Abstract

Petroleum hydrocarbons are widely contaminating the terrestrial and marine environments. Alkanes, cycloalkanes, aromatics, polyaromatics, and other chemicals make up petroleum. While all of these hydrocarbon groups pose a risk to the environment, the carcinogenic qualities of polyaromatics and their breakdown products are particularly well-known. This review attempts to identify the microorganisms that break down petroleum hydrocarbons and the factors that affect petroleum biodegradation to demonstrate the mechanism of microbial degradation of crude oil.

Petroleum hydrocarbons are broken down by the natural biochemical mechanisms of hydrocarbon-utilizing bacteria (HUB), which use them as a source of carbon and for other cellular functions. The common groups of HUB are iron-reducing bacteria, methanogenic archaea, fermentation bacteria, and sulfate-reducing bacteria.

By enzymatically functionalizing or activating the petroleum hydrocarbons to yield metabolites that are advantageous to their survival, or by producing biosurfactants to promote the breakdown of petroleum hydrocarbons, bacteria use biochemical mechanisms to break down petroleum hydrocarbons. The two main metabolic routes that HUB uses to break down petroleum hydrocarbons are anaerobic and aerobic.

Keywords: Biodegradation, Crude oil, Hydrocarbon

Introduction
Environmental pollution with crude oil has been a serious worldwide environmental concern. Crude oil causes drinking water pollution, decline in the water and air quality as well as soil fertility. The accidental spillage of crude oil or its byproducts is to blame for this harm to the ecology. By filling the soil pores, petroleum hydrocarbons may decrease the aeration and water permeability of the soil, which could have detrimental impacts on plants’ ecology and toxicity as well as destroy the oilfield’s natural state. Furthermore, because to their biological toxicity and endurance, carcinogenic and mutagenic crude oil chemicals have the potential to produce catastrophic genetic abnormalities even at low pollution levels. As a result, they have received attention, particularly due to their hydrophobicity and complicated structure (Yu et al., 2020).

Crude oil has been more significant as an energy source since the middle of the 1950s. Petroleum hydrocarbons are the alkanes, cycloalkanes, and aromatic alkanes that makeup between 50% and 80% of crude oil. However, the environmental degradation caused by these petroleum hydrocarbons has resulted in significant issues across the globe (Hanafy et al., 2016).

Environmental pollution can result from the extraction, transportation, and refining of petroleum, with crude oil leaks throughout these processes being the primary source of pollution. Crude oil leaks are dispersed throughout the terrestrial and aquatic environments. The everyday lives of the people living in the vicinity will be severely impacted by the dispersed crude oil if it is not managed promptly. Additionally, the land and water ecosystem will be negatively impacted. Physical methods, chemical methods, and combination approaches have been widely used for treating petroleum hydrocarbon pollution. However, the cost of treatment is higher and these procedures frequently fail to eliminate all of the dissolved and emulsified oil from soil and water. Physical or chemical approaches should only be utilized in an emergency since they will repeatedly pollute the surrounding environment. Certain oil-loving microbes in the environment have been known to break down petroleum hydrocarbon contaminants via biological techniques. This approach has cheap processing costs and doesn’t pollute repeatedly. In nature, a wide variety of microorganisms can break down contaminants made of petroleum hydrocarbons. Certain hazardous and poisonous petroleum hydrocarbon pollutants can be converted into innocuous compounds, and some hydrocarbon substances can even be completely broken down, by employing the right microbes that break down petroleum. (Li et al., 2019).

This review aims to show the microbial degradation mechanism of oil crude and determine petroleum hydrocarbon-degrading microorganisms and factors influencing petroleum biodegradation.
Petroleum biodegradation
The process by which naturally occurring microorganisms, such as bacteria, break down petroleum hydrocarbons is one of the main ways that petroleum contamination is removed from the environment. Microbes like yeast, fungi, bacteria, and microalgae can break down petroleum hydrocarbons. However, a variety of microorganisms can break down and/or make use of petroleum hydrocarbon substrates. More than 200 species of bacteria are capable of biodegrading petroleum hydrocarbons, and a larger Pollutants made of petroleum hydrocarbons are broken down in site by consortiums of microorganisms, i.e., the combination of several microbial species as opposed to depending solely on a single species’ ability to catabolize (Figure1) (Victor et al., 2020).

Figure 1: Crude oil’s basic elemental composition, which usually varies little from source to source

Hydrocarbon-degrading species have been identified in several genera, including Bacillus, Aeromonas,Thiobacillus, Acinetobacter, Lactobacter, Staphylococcus, Penicillium, Articulosporium, Halomonas, Klebsiella, Proteus, Aspergillus, Micrococcus, Neurospora, Rhizopus, Mucor, Trichoderma. These microorganisms are found in smaller quantities in uncontaminated environments, but have been isolated in high numbers from numerous oil-polluted streams and soils. The processes and capacities of degradation vary amongst microbial species. By using both aerobic and anaerobic degradation pathways, hydrocarbon-utilizing bacteria (HUB) convert petroleum-based organic contaminants into a source of carbon and energy for their cellular processes. (Pandolfo et al., 2023).

Petroleum Bacterial Degradation
Bacteria represent the active microorganisms in the petroleum breakdown biochemically, acting as a main oil degraders spills into the environment. When it comes to the biodegradations of oil crude, a combination of bacterial strains with diverse enzymatic capacities works better (Zulfiqar and Safia, 2012). Groups of bacteria known as hydrocarbon-utilizing bacteria (HUB) have the innate ability to break down petroleum hydrocarbons. They accomplish these tasks by using petroleum hydrocarbons as a source of energy and carbon for biological functions. According to Victor et al. (2020), the petroleum-degrading bacteria can be divided into four groups according to their biological activities: iron-reducing bacteriafermentation bacteria, , methanogenic archaea, and sulfate-reducing bacteria.

1. Bacteria in fermentation
It has been discovered that crude oil contains a wide variety of fermentation microorganisms. These microbes can accept electrons from hydrocarbons, proteins, carbohydrates, and H2. Carbon dioxide and other gases, as well as organic acids, are the end products of metabolic processes.

2. Bacterial Reducing Sulfate
Sulfates and other oxygenated of sulfur compounds (sulfites, thiosulfate, elemental sulfur, , tetrathionate) are used by bacterial reducing sulfate, which are heterotrophic organism and absolute anaerobes, as final electron acceptors in respiration processes.

3. The Archaea Methanogenic
An important class of microorganisms found in crude oil reservoir environments are the methanogenic archaea bacteria. Since methane is the byproduct of their activity, the rate or volume of methane produced serves as a proxy for the biological activity of these bacteria.

4. Bacterial reducing iron
In addition to reducing iron, Shewanella putrefaciens may also convert thiosulfates, sulfites, and elemental sulfur into sulfides. Oil reserves present extreme circumstances that this bacteria can endure. Iron hydroxides and oxides can be the acceptors of electrons, and H2 or formate can be the electron donor.

The Degradation Mechanism of Petroleum Hydrocarbons
The primary cause of microorganisms’ breakdown of petroleum hydrocarbons is the catalysis of intracellular enzymes. There are four primary phases in the microbial breakdown of petroleum hydrocarbons: First, surfactants secreted by microorganisms emulsify petroleum pollutants, next, the surface of the microorganism adsorbs the emulsified petroleum hydrocarbon, finally, the petroleum hydrocarbon adsorbed on the cell membrane surface enters the cell membrane either actively or passively. To degrade the pollutant, the petroleum hydrocarbon that enters the cell finally engages in enzymatic process with a matching enzyme (Li et al., 2019). Under aerobic conditions, the bulk of organic pollutants degrade most quickly and completely. The basic idea of aerobic hydrocarbon breakdown is depicted (Figure 2(. The enzymatic response that is performed by oxygenases and peroxidases is the activation and incorporation of oxygen during the early intracellular attack of organic contaminants, which is an oxidative process (Das and Chandran, 2011).

Figure 2: Fundamental concept of microorganisms’ aerobic hydrocarbon decomposition

It is possible to understand the intrinsic mechanisms by which bacteria break down petroleum hydrocarbons from two different biochemical viewpoints :

The first mechanism is called activation/functionalization, and it is carried out by a particular system of bacterial enzymes that attack the organic pollutants intracellularly using addition, hydration, carboxylation, or oxygenation. This produces a substrate and metabolite which is then bio-transformed to central metabolites, which are used as carbone source for the cells and activities. Anaerobic and aerobic microbes have distinct mechanisms for activating hydrocarbons (Boll and Heider,2010) .

Table1: Summarizes the anaerobic and aerobic processes by which bacteria activate hydrocarbons

Hydrocarbon absorption by bacteria via the synthesis of biosurfactants:
A diverse range of microorganisms produce a heterogeneous set of chemical molecules that are surface active, known as biosurfactants. They improve the organic contaminants’ solubilization and elimination. 90% of hydrocarbons could be broken down in six weeks in liquid culture by a microbial consortium made up of one strain of Rhodococcus erythropolis and two isolates of Pseudomonas aeruginosa from soil polluted with oily sludge. The utilized of crude biosurfactants for remediation of hydrocarbon was supported by these findings (Das and Chandran, 2011).
One of the most well-known bacteria that can use petroleum hydrocarbons as a source of carbon and energy is pseudomonads, which produce biosurfactants (Figure 2.3). Pseudomonads such as P. aeruginosa are extensively researched for their ability to produce biosurfactants of the glycolipid type. Biosurfactants give oil a larger surface area so that microorganisms can use it. By lowering surface tension and creating micelles, biosurfactants can function as emulsifying agents. According to Victor et al. (2020), the hydrophobic microbial cell surface encapsulates the micro droplets, which are then carried within and broken down.

Figure3: Hydrocarbon absorption involves the use of a biosurfactant (rhamnolipid) produced by Pseudomonas sp.

After emulsification and production of micelles to improve the bacteria’s absorption of organic contaminants and subsequent breakdown, bacteria can produce biosurfactants that solubilize the organic pollutant (Victor et al., 2020).
Factors Affecting the Degradation of Petroleum Hydrocarbons
The biodegradation of petroleum hydrocarbons is influenced by several limiting parameters, as reported by (Das and Chandran,2011,Victor et al.,2020).
1.Temperature
It is known that several limiting factors influence how quickly petroleum hydrocarbons biodegrade. Temperature is one of the physical parameters that affects the biodegradation of hydrocarbons the most since it directly affects the chemistry of the pollutants and the physiology and diversity of the microbial flora. The beginning of biodegradation was postponed at low temperatures because the oil’s viscosity rose and the volatile low molecular weight hydrocarbons became less volatile. Temperature has an impact on hydrocarbon solubility as well. Hydrocarbon biodegradation can happen over a wide variety of temperatures, although as the temperature drops, the rate of biodegradation usually slows down. The maximum degradation rates, which typically occur in the range of 30–40°C in soil habitats, 20–30°C in some freshwater environments, and 15–20°C in marine environments, are depicted in Figure 2.4.
2. Availability of Oxygen
Degradation of hydrocarbons occurs both with and without oxygen. However, since oxygenases are the main enzymes required for breakdown to occur, aerobic conditions are more favorable. Because oxygen is necessary for oxygenases to function, aerobic environments result in higher degradation rates than anaerobic ones.

3. Availability of Nutrient
The addition of necessary nutrients, such as phosphorus and nitrogen, can accelerate the biodegradation process. When there is a petroleum oil leak and there may be a severe nitrogen shortage, nitrogen, carbon, and phosphorus were added as a ratio of roughly 100/10/1 (C/ N/ P).

Figure 4: The rate hydrocarbon degradation in a soil, marine environment , fresh-water

4. Potential of hydrogen
At neutral pH, the highest rates of breakdown are often seen. Nonetheless, even at pH 2-3, bacteria that thrive on hydrocarbons have been isolated from previously contaminated areas.
5.Salinity
Depending on the kind of habitat and the species involved, the effect of salt content on degradation varies. Degradation is typically inhibited by higher salt concentrations. It seems that the natural variation in the salinity for the source sample determines the salinity variation to influence the pace of hydrocarbon dissolution.

6.Light
The presence of light can positively affect the hydrocarbons that photosynthetic microorganisms like algae break down. Petroleum compounds can also be directly photochemically broken down by light. The photochemical processes in petroleum compounds have an impact on their physical properties. Light can affect the solubility of petroleum compounds and the creation of emulsions.

Impact of Pollution from Petroleum Hydrocarbons
The primary consequences of hydrocarbon pollution are: increased temperatures worldwide, the extinction or endangered status of vulnerable animal species as a result of poisoning the air, land, and oceans, decreased agricultural land production if oil spills contaminate the soil, the poisoning and oxygen deprivation that kill fish in lakes, ponds, rivers, and other bodies of water when they are contaminated by petroleum pollutants, resulting in financial loss, The majority of hydrocarbons cause cancer, Inhaling hydrocarbons can irritate the respiratory system and trigger the onset of allergies such as asthma, According to Anwesha et al. (2016), polycyclic aromatic hydrocarbons (PAHs) can result in developmental abnormalities, low birth weight, and prenatal problems.
Conclusions
Petroleum hydrocarbons are bad for people’s health and the environment. One of the main, organic processes for removing petroleum hydrocarbons contaminants from environment is the breakdown petroleum hydrocarbons by certain bacteria. Both aerobic and anaerobic environments can support bacterial hydrocarbon decomposition. Numerous environmental factors, including temperature, pH, salinity, availability of nutrients and oxygen, and light, might affect biodegradation.
Acknowledgment
None
Conflicts Interest
None
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