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doi:10.2204/iodp.proc.311.103.2006

Microbiology

Site U1325 is located along the main transect of sites established during Expedition 311 across the northern Cascadia accretionary prism within a small basin on the lower slope between two accretionary ridges. It was the fourth site sampled for microbiology. We anticipated a shallow sulfate/methane interface and a gas hydrate stability zone (GHSZ) that extended to ~230 mbsf. Efforts continued to obtain live anaerobic methane oxidizers, potentially unknown gas hydrate–associated microbial communities, high-pressure adapted microorganisms, and methanogens. Subsamples for direct cell counts and biological contamination tests continued at this site.

Microbiological sampling

Sampling from the mudline (Sample 311-U1325B-1H-1, 0–3.0 cm) in Hole U1325B and the deepest core (Sample 311-U1325C-15X-2, 5–15 cm; 296.14 mbsf) in Hole U1325C targeted microorganisms for aerobic and anaerobic high-pressure culturing. The SMI in the upper sedimentary section of Hole U1325B was targeted for intensive, coupled microbiological and geochemical sampling. At this site, the SMI was placed between 4 and 5 mbsf (see "Interstitial water geochemistry" and "Organic geochemistry"). Coring resumed in Hole U1325C where coring in Hole U1325B ended (188 mbsf) and completed the targeted sediment section.

Methanogenesis can occur in most anaerobic environments, but it becomes the major metabolic strategy when other electron donors such as nitrate, Fe(III), and sulfate are depleted. We sampled regularly downhole to below the depth of the predicted BSR to quantify methanogenesis in these sediments (see "Microbiology" in the "Methods" chapter).

Contamination tests

Perfluorocarbon tracers

Samples for perfluorocarbon tracer (PFT) and microsphere analyses were taken immediately from the ends of whole-round cut sections on the catwalk. Because the freshly cut sediment surface was potentially contaminated by PFT smeared along the core liner, the sediment surface at the sampling spots was scraped away using a clean spatula before sampling with a syringe. Subsamples (~5 cm³) were taken from outer and inner layers from each core for gas chromatograph analysis as described in "Microbiology" in the "Methods" chapter. Samples were analyzed as described and raw data are presented in Table T8. We also measured the air on the catwalk to confirm background PFT levels. The results (Table T8) show that the interior portions of the APC cores from Hole U1325B are less contaminated with PFT than the exterior portions.

Fluorescent microspheres

Comparisons of fluorescent microsphere penetration in paired samples collected from the edge and center of the cores are summarized in Table T8. Microscopic analysis of the outer portion of the APC cores showed detectable numbers of 10⁴ microspheres/g of sediment, whereas microspheres were generally below the detection limit of 100 microspheres/g in samples taken from core interiors. Sediment samples from XCB cores were collected from the split cores to allow separate sampling of specific structures (e.g., sand layers, biscuits, and drilling slurry).

Shipboard analysis

We started an enrichment culture experiment for methanogens with a few sediment samples (see "Enrichment cultures" in the "Methods" chapter). Samples 311-U1325B-10X-5, 52–67 cm (78.8 mbsf) and 311-U1325C-4X-1, 55–70 cm (208.00 mbsf), and 8X-3, 97–112 cm (239.15 mbsf, from approximately the BSR depth) were used for this enrichment culture study.

Samples were taken from the top (Sample 311-U1325B-1H-1, 0–3 cm) and the bottom (Sample 311-U1325C-15X-2, 5–15 cm; 296.14 mbsf) of the sedimentary section for inoculation of enrichment cultures targeting high pressure–adapted heterotrophic and sulfate-reducing microorganisms. Samples were maintained at low temperature, and dilution series were inoculated to culture for microorganisms at 55.1 MPa and 4°C. Cultures for sulfate reducers required preparation in the anaerobic chamber and were fed formate, acetate, or lactate as a carbon source.

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