ROCK-HOSTED HYDROTHERMAL ECOSYSTEMS II: THE LOST CITY HYDROTHERMAL FIELD

Most of the well-studied hydrothermal vent systems in the deep-sea are located at or near mid-ocean ridge spreading centers on basaltic substrate. However, in December 2000, the Lost City Hydrothermal Field was discovered, which represents a new type of life-supporting submarine hydrothermal system hosted on 1-2 my old ultramafic material. In contrast to black smoker environments, this system hosts carbonate towers that rise >60 m above the surrounding seafloor. Unlike magmatically influenced black smoker vents, hydrothermal circulation at LC is believed to be driven primarily by exothermic serpentinization reactions in ultramafic oceanic crust. These reactions produce highly reduced, alkaline hydrothermal fluids and create byproducts that include hydrogen, methane, and small organic molecules. Although sites near active serpentinization fronts below the LC field cannot be easily accessed, the carbonate habitats with the chimney structures have been sampled and characterized. The chimneys are composed of calcium carbonate minerals and are extremely porous (>40%) relative to sulfide chimney structures. Microscopic examination of pore spaces within the carbonate structures and cell extraction and enumeration revealed highly-active microbial populations in the vicinity of 40-75°C venting fluids. A high percentage of cells within these regions contained factor 420 (F₄₂₀), a fluorescent coenzyme involved in anaerobic methane cycling. The F₄₂₀-fluorescing cells formed thick biofilms consisting of a single morphotype. Sequencing of the 16S rRNA gene from carbonate samples confirmed that the organisms observed were methane-metabolizing Archaea. Samples from peripheral parts of the LC field harbored less active microbial populations and contained fewer autofluorescent cells than those bounding the vent fluids. These data indicate that microbial communities within anaerobic habitats of the chimney walls are linked to energy sources created during subseafloor serpentinization reactions. The microorganisms found within the carbonate structures hint that similar metabolic processes may be operative in the subseafloor and highlight the discovery of a new type of deep-sea hydrothermal ecosystem.