B978-0-12-803810-9.00008-5.

[16] Sharrel Rebello, Vinod Kumar Nathan, Raveendran Sindhu, Parameswaran Binod, Mukesh Kumar Awasthi & Ashok Pandey (2021) Bioengineered microbes for soil health restoration: present status and future, Bioengineered, 12:2, 12839-12853, DOI: 10.1080/21655979.2021.2004645

[17] https://www.sciencerepository.org/bioremediation-brought-to-you-by-the-invisible-workforce