Life in extreme environments: Hydrothermal vents

Abstract

Eruption of volcanic rocks at the midocean ridges is the major mechanism by which heat is lost from the interior of the Earth. Approximately one-third of the heat is removed from the spreading centers by convective circulation of seawater (1). The magnitude of this heat loss requires that the entire volume of the oceans circulates through the midocean ridges in approximately 10 million years. Seawater interaction with volcanic rocks at near 400°C results in substantial chemical flux and makes an important contribution to buffering the composition of some elements in seawater. Cations from seawater (Mg ϩ2 , Ca ϩ2, and Na ϩ ) form hydroxyl-bearing alteration minerals in the volcanic rocks, releasing hydrogen ion to solution. The hot, acidic alteredseawater releases metals (Fe, Mn, Zn, and Cu) and reduced sulfur (H 2 S) from the volcanic rock; these are transported by hydrothermal solutions to the seafloor and form metallic mineral deposits.Seafloor hydrothermal vents support ecosystems with enormous biomass and productivity compared with that observed elsewhere in the deep oceans. What is the energy source that fuels these oases of life, and what adaptations allow them to exist in these extreme environments?Oxidation͞reduction (redox) reactions are key to supporting chemosynthesis. The atmosphere and hydrosphere are relatively oxidizing with an abundance of potential electron acceptors (O 2 , SO 4 ϭ , and NO 3 -). In contrast, the basaltic rocks that form the oceanic crust are relatively reduced because of the abundance of ferrous iron. High-temperature fluid͞rock interaction forms reduced gases (H 2 S, H 2 , and CH 4 ) that dissolve in hydrothermal fluid. Representative redox reactions that produce H 2 S include:where talc serves as a proxy for a Reduced magmatic gases (H 2 and H 2 S) also are released directly during intrusion and eruption of basaltic magma. Seafloor hydrothermal systems undergo cyclic variations in temperature, vent fluid composition, and biological activity in response to magmatic episodes (3, 4). Volcanic events initially are followed by relative increases in vent fluid temperature and magmatic gases, often accompanied by decreased salinity in hydrothermal fluids as a result of supercritical phase separation of seawater in the subsurface (5) and faster migration of the vapor-like component to the seafloor. Later in the cycle (on the order of a few years) the high-salinity component of the phase-separated fluids reaches the seafloor. The higher chloride content of these fluids enhances metal solubility, but H 2 S content drops, consistent with solubility control by metal sulfide minerals. Microbial blooms are observed at the initiation of the magmatic cycle, suggesting that the upper oceanic crust is inoculated with microbial communities poised to exploit the chemical energy carried by hydrothermal fluids. Although there is a potential abundance of chemical energy, deep-sea hydrothermal communities have had to adapt to extreme conditions to exploit this resource. Of particular interest are the...

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