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doi:10.2204/iodp.proc.331.203.2016

Introduction

Subseafloor microbes beneath active hydrothermal vents, in the “subvent biosphere,” are believed to live near the upper temperature limit for life on Earth. Hydrothermally influenced deeply buried marine sediments were sampled during Integrated Ocean Drilling Program (IODP) Expedition 331, Deep Hot Biosphere (see the “Expedition 331 summary” chapter [Expedition 331 Scientists, 2011a]).

Site C0013 is located ~100 m east of the main hydrothermal mound chains of the Iheya North hydrothermal field. In Hole C0013C, the core liner melted, indicating a temperature >82°C at 12.5 meters below seafloor (mbsf). Site C0014 is located ~450 m east of the high-temperature vents and mounds, and the temperature exceeded 210°C at only 50 mbsf. The temperature gradient was roughly linear from 0 to 47 mbsf, increasing from the bottom water temperature of 4.5°–145°C over that depth range, but it deviated greatly from this line at 0–9 and 47–50 mbsf, where it was clearly affected by high-temperature fluid pooling or lateral flow. Site C0015, located on a hill ~600 m northwest of the active vents, represents a potential migration path for hydrothermal fluid; however, hydrothermal alteration was absent in the cored interval. Site C0017 is located 1550 m east of the high-temperature vents of the Iheya North hydrothermal field, in an area of low heat flow. The overall temperature profile was exponential and concave upward, which is consistent with downwelling of cold water, implying that this was an area of recharge to the hydrothermal system. A maximum temperature of 90° ± 5°C was measured at the bottom of the deepest hole (C0017D) at 151 mbsf (see the “Expedition 331 summary” chapter [Expedition 331 Scientists, 2011a]).

Temperature gradients decrease greatly with distance from the active vents, from >7°C/m at Site C0013 to 3°C/m at Site C0014 and 0.6°C/m at Site C0017. Site C0015 has a relatively low heat flow and a surficial temperature gradient of ~1°C/m. At Sites C0013, C0014, and C0017, the relatively high, moderate, and low heat flow areas east of the Iheya North hydrothermal field were drilled to investigate subseafloor microbial habitats and communities. Broad gradients of physical and chemical variation, both laterally and vertically, that could be affected by mixing between discharging hydrothermal solutions and recharging ambient bottom seawater (see the “Expedition 331 summary” chapter [Expedition 331 Scientists, 2011a]) characterizes this habitat.

Sampling of the sediments was done while applying contamination tests (Yanagawa et al., 2013). Using molecular ecology approaches, the deep biosphere of these sediments was analyzed (Yanagawa et al., 2014). Here, cultivation of bacteria with different media was done following previous Ocean Drilling Program (ODP)/IODP approaches (Batzke et al., 2007), and microbial activity was measured by isothermal microcalorimetry. The latter allows the sensitive detection of heat output generated by the sum of chemical and biological reactions in a sample. A chloroform treatment to kill the microorganisms allows for differentiation between chemical and biological activity. The higher the heat output, the higher is the activity of a sample (Schippers et al., 1995; Braissant et al., 2010; Schippers and Bosecker, 2005). To study hydrothermal influence on activity, measurements were carried out at 25°C and 90°C for comparison.

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