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Mohammadikia, R., Fallah Nosratabad, A., Khoshrou, B. (2025). The impact of soil moisture on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) by bacterial strains. , 13(1), 107-125. doi: 10.22092/sbj.2025.370068.282
Reza Mohammadikia; Alireza Fallah Nosratabad; Bahman Khoshrou. "The impact of soil moisture on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) by bacterial strains". , 13, 1, 2025, 107-125. doi: 10.22092/sbj.2025.370068.282
Mohammadikia, R., Fallah Nosratabad, A., Khoshrou, B. (2025). 'The impact of soil moisture on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) by bacterial strains', , 13(1), pp. 107-125. doi: 10.22092/sbj.2025.370068.282
Mohammadikia, R., Fallah Nosratabad, A., Khoshrou, B. The impact of soil moisture on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) by bacterial strains. , 2025; 13(1): 107-125. doi: 10.22092/sbj.2025.370068.282

The impact of soil moisture on the bioremediation of polycyclic aromatic hydrocarbons (PAHs) by bacterial strains

زیست شناسی خاک
Article 7, Volume 13, Issue 1, September 2025, Pages 107-125    PDF (1.29 M)
Document Type: Research Paper
DOI: 10.22092/sbj.2025.370068.282
Authors
Reza Mohammadikia* 1; Alireza Fallah Nosratabad2; Bahman Khoshrou3
1Researcher, Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran.
2Professor, Department of Biology and Biotechnology Research, Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran.
3Postdoctoral Researcher, Department of Biology and Biotechnology Research, Soil and Water Research Institute (SWRI), Agricultural Research, Education and Extension Organization (AREEO), Karaj, Iran.
Abstract
Background and Objectives: Soil contamination by petroleum hydrocarbons, particularly Polycyclic Aromatic Hydrocarbons (PAHs), due to industrial activities, poses significant environmental and health risks. PAHs are persistent, toxic organic compounds, and their presence in soil alters its physicochemical and biological properties, threatening ecosystem health and food security. Bioremediation, utilizing microorganisms to degrade pollutants, offers an eco-friendly and cost-effective solution. The efficacy of bacterial degradation of PAHs is, however, critically influenced by environmental factors, especially soil moisture, which affects microbial activity, nutrient and oxygen availability, and pollutant accessibility. While optimal moisture is known to enhance degradation, a comprehensive understanding of the interactive effects of specific bacterial strains and varying moisture levels on PAH bioremediation remains incomplete. This study aimed to address this gap by investigating the effects of different soil moisture levels and three selected bacterial strains on PAH bioremediation in a contaminated loamy-clay soil under laboratory conditions. The main hypothesis was that both bacterial strain type and soil moisture level, along with their interaction, would significantly impact PAH degradation efficiency.
Materials and Methods: A factorial pot experiment based on a Randomized Complete Block Design (RCBD) with three replicates was conducted. Treatments included four bacterial applications (three individual strains: Pseudomonas alcaligenes (B1), Pseudomonas stutzeri (B2), Enterobacter cloacae (B3); and an uninoculated control (B0)) and four soil moisture levels. Surface soil (0–30 cm) from an oil-contaminated site near Tehran refinery was sieved, characterized (loamy-clay), and autoclaved at 121°C for 60 minutes. Bacterial strains were cultured in Nutrient Broth, harvested, washed, and resuspended to an OD600 of 0.7 (approx. 1.5–2.8 × 10⁹ CFU/mL). Thirty mL of inoculum (or sterile water for control) were added to each pot. The four moisture levels were: I1 (30% depletion of available water, AWC), I2 (50% AWC depletion), I3 (70% AWC depletion), and I4 (90% AWC depletion), maintained daily by weighing. Pots were kept at 25 ± 2°C. After a 20-day bacterial stabilization period, the moisture levels were imposed for 65 days. Bacterial populations were enumerated (CFU/g dry soil) on Nutrient Agar. Concentrations of 14 target PAHs were determined by HPLC (Agilent 1260, fluorescence detector, C18 column) following USEPA method 8310 after Soxhlet extraction (hexane/acetone 1:1) and silica/alumina column cleanup. Data were analyzed by two-way ANOVA, and means were compared by Duncan's Multiple Range Test (P≤0.05) using SAS and SPSS software.
Results: The initial concentration of total PAHs in the soil was 33.4 mg/kg, classifying it as highly contaminated. Fluoranthene was the dominant compound among the 14 PAHs analyzed. Total bacterial populations did not show significant changes across treatments throughout the experiment. However, the performance of the inoculated strains in reducing PAH concentrations was notable; treatments with P. alcaligenes (B1) and E. cloacae (B3) achieved total PAH reductions of 45.65% and 50.57%, respectively, although these differences were not statistically significant (P>0.05). In contrast, soil moisture had a statistically significant effect (P<0.05), with the highest PAH reduction (56.8%) observed in the optimal moisture treatment (I1). Analysis of individual compounds revealed that lighter PAHs, such as phenanthrene and fluorene, were more biodegradable. Conversely, the concentrations of heavier and more complex compounds like benzo(a)pyrene increased in some treatments, likely due to the incomplete degradation of larger molecules.
Conclusion: The present study demonstrated that soil moisture is the main determining factor in enhancing the biodegradation efficiency of polycyclic aromatic hydrocarbons (PAHs), with the greatest reduction of contaminants observed at moisture levels close to field capacity. Optimal moisture improves soil aeration, increases contaminant mobility, and provides a suitable environment for microbial activity, thereby facilitating effective degradation of pollutants. Regarding bacterial strains, Enterobacter cloacae and Pseudomonas alcaligenes exhibited a high potential in reducing PAH concentrations, highlighting the importance of functional capacity and adaptability of microorganisms rather than merely their population size. However, the observed increase in the concentration of certain toxic compounds in some treatments raises the possibility of incomplete degradation and the formation of hazardous intermediates. Overall, successful microbial remediation of petroleum-contaminated soils requires simultaneous consideration of optimal moisture, effective microbial strain selection, and precise monitoring of contaminant behavior. Future studies should employ more specific indicators such as catabolic genes and bioavailability assays, and also take into account the impact of soil sterilization on contaminant structure and bioavailability. It should be noted that although soil autoclaving in this study was necessary to accurately assess the performance of inoculated strains, it may have influenced the initial bioavailability of PAHs.
Keywords
Microbial Remediation; Soil Moisture; Polycyclic Aromatic Hydrocarbons (PAHs); Oil Pollution; Biodegradation
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