An ecosystem without a volcano
Discovered by accident on December 4, 2000, the Lost City Hydrothermal Field is a unique environment on the seafloor. Located 750 meters deep on an underwater mountain called the Atlantis Massif, it differs from the common "black smoker" vents, which are driven by volcanic heat. Instead, Lost City's energy comes from a chemical reaction between seawater and mantle rock, a process called serpentinization.
This field consists of about 30 active and inactive chimneys made of carbonate minerals. They range from small mounds to the massive Poseidon tower, which stands 60 meters (nearly 200 feet) tall. The fluids venting from these structures are not acidic like those from black smokers; they are highly alkaline, with pH values between 9 and 11. Temperatures are also moderate, ranging from 40°C to 91°C. Radiocarbon dating shows the field has been active for at least 120,000 years, making it the longest-lived vent environment known.
The chemistry of rock-powered life
The engine driving Lost City is serpentinization, an exothermic reaction where seawater infiltrates the Earth's upper mantle rock, primarily peridotite. This process alters the rock's minerals, consumes water, and releases significant heat, capable of raising rock temperatures by 260°C. The chemical byproducts are hydrogen gas (H₂) and methane (CH₄), which become the primary energy source for life in this dark environment.
When the warm, calcium-rich hydrothermal fluids mix with the cold seawater, minerals precipitate to build the towering chimneys. Young, active structures are composed mainly of aragonite and brucite, giving them a brilliant white color. As the chimneys age and become inactive, they turn gray or cream-colored, and their composition shifts to calcite. This entire system produces simple hydrocarbons abiotically—without biological input, it is a model for how the building blocks of life could have formed on early Earth.
The ecosystem is dominated by microbes. Dense biofilms of archaea and bacteria thrive inside the porous chimney walls. The most active, warmest parts of the chimneys host a single type of methane-producing archaea from the order Methanosarcinales. This life differs from that at black smokers, which relies on carbon dioxide and hydrogen sulfide. While larger animals are rare, the chimneys host snails, crustaceans, crabs, and shrimp. The unique conditions make Lost City a analog for searching for life on other worlds, such as Jupiter's moon Europa or Saturn's moon Enceladus, which may harbor similar subsurface oceans interacting with rock.