Marine chemosynthesis in the Mediterranean Sea

The Mediterranean Sea contains a vast spectrum of chemosynthetic habitats from shallow marine to bathyal depths. These habitats (hydrothermal vents, cold seeps, reducing sediment) are home to bacteria and archaea acting as primary producers using the energy obtained by oxidizing reducing compounds in fluids (e.g., H2S and hydrocarbons such as CH4) to synthesize organic matter. Such sites may make a large microbial biomass available to consumers and promote the development of complex symbiotic relationships between prokaryotes and hosting eukaryotes. Shallow water (<200 m) chemosynthetic niches are pervasive in the Mediterranean where reduced sediment are present (lagoons, seagrass beds, prodeltaic settings etc.) being exploited primarily by sulphur-oxidizing bacteria, some of which are symbiotic with metazoans. Particular cases of shallow chemosynthetic habitats are hydrothermal vents, submarine caves with sulphur springs, cold seeps including active pockmarks and reducing sediments in areas of high organic deposition. Deep-water chemosynthetic habitats (>200 m) with also metazoans with chemoautotrophic symbionts are present at various sites in the Mediterranean, encompassing both cold seeps and hydrothermal vents. Cold seeps are diffuse and geologically diverse, and comprise mud volcanoes, brine pools and active pockmark fields, whose reducing environments are often exploited by endemic chemosymbiotic metazoans, including bivalves and siboglinid polychaetes. Such environments are clustered in the Eastern Mediterranean (Nile Deep Sea Fan, Anaximander mud volcano field, Olimpi mud volcano field, Eratosthenes Seamount, Calabrian Arc, Marmara deep fault systems) but equally occur in the Strait of Sicily, Adriatic, Tyrrhenian and Alboran Seas. Deep hyperhaline anoxic basins occur in the Eastern Mediterranean and even host metazoans that live in anoxic conditions. Deep-water hydrothermal vents characterized by microbial communities and metazoans (siboglinid polychaetes) are documented so far only from some Tyrrhenian seamounts (Marsili, Palinuro) and Aegean submerged volcanic craters (Santorini, Kolumbo). The geo-biological relevance of such complex habitats often situated in the high seas, calls for an international effort towards the implementation of proper protection and sustainable management.