Bioremediation of Petroleum Tar and Characterization of the Associated Microbial Community

Abstract

The heavy oil tanker traffic and planned oil and gas activities off the Mediterranean Sea shorelines have increased the risks of oil spills and the subsequent contamination of the marine environment with crude oil and tar residues. A recent oil spill that occurred in the south of Lebanon has led to tar residues washing up its southern beaches, affecting natural reserves and fishing areas. Such pollution poses a threat to the marine environment, human health, and biodiversity, and highlights the necessity of understanding the fate of tar residues in contaminated marine environments. In this study, the biodegradation potential of tar residues by the shoreline of Lebanon was assessed. Two sets of biodegradation experiments were conducted in nutrient-amended laboratory microcosms using tar-contaminated beach sediments. The microcosms were incubated for 56 days at two different temperatures, simulating average seawater temperatures in the cold (18 degrees C) and warm (28 degrees C) seasons. The biodegradation of tar constituents, namely n-alkanes and polycyclic aromatic hydrocarbons (PAHs), was monitored and quantified periodically using gas chromatography-mass spectrometry (GC-MS). Microbial community characterization and evolution throughout the experiments were also assessed to identify the populations associated with the biodegradation of tar. Biodegradation rates of total alkanes and total PAHs were 0.0353 day-1 and 0.0238 day- 1 respectively, at 18 degrees C, and increased to 0.1106 day-1 and 0.0556 day-1 at 28 degrees C, demonstrating the enhancing effect of increased temperatures on tar biodegradation. Measured biodegradation rates were half those reported in a previous study for the biodegradation of crude oil, which is associated with the increased proportion of higher molecular weight alkanes and PAHs in tar as demonstrated by its weathering diagnostic ratios (L/H [alkanes] = 0.82, and L/H [PAHs] = 0.68). Microbial analysis showcased the evolution of the microbial community from generalist hydrocarbon degraders at the onset of the biodegradation process to more specialized groups as the experiments progressed. Erythrobacter, Bacillus, Alcanivorax, Marinobacter, Actinomarinales, and Pseudomonas were the key genera involved in the biodegradation of tar components. The results obtained from this study demonstrate the potential of the shoreline of Lebanon to biodegrade tar contamination and serve as a guide for policymakers and spill responders in planning the cleanup of potential future tar pollution of the Lebanese shoreline.