Abstract
During the Integrated Ocean Drilling Program (IODP) Expedition 331 at the Iheya North hydrothermal system in the Mid-Okinawa Trough by the D/V Chikyu, we conducted microbiological contamination tests of the drilling and coring operations. The contamination from the drilling mud fluids was assessed using both perfluorocarbon tracers (PFT) and fluorescent microsphere beads. PFT infiltration was detected from the periphery of almost all whole round cores (WRCs). By contrast, fluorescent microspheres were not detected in hydrothermally active core samples, possibly due to thermal decomposition of the microspheres under high-temperature conditions. Microbial contamination from drilling mud fluids to the core interior subsamples was further characterized by molecular-based evaluation. The microbial 16S rRNA gene phylotype compositions in the drilling mud fluids were mainly composed of sequences of Beta- and Gammaproteobacteria, and Bacteroidetes and not archaeal sequences. The phylotypes that displayed more than 97% similarity to the sequences obtained from the drilling mud fluids were defined as possible contaminants in this study and were detected as minor components of the bacterial phylotype compositions in 13 of 37 core samples. The degree of microbiological contamination was consistent with that determined by the PFT and/or microsphere assessments. This study suggests a constructive approach for evaluation and eliminating microbial contamination during riser-less drilling and coring operations by the D/V Chikyu.
Highlights
Scientific ocean drilling has demonstrated that microbial populations are ubiquitously detectable in deep marine subsurface environments, even at a depth of 1626 m below the seafloor (Roussel et al, 2008)
In the contamination test using fluorescent microspheres of core samples obtained from high-temperature regions of the subseafloor environments at Integrated Ocean Drilling Program (IODP) Sites C0013 and C0014, no microspheres or only very few microspheres were observed, even in the most outer surface of the whole round cores (WRCs) (Table 1)
Because the fluorescent microsphere were latex beads formed from an amorphous polymer, most of the microspheres would be thermally degraded under high-temperature conditions, i.e., a glass transition temperature of ∼95◦C and melting temperature of ∼240◦C
Summary
Scientific ocean drilling has demonstrated that microbial populations are ubiquitously detectable in deep marine subsurface environments, even at a depth of 1626 m below the seafloor (mbsf) (Roussel et al, 2008). Functions, metabolic processes and geochemical interactions in subseafloor microbial ecosystem, core samples from scientific drilling expeditions are conventionally investigated via molecular biological analyses targeting 16S rRNA and functional genes, metabolic activity measurements and cultivation experiments (Fry et al, 2008; Orcutt et al, 2011). These attempts have revealed the presence of uncultivated, phylogenetically diverse and physiologically unknown microorganisms.
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