# Biodegradation of petroleum tar in contaminated sediments of the Eastern Mediterranean shores and associated microbial dynamics

## TL;DR

This study explores how petroleum tar from oil spills is naturally broken down in Mediterranean beach sediments and how the involved microbes change over time.

## Contribution

The study provides novel insights into the biodegradation of petroleum tar in the Eastern Mediterranean and identifies key microbial genera involved in the process.

## Key findings

- Tar biodegradation rates increased significantly at higher temperatures (28°C compared to 18°C).
- Microbial communities shifted from generalist hydrocarbon degraders to specialized groups during biodegradation.
- Key genera like Erythrobacter, Bacillus, and Pseudomonas were identified as important in tar degradation.

## Abstract

The Eastern Mediterranean coasts were impacted by several oil spills over the past two decades where tar residues were encountered on the shoreline. The majority of research on marine tar residues has focused on tar formation, chemical composition, transport, and fingerprinting to trace petroleum oil spills. Only a few studies have assessed the biodegradation of tar in the marine environment, and no studies have examined the evolution of the associated microbial communities. This research evaluated the biodegradation of tar residues in contaminated beach sediments of the Eastern Mediterranean coast of Lebanon. Biodegradation experiments were conducted in laboratory microcosms over a period of 56 days at 18°C and 28°C. Tar removal throughout the incubation period was monitored by the measurement of residual n-alkanes and polycyclic aromatic hydrocarbons (PAHs) using gas chromatography-mass spectrometry. Additionally, the diversity and evolution of the microbial community structure throughout the tar biodegradation experiments were determined using 16S rRNA gene sequencing. The biodegradation rates of total alkanes and total PAHs were 0.035 day−1 and 0.023 day−1, respectively, at 18°C, and increased to 0.110 day−1 and 0.055 day−1 at 28°C. Microbial analysis revealed a shift in the microbial community from generalist hydrocarbon degraders at the beginning of the biodegradation process to more specialized groups as the experiments progressed. Key genera involved in tar biodegradation included Erythrobacter, Bacillus, Alcanivorax, Marinobacter, Actinomarinales, and Pseudomonas. These findings highlight the potential of the Eastern Mediterranean coast to naturally degrade tar contamination and enhance our understanding of the microbial dynamics associated with tar biodegradation.

The planned oil and gas extraction activities of the Eastern Mediterranean coasts increase the risk of potential oil spills and threaten the Mediterranean shoreline with devastating impacts. A recent oil spill has resulted in huge amounts of tar residues washing up along the Lebanese southern coastline, affecting Nature Reserve shores known to be a nesting ground for several species of endangered turtles. The majority of research conducted on marine tar residues has studied tar formation, distribution and prevalence, chemical composition and tracing, transport mechanisms, as well as human and ecological effects. The biodegradation of spilled petroleum tar in aquatic media and the associated microbial dynamics are still poorly addressed in the literature. This study contributes to the state of knowledge and current scarce literature on petroleum tar biodegradation in marine environments and provides guidelines to spill responders for an effective bioremediation response plan to address future potential tar contamination.