Vent Habitat

Riftia pachyptila inhabits hydrothermal vent sites along the East Pacific Rise and the Galapagos Rift in the Eastern Pacific. The distribution of the tubeworm is intimately tied to the unique physiochemical characteristics of hydrothermal vents. Vent sites are typified by steep gradients between cold (~1.8°C), oxygen-rich (110 |M) bottom water and hot (up to 400°C), acidic (pH ~3 to 6) vent fluid. Hydrothermal fluids are usually laden with volcanic gases (e.g., methane and carbon dioxide) and reduced chemicals, including heavy metals and hydrogen sulfide (H2S, HS-, S2-). Sulfide in vent fluids, produced by the geothermal reduction of seawater sulfate and the interaction of geothermally heated water with sulfur-containing rocks (e.g., basalt; Alt 1995; Elderfield and Schultz 1996; Rouxel et al. 2004), usually occurs at concentrations (3-12 mmol/kg) orders of magnitude higher than those in ambient seawater. It is at the interface (the chemocline) between the anoxic, reduced vent effluent and oxic bottom water that chemosynthetic vent symbioses thrive. Here, chemosynthetic symbionts access both the reduced compounds (e.g., sulfide) used as an energy source and the oxygen to which electrons are shuttled in aerobic energy metabolism.

While some vent symbioses, including those involving alvinellid polychaete worms and episymbiotic bacteria, congregate around sites where effluent (up to 400°C, pH ~3) directly exits the seafloor, R. pachyptila is typically clustered around diffuse or low flow vents. These vents, formed by ambient seawater mixing in the shallow subsurface with vent fluid, generally have a higher pH (~6), lower temperatures (1.8 to ~40°C), and, consequently, lower concentrations of reduced chemicals (e.g., sulfide up to 300 mM; Fisher 1995; van Dover 2000). But the physiochemical environment of diffuse vents is rarely stable; flow rate, temperature, and sulfide concentration may vary over timescales measured in seconds (Johnson et al. 1988a, 1994). To exploit this stochastic environment R. pachyptila relies on anatomical and physiological adaptations for sequestering sulfide, oxygen, inorganic carbon, and nitrogen from the chemocline and on the ability of its endosymbionts to use these substrates for energy metabolism and biomass synthesis.

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