Tracer-based estimates of drilling-induced microbial contamination of deep sea crust

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Progress in studying the active biosphere in the deep seafloor is currently limited by the unknown amount of drilling-induced microbial contamination. Contamination tests were conducted during Leg 185 of the Ocean Drilling Program on the drillship JOIDES Resolution to assess the suitability of this platform for deep biosphere research. Tests using both a chemical tracer [perfluoro(methylcyclohexane)] and a particulate tracer (0.5-μm-diameter fluorescent microspheres) were conducted during coring with the advanced hydraulic piston core in unconsolidated sediments and with the rotary core barrel and diamond core barrel in igneous rock. Detection of both types of tracers on the exterior of recovered cores confirmed successful delivery. The particulate tracer was never detected in the interior of unconsolidated sediment (n = 24). The average concentration of the chemical tracer (n = 12) was equivalent to 0.35 μL of drilling fluid per gram of sediment. The particulate tracer was not found in the interiors of igneous rock samples that were crushed (n = 4) but was found in the interiors of 64% of the thin sections examined (n = 12), indicating that the samples were contaminated during sectioning rather than drilling. Perfluorocarbon data indicate that drilling fluid in the igneous rock samples averaged 0.01 μL g-1 rock. Based on the abundance of bacteria in the surface seawater (4.2 x 108 L-1), which was used as the drilling fluid, the potential contamination of both sample types is on the order of a few bacteria per gram of cored material. This estimate is conservative, given the high pervasiveness of the perfluorocarbon tracer relative to microbes.

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Geomicrobiology Journal