Natural microbial diversity in superficial sediments of Milazzo Harbor (Sicily) and community successions during microcosm enrichment with various hydrocarbons

<jats:title>Summary</jats:title><jats:p>Hydrocarbon‐contaminated superficial sediments collected from the Harbor of Milazzo (Tirrenean Sea, northern Sicily), a zone strongly affected by anthropogenic activities, were examined for <jats:italic>in situ</jats:italic> biodegradative capacities. A culture‐independent molecular phylogenetic approach was used to study the influence of hydrocarbon and nutrient addition on the activity and diversity of the indigenous microbiota during a microcosm evaluation. The autochthonous microbial community in non‐polluted sediments was represented by eubacterial phylotypes grouped within <jats:italic>Proteobacteria</jats:italic>, <jats:italic>CFB</jats:italic> and <jats:italic>Firmicutes</jats:italic>. The archaeal domain was represented by members of Marine Group I of <jats:italic>Crenarchaeota</jats:italic>. The majority of recovered sequences was affiliated with heterotrophic genera <jats:italic>Clostridium</jats:italic> and <jats:italic>Vibrio</jats:italic>, typical members of eutrophic coastal environments. Amendments of hydrocarbons and mineral nutrients to microcosms dramatically changed the initial diversity of the microbial community. Only bacterial phylotypes affiliated with <jats:italic>Proteobacteria</jats:italic> and <jats:italic>CFB</jats:italic> division were detected. The decrease in diversity observed in several microcosms could be explained by the strong selection for microorganisms belonging to group of marine hydrocarbonoclastic γ‐<jats:italic>Proteobacteria</jats:italic>, namely <jats:italic>Alcanivorax</jats:italic>, <jats:italic>Cycloclasticus</jats:italic>, <jats:italic>Marinobacter</jats:italic>, <jats:italic>Marinobacterium/Neptunomonas</jats:italic> and <jats:italic>Thalassolituus</jats:italic>. This study demonstrated that nutrient amendment to hydrocarbon‐contaminated superficial sediments enhanced the indigenous microbial biodegradation activity and that highly specialized marine hydrocarbonoclastic bacteria, representing a minor fraction in the natural microbial community, play an important role in the biodegradation of petroleum hydrocarbons accidentally entering the coastal environment.</jats:p>