Microbiological Assessment of Circulation Mud Fluids During the First Operation of Riser Drilling by the Deep-Earth Research Vessel Chikyu

Abstract

Quality assurance and control (QA/QC) is significant for the scientific drilling in order to accurately characterize physical, geochemical, and biological properties in the cored deep subseafloor materials. To explore the deep subseafloor life and its biosphere, identification and control of microbial contamination in drilling cores is critical for highly sensitive molecular analyses as well as cultivations, especially for the evaluation of low biomass and/or extremely harsh deep environments. Here we report some microbiological characteristics of circulation mud fluids before and after the first riser drilling operation by the newly constructed deep-earth research vessel Chikyu. During the Chikyu shakedown expedition CK06-06 in 2006, we used the riser system for drilling 547 to 647 meter below the seafloor into the sediments offshore the Shimokita Peninsula of Japan. Cultivation experiments showed that no microbial growth was observed in the precirculation mud fluid, while 4 × 105 colonies per 1 ml were observed in the postcirculation mud fluid; all cultured bacterial isolates were found to be Halomonas. Using culture-independent molecular analysis, 16S rRNA gene sequences of Xanthomonas, which is used for industrial production of the mud fluid viscosifier “xanthan gum”, were predominantly detected in the precirculation mud fluid, while Halomonas sequences consistently dominated the clone library constructed from the postcirculation mud fluid. Archaeal 16S rRNA genes were amplified only from the postcirculation mud fluid; these archaeal clone sequences were affiliated to the Marine Crenarchaeota Group I (MGI), Marine Euryarchaeota Group II (MGII), Miscellaneous Crenarchaeotic Group (MCG), South African Gold Mine Euryarchaeotic Group (SAGMEG), Soil Group, and Methanococcus aeolicus. These results suggest that Halomonas contaminated and grew in the tank of circulation mud fluids, and other indigenous deep subseafloor microbial components, especially deep subsurface archaea, were also mixed into the post-circulation mud fluid.