Hydrothermal Vent Habitat

Chapter 1: Hydrothermal vent habitats

As light penetration at depths below 300 meters is inadequate to support photosynthesis, ambient seawater temperatures below 1000 meters lie between 1 and 5⁰C and hydrostatic pressure increases with 1 atm for every 10 meters of water depth, for a long time the deep sea was not regarded to be a very habitable environment (van Dover, 2000). However, since the discovery of hydrothermal vents in 1977 this view has drastically changed (Corliss et al., 1979; German et al., 2011). Different hydrothermal vents offer a range of different environments, and at particular venting sites teeming oases of life can be found with biomass estimates of 500-1000 times higher than at the surrounding non-vent deep sea (Lutz

Mid-ocean ridges are sub-surfaced mountain ranges located at the boundaries between the tectonic plates that make up the Earth’s crust (van Dover, 2000; Tivey, 2007; Martin et al., 2008). As the plates are pulled apart by tectonic forces, hot soft rock from the deep Earth emerges to fill the fissure. As the generated ocean crust laterally moves away from the spreading-axis, the age of the crust and the depth of the sediment cover increase systematically (van Dover, 2000). Therefore, different types of hydrothermal vents occur at different areas of the ultramafic sea floor. The simplest of these are the columnar sulfide chimneys at the East Pacific Rise (EPR). These chimneys are often the result of volcanic eruptions and are characterized by high temperatures (>4000C) (Tivey, 2007). Formation of these black smoker hydrothermal vents begins when metal- and sulfide-rich acid fluids mix with seawater, causing the metal sulfides to precipitate and form particle rich black plumes. The microbiological communities at the EPR sites are dominated by sulfide as the main electron donor for respiration (Jørgensen & Boetius, 2007). More complex hydrothermal vent systems are found at the Trans-Atlantic Geotraverse (TAG), where vigorously venting black smokers and entrainment of seawater beneath the mound trigger the remobilization of zinc (Zn) and other trace metals which are subsequently deposited

These habitats are characterized by high bacterial and archaeal diversity (Kelley et al., 2002), but the factors shaping community structure and composition are often not well understood, as diffuse venting sites are far less studied than plume and chimney habitats (Campbell et al., 2013). The wide, extensive plumes typical of black smoker vents contain chemicals that can either sink back to the ocean floor and become hydrothermal sediment or be scavenged by microbial communities living within the plumes (Kelley et al., 2002; Tivey, 2007). Generally, hydrothermal plumes are characterized by near ambient sea-water temperatures and low nutrient concentrations (Kelley et al., 2002) but they contain distinct regions (e.g., the rising plume and neutrally buoyant plume) with steep physical and chemical gradients that define distinct microbial communities (Dick et al., 2013). Microbial communities within plumes are generally derived from 3 sources (fig 1.3): (1) seafloor communities that are transported upwards, (2) background deep-sea water communities and (3) microbes that grow within the plume. The latter category shapes the plume community most, as these microbes both utilize the hydrothermally sourced electron donors for chemosynthesis, and consume the organic carbon produced in a