The inevitable and disastrous consequence of crude oil pollution for the biotic and abiotic components of the ecosystem has been a major source of concern to the government and people living in oil producing and industrialized countries. This had led to ethnic and regional crises in the Niger Delta region that generated significant tension between them and the multinational oil companies operating in the region (Vidal, 2010).Crude oil exploration, production and transportation in the Niger Delta region have increased tremendously since its discovery in Nigeria in 1956 and has become a veritable source of economic growth and the main stay of the Nigerian economy (Okoh, 2006).The global scale of oil production is staggering and its demand is in the order of 3.25 x 109 tones or 3.8 x 1012 liters per year and much of it is transported thousands of kilometers before it is used (Prince and Lessard, 2004).
Crude oil is a complex mixture of organic compounds including volatile aromatic fractions and less volatile aliphatic fractions. The main constituents of crude oil are the elements hydrogen (10 – 40%) and carbon (83 – 87%). Various types of crude oil contain small quantities of sulphur, nitrogen, oxygen and trace metals such as vanadium, nickel, iron and copper which are not usually found in refined petroleum (Atlas and Bartha, 1973). Individual chemical composition of each crude petroleum however, depends on its origin and location and has a unique mixture of molecules which defines its physical and chemical properties. Crude oil has been part of the biosphere for millennia and has been used since ancient times in one form or the other and has risen in importance due to rise in commercial aviation, invention of internal combustion engines and the increasing use of pesticides, fertilizers and plastics which are mostly made from oil (Okoh, 2006).
Soil is an extremely complex, dynamic and living medium, formed by mineral particles, organic matter, water, air and living organisms. It establishes the interface between earth, air and water and performs many vital functions. The importance of soil for the survival of plants has become apparent due to numerous services it renders, ranging from filtration of ground water, removal of pathogens, degradation of organics, recycling of nutrients on which agriculture thrives and provision of raw materials for industries which are of economic value. Human activities such as the production, transportation, storage and sometimes vandalization of oil facilities accidentally release large quantities of crude oil and its fractions to marine and terrestrial environments thereby posing a long term threat to the soil and the services it renders (Blum, 1997).
Crude oil is a fossil fuel derived from ancient fossilized organic material. The fossilization processes include the initial process of diagenesis and the final or completion process called catagenesis. The initial process of diagenesis occurs at temperatures at which microbes partially degrade the biomass and result in dehydration, condensation, cyclisation and polymerization of the biomass. Subsequent burial under more sediments at higher temperature and pressure allows catagenesis to complete the transformation of the biomass to fossil fuel by thermal cracking and decarboxylation (Prince and Lessard, 2004).
1.1 CRUDE OIL POLLUTION OF THE ECOSYSTEM
The release of crude oil and its fractions into the natural environment has adverse ecological impacts on both terrestrial and aquatic ecosystems. In Nigeria, it has been reported that, annually an estimated quantity of 2,105,393 barrels of oil was spilled on land, coastal and offshore marine in the Niger Delta region between 1976 and 1990 (Kontagora, 1991). The impact of oil exploration and exploitation on the environment is one of the inevitable consequences of industrialization and economic development in Nigeria (Osuji and Onojake, 2006). In aquatic ecosystem, crude oil floats and blocks out sunlight, thus initiating the death of phytoplanktons and seaweeds, which are at the base of the aquatic food chain, thereby starving organisms that depend on them. Crude oil also has become one of the most frequently detected underground water pollutant caused by leakages from underground tanks, pipelines and other components of crude oil distribution (Kharoune et al., 2001; Gwendoline, 2010). Soil soaked with crude oil loses fertility and initiates environmental degradation and ecological succession. Crude oil pollution changes the composition of soil microorganisms and alters the physicochemical properties of the soil rhizosphere which affects plant growth and development (Gesinde et al., 2008; Ebere et al., 2010).
1.2 THE PHYSICOCHEMICAL PROPERTIES OF RHIZOSPHERE
The physicochemical properties of rhizosphere are the physical and chemical characteristics of the soil surrounding plant roots, which differ from those of the bulk soil. Such properties include soil texture, soil porosity, bulk density, cation exchange capacity, mineral composition, soil organic matter, soil pH, soil water or moisture content (Luthy et al., 1997; Gogoi et al., 2003). Densification of soil particles due to crude oil contamination gives rise to compaction, formation of organic ligands and binding of clay particles. This leads to increase in soil bulk density, low soil porosity and low soil moisture content (Xu and Johnson, 1995). Moisture level affects soil respiration, nutrient transport and availability and hence limits the metabolism and growth of microorganisms in the soil (Smith et al., 1998). The growth rate of microorganisms on crude oil contaminated soil is limited by the availability of nutrients such as nitrogen and phosphorous (Pritchard and Costa, 1991). The rate of microbial growth depends on soil pH. Soil pH is a critical factor for microbial growth and survival. Different microbial strains exhibit their maximum growth potential in a limited pH range. Soil pH value near neutral is suitable for growth of diverse bacterial population (Barua et al., 2011).
1.3 RHIZOSPHERE MICROFLORA
Rhizosphere is the zone of the soil surrounding the root of plants where the biological, physical and chemical properties of the soil are greatly influenced by the roots (Frick et al., 1999). It is the soil matrix and can be described as the longitudinal and radial gradients occurring with expanding root growth, nutrients and water uptake, exudation and subsequent microbial growth (Uren, 2000). The environment of plant rhizosphere is the most favorable microhabitat for microorganisms compared to the surrounding bulk soil (Bias et al., 2006). The rhizosphere has been reported to harbor more oil utilizing bacteria than adjacent non rhizospheric soil (Sorkhoh et al., 2010). Microorganisms commonly associated with the rhizosphere are Pseudomonas sp., Bacillus sp., Sphingomonas sp., Streptomyces sp., Micrococcus sp., Aspergillus sp.and Penicillum sp.(Radwan et al., 1998; Obire et al., 2008).
1.4 BIOREMEDIATION OF CONTAMINATED SOIL
Biomediation is the use of living organisms to manage or remediate polluted soil (Bossert and Bartha, 1984). Bioremediation is the elimination, attenuation or transformation of polluting or contaminating substances by the use of biological processes (Wenzel, 2009). Biomremediation technologies include land farming, bioreactor, composting, landfilling, biopilling, biostimulation, bioaugmentation and phytoremediation (Siciliano and Germida, 1998).
1.4.1 LAND FARMING
Land farming is a waste disposal technology for handling hazardous chemical wastes. It involves simultaneous treatment and disposal. Land farming is a biotechnology application that uses soil aerobic microorganisms to degrade petroleum hydrocarbons and their derivatives to carbon dioxide and water or other less toxic intermediaries (USEPA, 1990).
This is a periodic treatment of contaminated groundwater or industrial effluent, using an engineered bioprocess. Bioreclamation of soil contaminated by petroleum hydrocarbons have been carried out, using bioreactors such as the conventional suspended growth sequencing batch reactors (SBRs), sequencing batch biofilm reactors (SBBRs) and soil slurry sequencing batch reactors (SS-SBRs). These bioreactors boost the population of petroleum degrading microorganisms and thus increate bio-oxidation rate of the pollutants (Irving and Ketchum, 1983).
This technology, which seems to operate more under thermophilic conditions, is used mainly for bioremediation of resistant (recalcitrants) chemicals and explosives such as 2,4, 6, trinitrotoluene (TNT). The approach of this technology is mixing of the hazardous chemicals with the soil and the compostable materials before treatment (USEPA, 1990).
1.4.4 LAND FILLING
Land filling is an engineered and controlled treatment operation on land. The procedure is technically simple, less costly and easily managed. Generally, the waste is spread out in layers (in a pit) and compacted down using either a tractor or landfill compactor. The compacted waste, which may be about 2.4m thick, is eventually covered with inert material (USEPA, 1994).
This requires heaping or piling up of the pollutant in an enclosure such as tunnel or greenhouse structure or on top of a liner. The “treatment’ heap can be aerated from time to time either by tilling or by forcing air through perforated pipes installed at the base of the heaps (Balba et al., 1991).
This technology can be used alongside those already listed above but it is an important process that requires consideration on its merit. This innovative technology is employed for optimization of the environmental conditions in order to maximize the contaminant degrading potential of the native or indigenous bacteria (Atlas and Bartha, 1973).
Seeding a contaminated environment with strains of bacteria that are tolerant (adapted) and capable of degrading a given contaminant and thus supplementing the natural resident microbiota, has proven to be useful in bioremediation. The relative success of such adapted bacteria when added to the polluted site depends on many factors including competitive interactions with native bacteria, their rate of growth in the system as well as their tolerance to the physicochemical environment (Leahy and Colwell, 1990).
Initially, bioremediation employed microorganisms to degrade organic pollutants, but since the use of green plants was proposed for in situ soil remediation, phytoremediation has become an alternative topic of research and development (Salt et al., 1995). Phytoremediation appears attractive because in contrast to most other remediation technologies it is not invasive and, in principle delivers intact, biologically active soil (Wenzel, 2009).
The fundamental technologies applicable in phytoremediation of contaminated soil are phytostabilisation, phytoextraction, phytovolatilisation (rhizovolatilisation) and phytodegradation (rhizodegradation), (Salt et al., 1995).
Phytostabilization is a contaminant process using plants often in combination with soil additives to assist plant to mechanically stabilize the site and reduce pollutant transfer to other ecosystem compartments and the food chain (Wenzel, 2009).
Phytoextraction is a removal process that takes advantage of unusual ability of some plants to hyper accumulate metals/metalloids in their shoots (Wenzel, 2009).
Phytovolatilisation (rhizovolatilization) is a removal process that employs metabolic capabilities of plants and rhizospheric microorganisms to transform pollutants into volatile compounds that are released into the atmosphere (Wilber, 1980). Pollutant, toxicity, adverse soil conditions, water stress and nutrient deficiency are typical problems challenging the establishment of vegetation in contaminated sites (Tordoff et al.,2000).
Phytodegradation (rhizodegradation) refers to the use of metabolic capabilities of plants and rhizospheric microorganisms to degrade organic pollutants (Wenzel, 2009). Plants and microorganisms are involved, both directly and indirectly in the degradation of petroleum hydrocarbons into products that are less persistent in the environment than the parent compounds (Cunningham et al., 1996). Low availability of pollutants is the main challenge in the rhizodegradation of field- contaminated and aged spiked soil (Wenzel, 2009). Limited pollutant bioavailability can be overcome by the design of plant microbial consortia that are capable of mobilizing pollutants by modification of rhizosphere pH (Siciliano and Germida, 1998).
1.4.9. RHIZODEGRADATION OF CRUDE OIL CONTAMINATED SOIL
The interaction between plants and microbial communities in the rhizosphere is exploited in the specific use of plants to enhance microbial degradation of organic compounds in the soil. The fibrous root structure of grasses is known to provide a large surface area for colonization by microorganisms than the taproot system (Atlas and Bartha, 1993). Plant establishment during the process of phytoremediation follows many standard procedures that include adaptability of the plants to climatic conditions of the region, maximum root density and stress tolerance (Cuningham et al., 1996) and these are not uncommon with Sorghum vulgare .
1.4.10 ROLE OF MICROORGANISMS IN RHIZODEGRADATION
Soil microorganisms are known to convert some organic and inorganic pollutants (hydrocarbon, arsenic, boron, antimony, selenium, tin, lead and mercury) to their volatile species (Meyer et al., 2007). This microbial conversion is usually considered as a detoxification mechanism by which the microorganism decreases the toxicity of the surrounding microenvironment (Wenzel, 2009). Microorganisms can increase solubility and change speciation of metals/metalloids through the production of organic ligand via microbial decomposition of soil organic matter and exudation of metabolites and microbial siderophores that can complex cationic metals or desorb anionic species by ligand exchange (Gadd, 2004). Beneficial interactions between phytoremediation crops and bacteria have been demonstrated to alleviate pollutant toxicity and nutrient deficiency. Ectomycorrhizal associations can display considerable resistance against toxicity in the soil polluted with metal, organic compound and petroleum (Sarand et al., (1998). Microbial degradation of organic contaminants normally occurs as a result of microorganisms using the contaminant for their own growth and reproduction. (Cunningham et al., 1996). Another role played by microbes involves their ability to reduce the phytotoxicity of contaminants to the point where plants can grow in adverse soil conditions, thereby stimulating the degradation of other, non phytotoxic contaminants (Siciliano and Germida, 1998). The defence of plants to contaminants may be supplemented through the external degradation of contaminants by micro organisms in the rhizosphere (Anderson and Coats, 1997). Microorganisms are the primary mechanism responsible for petrochemical degradation in phytoremediation efforts (Siciliano and Germida, 1998).
1.5 RHIZOSPHERIC MICROORGANISMS ASSOCIATED WITH PHYTOREMEDIATION
A variety of microorganisms are reportedly involved in the degradation of petroleum hydrocarbons. Bossert and Bartha (1984) reported the use of bacteria such as Pseudomonas sp., Arthrobacter sp., Bicaligenes sp., Combacteriu sp.,, Flavobacterium sp., Achromobacter sp., Micrococcus sp., Mycobacterium sp. and Norcardia sp., as the most actual bacterial species in the degradation of hydrocarbons in soil. Pseudomonas, Arthrobacter sp. and Achromobacter sp. often occur in greater numbers within the rhizosphere than in the bulk soil (Anderson and Coats, 1997). Soil fungi also play role in the degradation of petroleum hydrocarbons. Surtherland (1992) reported that a diversity of fungi including Aspergillus ochraceus, Cunninghamella elegans, Phanerochaete chrysosporium, Saccharomyces cerevisiae and Syncephalastrum racemosum can oxidize various polycyclic aromatic hydrocarbon. Higher microbial numbers and increased degradation of hydrocarbon – contaminated soil were observed in contaminated soil plant with ryegrass compared to unplanted soil (Gunther et al., 1996).
1.6 A BRIEF IN SORGHUM VULGARE
Sorghum vulgare Pers. also known as guinea corn is a member of the grass family (Poaceae) under the class Angiospermeae and sub class Monocotyledonae. It is self pollinated, more drought and temperature resistant than maize, soybeans and wheat. It has deep fibrous root system, grows fast and can withstand conditions of stress which are important characteristics that make it useful in phytoremediation (Siciliano and Germida, 1998). S. vulgare is grown primarily in the semiarid tropics of Africa, India, China, South America, and stress- prone areas of United States (Subudhi and Nguyen, 2000). S. vulgare and other related Sorghum species can accumulate biomass very rapidly and attain heights greater than 2.5 m in less than 6 weeks (Subudhi and Nguyen, 2000).
SIGNIFICANCE OF THE STUDY
This study may help to foster good neighborliness, profitable dependability on nature’s rich ecosystem and create conducive environment for economic activities in polluted environments through efforts to remediate crude oil contaminated soil, assessing its effects on bacterial population and physicochemical properties of rhizosphere which affect the degradation potential of hydrocarbon-utilizing bacterial community. The result of the study will provide data for assessing the changes in bacterial population and physicochemical properties of soil rhizosphere in response to crude oil contamination. The data may be used to design effective remediation system and drive further improvement and innovation in the field of biotechnology.
OBJECTIVES OF THE STUDY
The objectives of this study are:
- To determine the physicochemical properties of the rhizosphere of S. vulgare contaminated with crude oil.
- To quantify the bacterial population associated with the rhizosphere of S. vulgare contaminated with crude oil.
- To identify the bacteria within the rhizosphere of S. vulgare contaminated with crude oil.