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doi:10.2204/iodp.proc.336.201.2013

Results and conclusions

In agreement with published data for other oligotrophic sediments (Kallmeyer et al., 2012), our data show overall low cell numbers decreasing with sediment depth (Fig. F3). An average ~1–2 orders of magnitude higher abundance of Archaea than of Bacteria was found, in contrast to the previously studied, rather organic carbon–rich sediments with a much higher qPCR abundance of Bacteria (Schippers et al., 2005, 2012). The near-surface sediment samples up to 15 cm depth showed mean values (and standard deviation) of 2.2 × 10⁸ (1.9 × 10⁸) cells/mL for Archaea and 1.5 × 10⁷ (1.8 × 10⁷) cells/mL for Bacteria. These qPCR data reflect the dominance of Archaea even at the sediment surface. As an explanation for their dominance, Archaea are likely better adapted to low energy flux (Valentine, 2007) and therefore have an advantage over Bacteria in oligotrophic sediments in contrast to eutrophic sediments (Schippers et al., 2005, 2012). Another explanation is the deposition of archaeal cells from the water column and their preservation in the sediment. Karner et al. (2001) counted pelagic cells of Crenarchaeota, Euryarchaeota, and Bacteria by fluorescence in situ hybridization in the open ocean up to 5000 m water depth (station in North Pacific subtropical gyre). In their results, the fraction of Crenarchaeota relative to total DNA containing prokaryotes equaled or exceeded the bacterial fraction below 1000 m. Further work on the composition of the microbial communities should reveal their origin and function in the oligotrophic subsurface sediments.

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