The impact of long-term hydrocarbon exposure on the structure, activity, and biogeochemical functioning of microbial mats

• Low seasonal variation was observed for chronically hydrocarbon-contaminated mats.
• Hydrocarbon contents explained the differences in the microbial community structure.
• Hydrocarbonoclastic bacteria are rare or transient in chronically polluted systems.
• Obligate hydrocarbonoclastic bacteria should not be good indicators of chronic pollution.

Photosynthetic microbial mats are metabolically structured systems driven by solar light. They are ubiquitous and can grow in hydrocarbon-polluted sites. Our aim is to determine the impact of chronic hydrocarbon contamination on the structure, activity, and functioning of a microbial mat. We compared it to an uncontaminated mat harboring similar geochemical characteristics. The mats were sampled in spring and fall for 2 years. Seasonal variations were observed for the reference mat: sulfur cycle-related bacteria dominated spring samples, while Cyanobacteria dominated in autumn. The contaminated mat showed minor seasonal variation; a progressive increase of Cyanobacteria was noticed, indicating a perturbation of the classical seasonal behavior. Hydrocarbon content was the main factor explaining the differences in the microbial community structure; however, hydrocarbonoclastic bacteria were among rare or transient Operational Taxonomic Units (OTUs) in the contaminated mat. We suggest that in long-term contaminated systems, hydrocarbonoclastic bacteria cannot be considered a sentinel of contamination.