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doi:10.2204/iodp.proc.336.108.2012

Discussion

In an effort to minimize perturbation of subsurface borehole environments for microbiology-related experimentation and observation, several alternative materials (to low-alloy steel) for observatory casing were examined for suitable properties. These examinations began in 2008, prior to Expedition 327, to guide the selection of materials to be used in CORK observatories. Early published results from leaching experiments with fiberglass and several resin coatings suggested that fiberglass had the best (i.e., lowest) rates of carbon and nitrogen leaching and that all materials had improved iron leaching characteristics as compared to bare steel pipe (Orcutt et al., 2010). Due to budgetary, risk, and time restraints, Expedition 327 CORK steel materials were coated with Amerlock 400 and TK-34XT resins, and fiberglass was not used (Fisher et al., 2011). In contrast, an aromatic amine–cured fiberglass product was used in Expedition 336 CORK observatories in addition to Amerlock-, Xylan-, and TK-34XT-coated steel pipe (Expedition 336 Scientists, 2012). It is expected that these new CORK observatories should have significantly improved experimental conditions for microbiological investigations, especially in relation to the leaching of iron in the borehole environment, although Xylan-coated steel does seem to allow for a significant amount of rust to be released into surrounding fluids. Further studies are necessary to examine the properties, fate, and impact of the low levels of organic carbon and nitrogen compounds expected to leach from the alternative materials used in the Expedition 336 observatories. It should be cautioned that, although these experiments were conducted at the higher temperatures expected in some crustal environments like the Juan de Fuca Ridge flank, we did not replicate the reducing (i.e., anoxic) fluid conditions present there; thus, it is unknown how these materials behave under reducing conditions.

The observation of significant reaction of common nucleic acid stains with the dopes used in observatory construction suggests caution may be necessary in interpretation of borehole and crustal samples investigated for cell density measurements based on microscopy. The nonspecific binding of nucleic acid stains to noncellular material has been observed previously; for example, in the binding of SYBR Green I nucleic acid stain to the mineral stalks produced by iron oxidizing bacteria in borehole colonization experiments (Orcutt et al., 2011) and in the binding of SYBR Green I to debris in sediments (Morono et al., 2009). In the past, some researchers have described the presence of DNA in “microtubes” in crustal thin sections based on nucleic acid staining (e.g., Banerjee and Muehlenbachs, 2003), and have used such observations as evidence for microbial life in these microtubes. We suggest that caution must be used in such extrapolations, considering that our results show the common nonspecific binding of DNA stains to dopes and other materials that would be used during the drilling process used to collect the hard rock samples.

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