Submarine Hydrothermal Vents: Limits of Life, Biofilms and Early Evolution
John A. Baross (U of Washington)
The two types of hydrothermal vent environments, magma-driven and peridotite-hosted, offer many contrasting habitat conditions for microbial communities. These environments span a wide range of chemical and physical conditions that include almost all of the extremes in temperature, Eh, salinity and heavy metal concentrations that limit where life can exist. Moreover, vent microorganisms have adapted to habitat conditions that include flowing fluids, porous spaces within basalt, sulfides and sediments, the surfaces of rocks and animals and the subseafloor potentially to depths in the crust exceeding 6 km. Hydrothermal systems produce volatiles, such as H2, H2S, CH4, CO, CO, and trace metals that are important sources of carbon and energy, and nutrients for organisms. The sources of volatiles include magma degassing, water/rock reactions, and abiotic reduction of CO2 to methane and possibly other organic compounds. All of these reactions take place in the subseafloor and are not always dramatically expressed on the seafloor. Recently, a peridotite-hosted hydrothermal vent environment was discovered on the Mid-Atlantic Ridge. This environment, named the "Lost City Hydrothermal Field" is a source of high concentrations of hydrogen and methane, hydrocarbons and possibly organic acids produced abiotically from serpentinization reactions that take place in the crust. Hallmark characteristics of both types of high temperature hydrothermal vent microbial communities are that they utilize hydrogen as a primary energy source and they exist as biofilms. This is interesting in that there are parallels between the energy metabolic reactions of these microbial biofilms and the chemistry of the H2-CO2 redox couple that are present in hydrothermal systems, thus indicating the possibility that vent autotrophs might provide clues about the kinds of reactions that initiated the chemistry of life. Hydrothermal vents thus unite microbiology and geology to address the important question - What is the origin of life? - And by inference, a model for understanding the environmental conditions that could support a second genesis on other planets and moons.