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For more than two decades, Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP) expeditions to the eastern flank of the Juan de Fuca Ridge have been conducted for the purpose of describing the hydrogeological, geochemical, and microbiological processes and properties within a young ridge flank environment (e.g., Shipboard Scientific Party, 1992, 1997; Expedition 301 Scientists, 2005; Wheat et al., 2013; also see the “Expedition 327 summary” chapter [Expedition 327 Scientists, 2011a]). Driven by lithospheric cooling of the ridge flank, the porous and permeable igneous ocean basement acts as a subsurface conduit for hydrothermal fluid flow between exposed rocky outcrops (Wheat et al., 2013). Although the nature of microbial life in the sediment-covered basement is the focus of much active research on the Juan de Fuca Ridge, the microbial diversity within marine sediment is still poorly cataloged, due in large part to the restriction of most marine sediment diversity surveys to shallow depths (<1 m) and continental margins (Orcutt et al., 2011a).

Microbial diversity surveys on the Juan de Fuca Ridge have focused at diffuse flow hydrothermal vents at Axial Seamount (e.g., Lynch, 2000; Alain et al., 2002; Pagé et al., 2004; Huber et al., 2006a, 2009; Kaye et al., 2011; Meyer et al., 2013), high-temperature hydrothermal vents at Endeavor Segment (e.g., Wang et al., 2009a, 2009b; Zhou et al., 2009; Anderson et al., 2013), a hydrothermal mound located at off-axis Cleft Segment (Davis et al., 2009), seafloor basalt (Mason et al., 2007, 2009), sediment-buried basement fluid (Cowen et al., 2003; Huber et al., 2006b; Nakagawa et al., 2006; Jungbluth et al., 2013), and sediment-buried rock (Orcutt et al., 2011b; Smith et al., 2011; Lever et al., 2013). The limited number of microbiological studies focused on sediment of the Juan de Fuca Ridge have typically not incorporated phylogenetic marker-based surveys of community diversity such as small subunit ribosomal RNA (SSU rRNA) gene sequencing (e.g., Cragg et al., 2000; Mather and Parkes, 2000; Engelen et al., 2008); only studies focused on Cascadia margin methane hydrate–bearing sediment have incorporated such an approach (e.g., Bidle et al., 1999; Marchesi et al., 2001; Knittel et al., 2003; Inagaki et al., 2006; Nunoura et al., 2008; Briggs et al., 2012). Descriptions of the phylogenetic and metabolic composition of microbial communities from other Juan de Fuca Ridge sediment environments are important for comprehensively investigating patterns in microbial distribution and community evolution with respect to gradients in the physical and chemical properties of the system.

During IODP Expedition 327, five boreholes were cored at Site U1363 (Fig. F1) (see the “Expedition 327 summary” chapter [Expedition 327 Scientists, 2011a]) adjacent to an exposed rocky seamount, Grizzly Bare outcrop (Fig. F2) (see the “Site U1363” chapter [Expedition 327 Scientists, 2011c]). Grizzly Bare serves as a point of seawater recharge into the igneous basement that is superimposed over basal basement fluid flow likely originating predominantly from the unsedimented ridge flanks. Sediment in this area is composed of turbidites interspersed with hemipelagic clay, consistent with core recovery from ODP and IODP boreholes at Sites 1026 and U1301 located >50 km to the north-northeast (see the “Site U1363” chapter [Expedition 327 Scientists, 2011c]). Three sample types were collected from Holes U1363B and U1363G for the purpose of investigating microbial community structure:

  • Raw uncompressed sediment samples,

  • Squeezed sediment (i.e., squeeze cakes), and

  • Interstitial water derived from the squeeze cakes.

It is possible that the three sample types differ with respect to their resident microbial community structure based on strength of attachment of individual microbial lineages to sediment substrate. For example, squeeze cakes may represent communities that might be more tightly attached to the sediment, whereas interstitial water derived from the squeeze cakes may reflect a more loosely associated microbial community. In this investigation, the variety of samples from Holes U1363B and U1363G (Fig. F2) (see the “Site U1363” chapter [Expedition 327 Scientists, 2011c]) were investigated using SSU rRNA gene cloning and sequencing.