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doi:10.2204/iodp.proc.314315316.134.2009

Microbiology and biogeochemistry

Sample processing

To study microbiological and biogeochemical characteristics in sediments at Site C0006, samples were obtained from 143 different depth locations (Table T18). Almost all sample processing was carried out after X-ray CT scanning and was completed within 1 h after core recovery on deck.

Cell abundance

Using paraformaldehyde-fixed slurry samples, preliminary microbial cell abundances were enumerated by visual inspection using the SYBR Green I staining method (Lunau et al., 2005, see “Microbiology and biogeochemistry” in the “Expedition 316 methods” chapter). At Site C0006, ~10⁹ cells/cm³ were detected in the upper lithologic unit above 100 m CSF. An apparent increase of microbial biomass was observed in sand layers, possibly suggesting the occurrence of energy supply via fluid circulation. Below the upper lithologic unit, microbial population decreased with depth in the accretionary prism, which is in good agreement with typical cell abundance tendencies that have been observed in global marine subsurface sediments (Parkes et al., 2000). In the accretionary prism, no or very small proliferation of cell abundance was observed in sand layers and at the Unit II/III boundary (Fig. F46). Most cells were tiny coccoids, but rod-shaped cells were also observed as minor components (Fig. F47). Generally, the microbial population in the accretionary prism at Site C0006 was found to be low (10⁷ cells/cm³), which is in clear contrast to the constant and abundant microbial population (10⁹ cells/cm³) at Site C0004. Average cell abundance in sediments at Site C0006 was estimated to be 6.19 × 10⁸ ± 2.27 × 10⁸ cells/cm³ (N = 27).

Given the cell abundance profile at Site C0006, it is hypothesized that the microbial population size may indicate the energy status of the subsurface. The microbial population generally decreases with increasing depth because of the limited availability of nutrient and energy sources that support microbial growth activity. If the decreasing population ratio indicates the balance point between community size and life maintenance energy (Hoehler, 2004), the accretionary prism at Site C0006 may be a harsh and energy-starved habitat for subseafloor life. It is a clear contrast to the accretionary prism at Site C0004, where relatively high biomass is present throughout the cored materials (Fig. F46). In addition, it is worth noting that the energy flux does not always indicate flux or hydrological movement of interstitial water; dehydration of smectite or physical compaction may induce water flux along faults and/or fractures, but the fluids do not always contain the energy (nutrient) substrates required for microbial life. On the other hand, porous sand layers in the upper unit harbor abundant microbial populations, suggesting that the environment is rich in available energy sources and habitable space. The potential energy sources in the sand layers are probably buried consumable organic matter (e.g., organic acids) and/or substrates derived from fluid circulation.

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