Abstract

Microorganisms inhabiting sediment in close proximity to recharging basement outcrops are of interest because of the enhanced advective fluid flow in these locations, which is expected to exert unique selective pressures on the resident microbial communities. Here, Integrated Ocean Drilling Program (IODP) boreholes were used to access sediment microbial communities near Grizzly Bare recharge seamount on the Juan de Fuca Ridge eastern flank. The two locations examined in this study, Holes U1363G and U1363B, are 50 and 177 m away from the center of the outcrop, respectively. In general, small subunit ribosomal RNA gene clones from all three domains of life were detected; these groups were predominantly related to microorganisms known to reside in marine sediment. A large fraction of environmental gene clones recovered from Hole U1363B and U1363G sediment were related to uncultivated, candidate phyla of Bacteria such as BHI80-139, BRC1, JS1, OPB41, and TA06. Hole U1363B and U1363G sediment clone libraries were generally dominated by the domain Bacteria and particularly the phylum Chloroflexi, which comprised approximately one-quarter of the total gene clones identified. However, borehole sediment also contained several archaeal lineages that were phylogenetically affiliated with the Miscellanenous Crenarchaeotal Group. Eukaryotic fungi were only detected within the interstitial water sample from Hole U1363B. Finally, a minor portion of clones recovered from sediment in this study were also recovered from basement fluid samples previously characterized from Baby Bare discharge seamount and Ocean Drilling Program and IODP borehole Circulation Obviation Retrofit Kit Observatories (CORKs) in Holes 1026B and U1301A, which are ~50 km to the north-northeast.