Isolation And Identification Of Crude Oil Degrading Bacteria From Soil Polluted With Automobile Lubricants

1.2.4. PHYSIOCHEMICAL VERSUS BIOLOGICAL METHODS FOR DEGRADING PETROLEUM HYDROCARBON POLLUTANTS

Several conventional engineering based physicochemical decontamination method are very costly due to the cost of transportation and excavation of large quantities of contaminated material of ex-situ treatment such as chemical inactivation, soil washing and incineration (Varjani, 2017). Other physicochemical methods include scorpion, abiotic transformations volatilization, comer, dispersion and dilution have been applied to return polluted sites to their pre-contamination (Jain et al., 2011).

The increasing cost and limited efficiency of these physicochemical treatment have initiated the development of alternative technologies for in-situ applications, particularly based on biological remediation capabilities of plants and microbes (Varjani, 2017). Biological methods for treatment of contaminated site is cost-effective and lead to complete mineralization (high efficiency) (Nilanjana and Preethy, 2011).

Physicochemical methods have limitations including ineffectiveness, cost of application, and release of toxic compounds. For example, incineration is a very effective soil treatment method, but it is very expensive, leading to loss of nutritional value and destruction of soil structure (Jain et al., 2011) while Bioremediation is safer because it does not lead to the release of toxic compounds to the environment; it is also cost effective and completely destroys the contaminant and applicable to large areas (Bento et al., 2005). Bioremediation uses environmentally sustainable techniques (Megharaj et al., 2005). The process does alter or modify the food chain in the treated environment: it respects the fauna and flora of the polluted environment. It is a biotechnological practice that has gained relevance worldwide, because of its environmentally friendly and very low in cost.

The application of bioremediation methods eliminates the costs for soil transportation because the ground can be remediated directly in the polluted area. It is a feasible approach that can be applied in areas that are difficult to access (Xenia and Refugio, 2016).

1.2.5. MICROBIAL DEGRADATION OF PETROLEUM HYDROCARBON

Microbial bioremediation is a widely used technique for remediating crude oil pollution in both aquatic and terrestrial environments (Abbasian et al., 2015). Microorganisms such as bacteria, fungi, algae are reported as primary degraders and most active agents in hydrocarbon degradation (Abbasian et al., 2015 and Meckenstock et al., 2016). It is important to access biodegradation in the light of a multi-domain community in order to comprehend complete microbial community (Widdel and Radus, 2001).

Biodegradation of petroleum hydrocarbon is a complicated process that depends on the amount and nature of the hydrocarbon. Several factors that affect hydrocarbon degradation have been reported. One of the important factors that limit petroleum hydrocarbon degradation is the availability of microorganisms.

Petroleum hydrocarbons differ in their vulnerability of hydrocarbons to microbial biodegradation can be ranked as follows: linear alkanes > branched alkanes > small aromatics > cyclic alkanes (Nilanjana and Preethy, 2011). Some compounds, such as the high molecular weight polycyclic aromatic hydrocarbons (PAHs), may not be degraded at all (Atlas and Bragg, 2009). Petroleum hydrocarbons in the ecosystem are biodegraded primarily by bacteria, yeast and fungi. The reported efficiency of biodegradation ranged from 6% to 82% from soil fungi, 0.13% to 50% for soil bacteria, and 0.003% to 100% for marine bacteria.

Scientists reported that mixed pollution with overall broad enzymatic capacities are needed to biodegrade complex mixture of hydrocarbons such as crude oil in soil marine environment and fresh water (Nilanjana and Preethy, 2011). Biodegradation of petroleum hydrocarbon pollutant involves series of steps using diverse enzymes (Abbasian et al., 2013). Consortium has been proved to be more effective than individual cultures for complete biodegradation of hydrocarbons (Varjani et al., 2013). It has been reported that n-alkanes are preferred for biodegradation by microbes than polycyclic aromatic hydrocarbons (Widdel and Rabus, 2001). This may be due to availability of petroleum hydrocarbon (sole source of energy and carbon) compounds subjected to bacteria degradation, adaption of petroleum hydrocarbon pollutant degraders to contaminated environment and presence of enzymes which contribute to different biodegradation pathways (Meckenstock et al., 2016). Microorganisms' enzymes are majorly encoded on plasmids.

Acinetobacter spp are exceptional with regards to location of biodegradative enzymes which is located on chromosome (Sallah et al., 2003). The degradation of petroleum hydrocarbons can be mediated by specific enzymes system (Das and Chandran, 2011). Initial attack is generally attained by various mechanisms such as attachment of microbial cells to the substrates and production of biosurfacants, solvents, gases and acid (Saeki et al., 2009). Microorganisms either metabolize petroleum hydrocarbon contaminants to obtain energy or assimilate them into cell biomass (Leahy and Colwell, 1990).

There are three possible methods for petroleum hydrocarbon degradation:

Phototropic anoxygenic;

Chemotheraphic aerobic; and

Chemotrophic anaerobic.

Hydrocarbons catabolism has been considered strictly aerobic process; however certain microorganisms are reported to degrade hydrocarbons anaerobically (Meckenstock et al., 2016). Several reactions such as oxidation, hydroxylation, reduction and dehydrogenation are common for both aerobic and anaerobic pathways of microbial degradation of petroleum hydrocarbon pollutants (Wilkes et al., 2016).

1.2.6. FACTORS AFFECTING PETROLEUM HYDROCARBON BIODEGRADATION

 Microorganisms are very sensitive to growth environment and respond to changes in their surrounding environment (Varjani, 2017). Biodegradation are affected by several factors: pollutant characteristics like availability, length and type of hydrocarbons, dispersion into aqueous phase and velarization (Beskoski et al., 2011); microbes; cell metabolic pathways and various structural changes from inclusion to complex extracellular polymeric spheres (Rocha et al., 2011); environmental conditions, such as pH, salinity, temperature, water content, oxygen availability and nutritional factors like carbon and nitrogen source and other nutrients (Chandra et al., 2013 and Aislabie et al., 2006); and physicochemical properties of soil such as pH moisture, density water holding capacity and texture (Beskoski et al., 2011). It is necessary to consider all these factors prior to selection of any alternative for bioremediation process (Okoh, 2006). Physicochemical properties and bioremediation (Varjani, 2017). Bioavailability is defined as amount of substance that is physicochemically accessible to microbes (Chauhdry et al., 2005).

1.2.6.1. TEMPERATURE

Elevated temperature increases solubility of hydrocarbon pollutants, decrease viscosity, and transfer long chain n-alkanes from solid phase to water phase (Okoh, 2006 and Aislabie et al., 2006). Petroleum hydrocarbon degradation occurs over a wide range of temperatures, the rate of biodegradation generally decreases with decreasing temperature. Highest biodegradation rate occurs in the range of 30°C- 40°C in soil environments, 20°C- 30°C in some freshwater environmenrs and 15°C- 20°C in marine environments. Biodegradation of petroleum hydrocarbons have been reported in psychrophilic environments in temperature regions (Nilanjana and Preethy, 2011).

1.2.6.2. NUTRIENTS

Nutrients are crucial ingredients for successful biodegradation of petroleum hydrocarbon pollutants especially nitrogen, phosphorus and sometimes iron. In marine and fresh water oil polluted environments, the supply of carbon was significantly increased and availability of nitrogen and phosphorus generally became limiting factor for oil, therefore limiting biodegradation. In marine environment, it was found to be more pronounced due to low levels of nitrogen and phosphorus in sea water. Freshwater wetlands are typically considered to be nutrient deficient due to heavy demands of nutrients by plants (Nilanjana and Preethy, 2011). Additions of nutrients are necessary to improve the biodegradation of oil pollutant. However, excess nutrient concentration can be inhibit biodegradation of petroleum hydrocarbons (Nilanjana and Preethy, 2011).

1.2.6.3. MOISTURE

Moisture is necessary for all biological process to help convey foods, nutrients and waste products in and out of microorganisms. For oceans, lakes, and other surface water environments, this poses no problems. For terrestrial environments, some moisture must be available for biodegradation to occur. Too much water can impede the soil type. Ratios range from 30-90% in one study and 12-32% in others and the aerobic biodegradation of hydrocarbons in soils is greatest in ranges of 50-70% of the soil water holding capacity (Leahy and Col well, 1990) whereas waves and tidal actions are essential in supplying aerated sea water to beaches and marshes, rainfall is necessary in the biodegradation of inland soils by supplying moisture and useful dissolved oxygen to the microbes (Shallu et al., 2014). (Varjani, 2017).