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ISOLATION OF CYANOBACTERIA FROM LOCAL WATER BODIES AND THEIR POTENTIAL IN BIOREMEDIATION OF CRUDE OIL-POLLUTED WATER AND HEAVY METAL REDUCTION
CHAPTER ONE
INTRODUCTION
1.1 Background to the Study
Nigeria is in a paradoxical situation, being the biggest crude oil producer in Africa and at the same time having some of the worst petroleum pollution burdens in the continent. The oil exploration and production operations in the Niger Delta over the decades have caused crude oil spillage, pipeline vandalism, bunkering operations and the discharge of effluents which have degraded vast surface waters, groundwater aquifers and freshwater wildlife habitats. However, the problem of crude oil pollution is not limited to the Niger Delta; artisanal oil refineries, road tanker accidents, and industrial effluents also pollute coastal waters in other states, such as Edo, Imo, Anambra, and Rivers. The remediation of these water bodies is further complicated by the co-contamination of the water bodies with heavy metals released into the water by mining runoff, industrial effluents, and also deposition by the atmosphere.
Traditional methods of petroleum hydrocarbon cleanup such as physical containment, pump-and-treat, thermal desorption, and chemical oxidation are expensive, technically challenging, and in many cases, environmentally disruptive, and out of the means of most states and communities in Nigeria (Ghoreishi et al., 2017). In alternative, biological remediation strategies especially the strategies that utilise the favorable microorganism and biosorbs that live locally that are found in the area have shown promise and are cost-effective. Cyanobacteria have garnered considerable scientific interest among the multifaceted collection of bioremediation-capable microorganisms due to their distinctive set of photoautotrophic metabolism, structural biochemical diversity, environment-wide adaptability, and well-characterized ability to degrade hydrocarbons and biosorb heavy metal (Frontiers in Microbiology, 2024).
The cyanobacteria (Three-domain name: Photovibrio benthii) are prokaryotic photosynthetic microorganisms that form one of the oldest phylogenetic branches on Earth, having been previously grouped with the blue-green algae. They are found everywhere in various ecosystems such as freshwater, marine, and brackish water bodies, damp soils, hot springs, and polar areas (Jassim et al., 2023). Cyanobacterial communities containing species of Oscillatoria, Anabaena, Nostoc, Microcystis and Synechococcus have been known to occur in inland water bodies of Nigeria, but there has been little systematic study of their diversity and bioremediation potential.
Both articles stated that Synechacoccus elongatus and Microcystis holsatica were isolated in Nigerian oil-contaminated areas at the state level and utilized in consortium with heterotrophic bacteria showed good ability in breaking down crude hydrocarbons and reducing heavy metal concentrations (Onyeneho et al., 2024). These results are scientific proof of the national cyanobacterial bioremediation capabilities that are the aim of the current study to describe within the local water bodies of the study region.
1.2 Statement of the Problem.
Although the bioremediation capability of cyanobacteria has been well documented in the international literature, the information available on the cyanobacterial communities living in particular systems in the Nigerian inland water bodies and their capacity to bioremediate a specific site is scanty. The ecological profiles of cyanobacteria in local water bodies are complicated due to the common co-ocurrence of bloom-forming species, morphological difficulties in distinguishing related taxa, and the lack of access to molecular phylogenetic tools in most institutions in Nigeria.
Among the cyanobacteria species present on the world, a small fraction of the species (approximately 10 percent) have been fully characterized and only a small number can be grown commercially (PMC, 2024). The interactions between cyanobacteria and the intricate chemical matrix of the Nigerian crude oil that has its own unique composition, including typical high levels of sulphur oil fractions, and aromatic hydrocarbons have not been studied systematically. Likewise, the kinetics of biosorption of heavy metal by locally isolated cyanobacterial strains in Nigerian environmental conditions of temperature, pH and speciation of metals remains undefined. This paper fills this particular knowledge gap as it seeks to categorize and describe the indigenous cyanobacteria of the local water bodies and objectively define their ability to do bioremediation and reduction of cruise oil-contaminated water and heavy metal with experimental conditions.
1.3. Objectives of the Study
The aim of this study is to isolate and characterize cyanobacteria from selected local water bodies and to evaluate their potential in the bioremediation of crude oil-polluted water and the reduction of heavy metal concentrations.
The specific objectives are:
(i) To isolate and identify cyanobacterial species from selected freshwater bodies using morphological characterization and molecular phylogenetic analysis (16S rRNA gene sequencing).
(ii) To determine the growth kinetics and environmental tolerance range (pH, temperature, and salinity) of isolated cyanobacterial strains.
(iii) To evaluate the crude oil biodegradation potential of isolated cyanobacterial strains through total petroleum hydrocarbon (TPH) quantification in treated microcosms.
(iv) To assess the capacity of isolated cyanobacterial strains to reduce concentrations of selected heavy metals (Pb, Cd, Cu, Zn, and Cr) in artificially contaminated water.
(v) To determine biosorption kinetics and isotherms for heavy metal uptake by selected high-performing cyanobacterial strains.
1.4 Research Questions
The following research questions correspond directly to each objective of the study:
(i) Which cyanobacterial species are present in the selected local freshwater bodies, and how do their morphological characteristics and 16S rRNA gene sequences confirm their taxonomic identity?
(ii) What are the growth kinetics of the isolated cyanobacterial strains, and within what ranges of pH, temperature, and salinity do they demonstrate optimal growth performance?
(iii) To what extent do the isolated cyanobacterial strains reduce total petroleum hydrocarbon (TPH) concentrations in crude oil-spiked water microcosms, and how does this compare with uninoculated controls?
(iv) What is the heavy metal removal efficiency of the isolated cyanobacterial strains for lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), and chromium (Cr) in artificially contaminated water, and which strains demonstrate the highest removal capacity?
(v) What biosorption kinetics and isotherm models best describe heavy metal uptake by high-performing cyanobacterial strains, and what do these indicate about the mechanisms and maximum sorption capacities involved?
1.5 Research Hypotheses
H0₁: There is no significant difference in cyanobacterial species diversity between polluted and unpolluted water bodies surveyed in the study area.
H0₂: Isolated cyanobacterial strains do not significantly reduce total petroleum hydrocarbon concentrations in artificially polluted water microcosms compared to uninoculated controls.
H0₃: There is no significant reduction in heavy metal concentrations in contaminated water treated with isolated cyanobacterial strains compared to untreated controls.
1.6 Scope of the Study
The study isolates cyanobacteria from at least four freshwater bodies in and around the study area, selected to represent unpolluted, moderately polluted, and highly polluted conditions. Identification utilizes both morphological keys and 16S rRNA gene sequencing. Bioremediation experiments are conducted at laboratory scale (microcosm) with both single-species and consortium treatments. The crude oil used is locally sourced Nigerian crude oil. Heavy metals assessed are limited to Pb, Cd, Cu, Zn, and Cr. The study does not extend to field-scale bioremediation trials or evaluation of cyanobacterial biosurfactant production.
1.7 Significance of the Study
This study will generate novel information on the diversity and distribution of cyanobacteria in Nigerian inland water bodies and provide the first systematic evaluation of the bioremediation capabilities of locally isolated strains against Nigerian crude oil and heavy metals. The isolation and preservation of high-performing strains will create an accessible biotechnological resource for bioremediation applications by government agencies, environmental consultancies, and community-based organizations working in polluted areas. The study will also contribute to the development of low-cost bioremediation protocols appropriate for resource-limited settings in Nigeria, and will inform the design of green infrastructure solutions including cyanobacterium-seeded constructed wetlands and bioreactor systems for treating petroleum-contaminated effluents prior to discharge into natural water bodies.
1.8 Definitions of Terms
The following terms are defined as used in the context of this study:
Cyanobacteria: Prokaryotic photoautotrophic microorganisms, formerly classified as blue-green algae, that perform oxygenic photosynthesis. They are among the most ancient life forms on Earth and inhabit diverse environments including freshwater, marine, and terrestrial ecosystems. Cyanobacteria include unicellular, colonial, and filamentous forms, several of which are capable of nitrogen fixation and heavy metal biosorption.
Bioremediation: The use of living organismsprimarily microorganisms including bacteria, fungi, and algaeto degrade, transform, or immobilize pollutants in contaminated environments such as water bodies and soils. Bioremediation may be in situ (at the contaminated site) or ex situ (in controlled reactor systems).
Crude Oil: A naturally occurring, unrefined petroleum liquid composed of complex mixtures of hydrocarbons (alkanes, cycloalkanes, and aromatic compounds) and lesser amounts of sulphur, nitrogen, oxygen, and heavy metals. In the context of this study, Nigerian crude oil refers to crude extracted from Niger Delta reservoirs, characterized by its specific hydrocarbon composition.
Total Petroleum Hydrocarbon (TPH): A measure of the total mass of petroleum-derived hydrocarbon compounds present in an environmental sample, expressed in milligrams per litre (mg/L) or milligrams per kilogram (mg/kg). TPH is used as an index of petroleum contamination and a parameter for monitoring bioremediation progress.
Heavy Metal: Metallic elements with high atomic masses and densities that are toxic to living organisms at low concentrations and tend to accumulate in biological tissues. In this study, the heavy metals of concern include lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn), and chromium (Cr), which commonly co-contaminate petroleum-polluted Nigerian water bodies.
Biosorption: The binding and accumulation of dissolved substances (particularly metal ions) from aqueous solution onto the surface or interior of biological material such as microbial cells. Biosorption is a passive, physicochemical process involving ion exchange, surface complexation, and electrostatic interaction with functional groups on the cell wall and extracellular polymeric substances.
Biosorption Isotherm: A mathematical model describing the equilibrium relationship between the concentration of a sorbate (metal ion) remaining in solution and the amount adsorbed onto the biosorbent at constant temperature. Langmuir and Freundlich isotherms are the most commonly applied models in cyanobacterial biosorption studies.
Biosorption Kinetics: The study of the rate at which biosorption of a contaminant (such as a heavy metal) occurs over time, used to elucidate the rate-limiting step and the mechanism of the sorption process. Pseudo-first-order and pseudo-second-order models are commonly applied to describe cyanobacterial metal uptake kinetics.
Extracellular Polymeric Substances (EPS): High molecular weight compounds secreted by microorganisms including cyanobacteria into their immediate environment, composed primarily of polysaccharides, proteins, nucleic acids, and lipids. EPS play a critical role in heavy metal biosorption by providing functional groups with high metal-binding affinity.
Photoautotroph: An organism that synthesises organic compounds using light as an energy source and carbon dioxide (CO₂) as a carbon source, through the process of photosynthesis. Cyanobacteria are photoautotrophs, distinguishing them from heterotrophic bacteria that require organic carbon substrates for growth.
Nitrogen Fixation: The biological conversion of atmospheric dinitrogen (N₂) to ammonia (NH₃) or related compounds, catalysed by the enzyme nitrogenase complex. In cyanobacteria, nitrogen fixation is carried out by heterocyst-forming genera such as Anabaena, Nostoc, and Calothrix.
Microcosm: A small, contained, and controlled experimental system designed to simulate conditions of a natural environment (e.g., a polluted water body) for laboratory-scale study of bioremediation processes. Microcosms in this study consist of sealed flasks containing crude oil-spiked or metal-spiked water inoculated with cyanobacterial cultures.
16S rRNA Gene Sequencing: A molecular phylogenetic technique that uses the sequence of the 16S ribosomal RNA gene, highly conserved across bacterial species but with variable regions permitting taxonomic discrimination, to identify and classify bacteria including cyanobacteria. It is the gold standard for cyanobacterial species identification in environmental microbiology studies.
Bacterial Consortium: A defined mixture of two or more bacterial species or strains that work cooperatively in a shared environment to achieve a metabolic outcomesuch as hydrocarbon degradationthat may not be achievable by any single member alone due to complementary metabolic capabilities.
Polycyclic Aromatic Hydrocarbons (PAHs): A class of chemical compounds consisting of multiple fused aromatic (benzene) rings, formed as components of petroleum and as products of incomplete combustion. PAHs are of particular environmental and toxicological concern as many are carcinogenic and highly persistent in aquatic environments.
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