Natural fluids with a pH (25°C) up to 12.3 were collected from a sub-seafloor borehole observatory (Ocean Drilling Program (ODP) Hole 1200C) on South Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc. We used systematic differences in the chemical compositions of pore waters from drilling operations during ODP Leg 195 and borehole fluids collected subsequently from Hole 1200C to define two endmember solutions, one of which was a sulfate-rich fluid with a methane concentration of 50 mM that ascends from the subduction channel and the other was a low-sulfate fluid. The sequence of sample collection and fluid compositions constrain subsurface hydrologic conditions. Deep-sourced, sulfate- and methane-rich, sterile fluids from the subduction channel can reach the seafloor unchanged within the central conduit, whereas other fluid pathways likely intersect the pelagic sediment that underlies the serpentinite mud volcano, providing potentially suitable conditions and inoculum for microbial anaerobic oxidation of methane (AOM). These AOM-affected, low-sulfate fluids also make it to the seafloor where they discharge. The source of the sulfate- and methane-rich fluid in the subduction channel is attributed to abiotic methane production fueled by hydrogen production from serpentinization and carbonate dissolution. This methane production includes a mechanism to raise the pH above values from serpentinization alone. Results from South Chamorro Seamount represent an end member along a transect defined by the distance from the trench. Results from this site are applied to other serpentinite mud volcanoes along this transect to speculate on likely chemical conditions within shallower and cooler portions of the subduction channel.
During the Integrated Ocean Drilling Program (IODP) Expedition 331 at the Iheya North hydrothermal system in the Mid-Okinawa Trough by the D/V Chikyu, we conducted microbiological contamination tests of the drilling and coring operations. The contamination from the drilling mud fluids was assessed using both perfluorocarbon tracers (PFT) and fluorescent microsphere beads. PFT infiltration was detected from the periphery of almost all whole round cores (WRCs). By contrast, fluorescent microspheres were not detected in hydrothermally active core samples, possibly due to thermal decomposition of the microspheres under high-temperature conditions. Microbial contamination from drilling mud fluids to the core interior subsamples was further characterized by molecular-based evaluation. The microbial 16S rRNA gene phylotype compositions in the drilling mud fluids were mainly composed of sequences of Beta- and Gammaproteobacteria, and Bacteroidetes and not archaeal sequences. The phylotypes that displayed more than 97% similarity to the sequences obtained from the drilling mud fluids were defined as possible contaminants in this study and were detected as minor components of the bacterial phylotype compositions in 13 of 37 core samples. The degree of microbiological contamination was consistent with that determined by the PFT and/or microsphere assessments. This study suggests a constructive approach for evaluation and eliminating microbial contamination during riser-less drilling and coring operations by the D/V Chikyu.
The origin, evolution, and distribution of life throughout the universe can be better understood by determining the limits to life on Earth. A broad range of many of the physical and chemical constraints that determine the limits to life, such as temperature, pressure, physical space, water content, and the availability of energy and nutrients, are found in subseafloor environments. In fact, several expeditions (Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP: now International Ocean Discovery Program)) have been at least partially motivated by the desire to explore the boundaries between the habitable and the uninhabitable parts of the subseafloor. In this chapter, the possible subseafloor environments and their physical and chemical characteristics that could signify the limits of the biosphere, particularly the hydrothermally active subseafloor environments, are reviewed. Although the nature and distribution of extreme or fringe biospheres are unknown, previous ODP- and IODP-expedition-based microbiological investigations have shown that the subseafloor hydrothermal systems with relatively abundant energy supplies (sediment-derived organic compounds and serpentinization-derived H2) provide targets for seeking the limits (boundary conditions) in subseafloor environments. Here, we also discuss predicted patterns of the abundance and composition of potential microbial catabolisms in the fringe microbial communities of subseafloor hydrothermal fluids based on the thermodynamic potential of particular catabolic strategies and the computed cost of anabolism in these settings.