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Hydrothermal circulation within oceanic crust depends on pressure (P) and temperature (T); the critical point (CP) of seawater at 298 bar and 407 °C represents the threshold between subcritical and supercritical conditions. Here we present data from the fi rst hydrothermal system in which the sampled fl uids fall on and above the CP. The vent system discovered at 5°S on the Mid-Atlantic Ridge is characterized by multiple fl uid emanations at variable temperatures in water depths of ~3000 m. Vigorous vapor phase bubbling, stable emanation of superhot fl uid at 407 °C, and decreased salinity indicate phase separation at conditions above the CP at one site. At another site the measured maximum T of 464 °C during a 20 s interval is by far the hottest fl uid ever measured at the seafl oor and falls into the vapor-phase supercritical region of seawater. Besides these two separate fi elds with ongoing phase separation and extremely hot fl uids, a third vent fi eld emanates non-phase-separated fl uids at 349 °C and is used as a reference site. Fluid chemistry shows that supercritical fl uids evolve differently than subcritical fl uids, making this vent system a unique natural laboratory to investigate processes at high P-T conditions. The stability of the high temperature and fl uid geochemistry measured in 2005 and 2006 after the assumed seismic trigger event in 2002 supports this as an exceptional site along the Mid-Atlantic Ridge.

The ultramafic-hosted Logatchev hydrothermal field (LHF) on the Mid-Atlantic Ridge is characterized by high hydrogen and methane contents in the subseafloor, which support a specialized microbial community of phylogenetically diverse, hydrogen-oxidizing chemolithoautotrophs. We compared the prokaryotic communities of three sites located in the LHF and encountered a predominance of archaeal sequences affiliated with methanogenic Methanococcales at all three. However, the bacterial composition varied in accordance with differences in fluid chemistry between the three sites investigated. An increase in hydrogen seemed to coincide with the diversification of hydrogen-oxidizing bacteria. This might indicate that the host rock indirectly selects this specific group of bacteria. However, next to hydrogen availability further factors are evident (e.g. mixing of hot reduced hydrothermal fluids with cold oxygenated seawater), which have a significant impact on the distribution of microorganisms.

SummaryLilliput was discovered in 2005 as the southernmost known hydrothermal field along the Mid-Atlantic Ridge. It is exceptional in that it lacks hightemperature venting probably because of a thickened crust. The absence of thermophilic and hyperthermophilic prokaryotes in emissions supports the argument against the presence of a hot subsurface at Lilliput, as is typically suggested for diffuse emissions from areas of high-temperature venting. The high phylogenetic diversity and novelty of bacteria observed could be because of the low-temperature influence, the distinct location of the hydrothermal field or the Bathymodiolus assemblages covering the sites of discharge. The low-temperature fluids at the Lilliput are characterized by lowered pH and slightly elevated hydrogen (16 nM) and methane (~2.6 mM) contents compared with ambient seawater. No typical hydrogen and methane oxidizing prokaryotes were detected. The higher diversity of reverse tricarboxylic acid genes and the form II RubisCO genes of the Calvin Benson-Bassham (CBB) cycle compared with the form I RubisCO genes of the CBB cycle suggests that the chemoautotrophic community is better adapted to low oxygen concentrations. Thiomicrospira spp. and Epsilonproteobacteria dominated the autotrophic community. Sulfide is the most abundant inorganic energy source (0.5 mM). Diverse bacteria were associated with sulfur cycling, including Gamma-, Delta-and Epsilonproteobacteria, with the latter being the most abundant bacteria according to fluorescence in situ hybridization. With members of various Candidate Divisions constituting for 25% of clone library sequences we suggest that their role in vent ecosystems might be more important than previously assumed and propose potential mechanisms they might be involved in at the Lilliput hydrothermal field.