Tracer-based estimates of drilling-induced microbial contamination of deepsea crust

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 Pr ogram on the drillship JOIDES Resolution to assess the suitability of this platform for deep biosphere research. Tests using both a chemical tracer [p erfluoro(methylcyclohexane)] and a particulate tracer (0.5-mu m-diameter fl uorescent microspheres) were conducted during coring with the advanced hydr aulic piston core in unconsolidated sediments and with the rotary core barr el and diamond core barrel in igneous rock. Detection of both types of trac ers on the exterior of recovered cores confirmed successful delivery. The p articulate tracer was never detected in the interior of unconsolidated sedi ment (n = 24). The average concentration of the chemical tracer (n = 12) wa s equivalent to 0.35 mu L of drilling fluid per gram of sediment. The parti culate tracer was not found in the interiors of igneous rock samples that w ere crushed (n = 4) but was found in the interiors of 64% of the thin secti ons examined (n = 12), indicating that the samples were contaminated during sectioning rather than drilling. Perfluorocarbon data indicate that drilli ng fluid in the igneous rock samples averaged 0.01 mu L g(1) rock. Based on the abundance of bacteria in the surface seawater (4.2 x 10(8) 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 e stimate is conservative, given the high pervasiveness of the perfluorocarbo n tracer relative to microbes.