Deep-sea hydrothermal ecosystems are considered oases of life in oceans. Since the discovery of these ecosystems in the late 1970s, many endemic species of Bacteria, Archaea, and other organisms, such as annelids and crabs, have been described. Considerable knowledge has been acquired about the diversity of (micro)organisms in these ecosystems, but the diversity of fungi has not been studied to date. These organisms are considered key organisms in terrestrial ecosystems because of their ecological functions and especially their ability to degrade organic matter. The lack of knowledge about them in the sea reflects the widely held belief that fungi are terrestrial organisms. The first inventory of such organisms in deep-sea hydrothermal environments was obtained in this study. Fungal diversity was investigated by analyzing the small-subunit rRNA gene sequences amplified by culture-independent PCR using DNA extracts from hydrothermal samples and from a culture collection that was established. Our work revealed an unsuspected diversity of species in three of the five fungal phyla. We found a new branch of Chytridiomycota forming an ancient evolutionary lineage. Many of the species identified are unknown, even at higher taxonomic levels in the Chytridiomycota, Ascomycota, and Basidiomycota. This work opens the way to new studies of the diversity, ecology, and physiology of fungi in oceans and might stimulate new prospecting for biomolecules. From an evolutionary point of view, the diversification of fungi in the oceans can no longer be ignored.Since the discovery of hydrothermal vent ecosystems 30 years ago, unexpected species diversity has been revealed that has shed light on the functional coupling between the geosphere and the biosphere. When submersibles dive to the seafloor, they bring numerous organisms back to the surface, and this has resulted in the description of nearly two new species per month (10). Deep-sea hydrothermal ecosystems are considered hotspots of microbial diversity on the seafloor. Indeed, they are ecosystems that produce biomass using the wide range of chemical compounds released by the polymetallic sulfite chimneys or "black smokers" that represent the huge quantity of chemical energy that is available (26). The vent fluid, having been heated close to a magma chamber, can have a temperature of 400°C when it is emitted. It is also characterized by a lack of dissolved oxygen, strong acidity (pH 2 to 3), a high concentration of electron donors (i.e., reduced compounds such as methane and hydrogen sulfide), and the presence of heavy metals (36). Continual mixing with the cold ocean water (2 to 4°C) that is rich in electron acceptors creates a dynamic chemical disequilibrium that is a source of energy for microorganisms that control the rates of redox reactions (16).Each ridge displays varied geochemistry, and the vent fluids differ, even at scales as small as the fractures, pipes, and porosities in the black smokers, creating diverse microhabitats for biota (10,

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The fungal kingdom is replete with unique adaptive capacities that allow fungi to colonize a wide variety of habitats, ranging from marine habitats to freshwater and terrestrial habitats. The diversity, importance, and ecological roles of marine fungi have recently been highlighted in deep-subsurface sediments using molecular methods. Fungi in the deep-marine subsurface may be specifically adapted to life in the deep biosphere, but this can be demonstrated only using culture-based analyses. In this study, we investigated culturable fungal communities from a record-depth sediment core sampled from the Canterbury Basin (New Zealand) with the aim to reveal endemic or ubiquist adapted isolates playing a significant ecological role(s). About 200 filamentous fungi (68%) and yeasts (32%) were isolated. Fungal isolates were affiliated with the phyla Ascomycota and Basidiomycota, including 21 genera. Screening for genes involved in secondary metabolite synthesis also revealed their bioactive compound synthesis potential. Our results provide evidence that deep-subsurface fungal communities are able to survive, adapt, grow, and interact with other microbial communities and highlight that the deep-sediment habitat is another ecological niche for fungi.