How do microbial communities deal with chronic hydrocarbon presence in oil seep soils? Data from historical hand-dug oil wells

Hand-dug oil wells, located in natural crude oil seep sites, are remnants of historical exploitation activities. Hydrocarbon pollution is regarded as the threat to soil ecosystem. On the other hand, there is no common environmental policies regarding these soils. The hypothesis was that natural attenuation processes might occur in seep soils since a diversified and stable bacterial community structure should be a result of its long-term (thousands of years) adaptation to hydrocarbon exposure and should be associated with eventual utilization of these compounds. To obtained this goal, we compared the structure, composition, and hydrocarbon-degrading potential of bacterial communities inhabiting soils with different hydrocarbon contents (seep, hydrocarbon-impacted, pristine soils), which were collected in two habitats (forest, meadow). 16S rRNA sequencing and isolation of hydrocarbon degraders were performed. The contaminant's presence shaped distinct and unique community structure and composition, and it enhanced physiologically and functionally adapted microorganisms. The most abundant community members were bacteria revealing a strong contribution in genetic potential toward aerobic hydrocarbon transformation (i.e., Mycobacterium/Mycolicibacterium and Pseudomonas). The strong hydrocarbon degraders population suggests that natural biodegradation occurs in situ in seeps and mitigates the pollution impact on adjacent soils. Seep and hydrocarbon-impacted soils are a great source for remedial bacterial populations. Twenty-four genera of degraders were isolated; however, strains belonging to the Mycobacterium/Mycolicibacterium, Rhodococcus, and Pseudomonas taxa were common. Our results underline the need to include undervalued microbiological aspects in remediation projects' guidelines for chronically polluted environments. The knowledge regarding seep communities should help to evaluate more efficient remediation strategies for anthropogenic spills.