Comparative Analysis of Bacterial Isolates for Bioremediation of an Oil-Contaminated Soil

Background and Objectives
Soil contamination with petroleum hydrocarbons is a critical environmental challenge, posing significant risks to ecosystems, agricultural productivity, and human health. The widespread release of petroleum compounds through industrial activities, accidental spills, and improper waste disposal has led to the accumulation of persistent and toxic pollutants in soil. Conventional remediation techniques, such as physical and chemical methods (e.g. thermal desorption, and chemical oxidation), often entail high costs, energy consumption, and secondary environmental damage. In contrast, microbial bioremediation has emerged as a sustainable and eco-friendly alternative, leveraging the metabolic capabilities of microorganisms to degrade hydrocarbons into less harmful byproducts. Among hydrocarbon-degrading microorganisms, bacterial isolates particularly those from the genera Pseudomonas, Bacillus, and Rhodococcus have demonstrated remarkable efficiency in breaking down complex petroleum compounds. Their enzymatic machinery, including oxygenases and dehydrogenases, enables the catabolism of alkanes, aromatics, and polycyclic aromatic hydrocarbons (PAHs). However, the effectiveness of bioremediation depends on factors such as microbial strain selection, environmental conditions (e.g., temperature, pH, and nutrient availability), and the composition of the contaminant.
 
Methodology
In this study, we evaluated the biodegradation potential of nine bacterial isolates (H921, H814, H712, H711, S21-1, C16-2O, 14A-4, P5, and Bio4) in a soil artificially contaminated with 6.5% petroleum hydrocarbons over a 60-day period. The experiment simulated natural conditions (room temperature) with controlled moisture, aeration (daily stirring), and NPK nutrients supplementation to enhance microbial activity. Key indicators such as reduction in hydrocarbon odor, visible color changes, and residual oil content were monitored to assess degradation efficiency. Hydrocarbon quantification was performed using Soxhlet extraction with diethyl ether, ensuring accurate measurement of residual contamination.
 
Results and Discussion
Preliminary results highlighted strain S21-1 (Pseudomonas sp.) as the most effective, achieving 26% hydrocarbon degradation, followed by 14A-4 and C16-2O. The superior performance of S21-1 was corroborated by its rapid colonization, biosurfactant production, and ability to thrive in hydrocarbon-rich environments. This study underscores the importance of strain-specific selection for optimizing bioremediation strategies and provides a comparative framework for future research on microbial consortia or genetically enhanced strains. By elucidating the degradative capacities of these isolates, we contribute to the development of scalable, low-cost solutions for mitigating petroleum pollution in contaminated soils.
 
Conclusion
Considering the distribution and dispersion of oil pollution in oil-rich regions, especially in the Persian Gulf area, and the prevalence of high temperatures in these regions, this experiment was conducted to investigate certain microbial species tolerant to 50 ^°C and their potential use in the bioremediation of oil-contaminated soils. Initially, Tween 80 was used to enhance the availability of petroleum compounds for biodegradation, followed by the application of NPK fertilizers and bacterial inoculation at specific intervals, along with maintaining appropriate moisture and active aeration. Analysis of the obtained data on the initial and residual TPH (Total Petroleum Hydrocarbons) levels in the control and bacterial-treated samples indicated that after 60 days, the initial TPH concentration of 6.5% decreased to 4.7%. Among the tested strains, S21-1, belonging to the genus Pseudomonas, exhibited the highest efficiency. GC-FID analysis was not performed on the samples to determine the proportion of aliphatic and aromatic compounds, but it appears that the distribution of these substances in the oil contamination may have been a limiting factor in the biodegradation of petroleum compounds. Alternatively, the tested bacteria may have required more time for the complete biodegradation of petroleum hydrocarbons. For oil bioremediation projects, obtaining highly efficient bacterial strains is a key factor, and the objective of this research was to achieve such a strain.
Author Contributions
Conceptualization and methodology, M.R.S; Performing experiments and measurements, M.R.S., A.L., and S.H.; formal analysis and data curation, M.R.S., A.L., and S.H.; writing-original draft preparation, M.R.S., writing- review and editing, M.R.S; supervision, M.R.S; project administration, M.R.S; All authors have read and agreed to the published version of the manuscript.
Data Availability Statement
Data is available on reasonable request from the authors.
Acknowledgements
This paper is published as a part of a PhDs thesis supported by the Vice Chancellor for Research and Technology of the University of Tabriz, Tabriz, Iran. The authors are thankful to the University of Tabriz for financial supports.
Conflict of interest
The authors declare no conflict of interest.
Ethical considerations
The authors avoided data fabrication, falsification, plagiarism, and misconduct.