Bacterial distributions were determined at Site 1149 in the Izu-Bonin Trench, which is a deepwater (5818 m) low-sedimentation-rate area. This was the first Ocean Drilling Program (ODP) leg where contamination checks were conducted for microbiology. These demonstrated that the inner portion of cores, where the microbiological samples were taken, were free from any potential sampling contamination. Bacterial populations were present in all samples (deepest at 171.2 meters below seafloor [mbsf]). The highest numbers were near the surface (1.4 mbsf; 7.2 x 106 cells/cm3), but these declined rapidly within the upper 10 mbsf. Below this, numbers decreased at a more gradual rate to 7.2 x 105 cells/cm3 at 172 mbsf, a 10-fold reduction. This two-stage bacterial depth distribution has been observed at several other ODP sites (e.g., Amazon Fan and Santa Barbara Basin). Bacterial depth distributions at this site were well below those predicted by the general equation for global deep-sediment bacteria and predominantly below the lower 95% prediction limits. These low bacterial populations were thought to reflect the low sedimentation rates and low input of bioavailable organic matter that is characteristic for deepwater sites. Consistent with this trend is that there was only limited removal of pore water sulfate and, thus, bacterial sulfate reduction. Most of this removal was in the top ~5 mbsf, coinciding with the highest bacterial populations, the presence of small amounts of methane, and an increase in pore water manganese and ammonia. In the deeper sediments, however, there was still indirect evidence of continuing low bacterial activity, with increases in pore water ammonia, soluble manganese, bioavailable acetate, and decreasing sulfate. Interestingly, manganese reduction, sulfate reduction, and a limited amount of methanogenesis seemed to be occurring simultaneously at depth in this low-organic matter site, and the coincident detection of bacterial populations provides further support for the universal presence of a deep biosphere in marine sediments.
1Cragg, B.A., Wellsbury, P., Murray, R.W., and Parkes, R.J., 2003. Bacterial populations in deepwater low-sedimentation-rate marine sediments and evidence for subsurface bacterial manganese reduction (ODP Site 1149, Izu-Bonin Trench). In Ludden, J.N., Plank, T., and Escutia, C. (Eds.), Proc. ODP, Sci. Results, 185 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/185_SR/008/008.htm>. [Cited YYYY-MM-DD]
2Department of Earth Sciences, University of Bristol, Queens Road, Bristol, BS8 1RJ, United Kingdom. Correspondence author: j.parkes@bristol.ac.uk
3Department of Earth Sciences, 685 Commonwealth Avenue, Boston University, Boston MA 02215, USA.
Initial receipt: 25 October 2001
Acceptance: 17 January 2003
Web publication:
10 December 2003
Ms 185SR-008