Ocean Drilling Program, United States

Bacterial populations and activity were quantified at three sites on the Blake Ridge, Ocean Drilling Program Leg 164, which formed a transect from a point where no bottom-simulating reflector (BSR) was present to an area where a well-developed BSR existed. In near-surface sediments (top approximately 10 mbsf) at Sites 994 and 995, bacterial profiles were similar to previously studied deep-sea sites, with bacterial populations (total and dividing bacteria, viable bacteria, and growth rates [thymidine incorporation]) highest in surface sediments and decreasing exponentially with depth. The presence of methane hydrate was inferred at depth ( approximately 190-450 mbsf) within the sediment at all three sites. Associated with these deposits were high concentrations of free methane beneath the inferred base of the hydrate. Bacteria were present in all samples analyzed, to a maximum of 750 mbsf, extending the previous known limit of the deep biosphere in marine sediments by approximately 100 m. Even at this depth, the population was substantial, at 1.8X10 (super 6) cells mL (super -1) . Bacterial populations and numbers of dividing and divided cells were stimulated significantly below the base of the inferred hydrate zone, which may reflect high concentrations of free gas. Localized increases in bacterial populations within the hydrate stability zone may also have been associated with free gas. Solid methane hydrate, recovered from 331 mbsf at Site 997, contained only 2% of the predicted bacterial population in a sediment from this depth, suggesting reduced bacterial populations in solid hydrate. Bacterial activity in near-surface sediments was dominated by sulfate reduction. Sulfate reduction rates and pore-water sulfate decreased rapidly with depth, concomitant with an accumulation of solid-phase sulfide in the sediment. Once sulfate was depleted ( approximately 20-30 mbsf), methane concentrations, methanogenesis, and methane oxidation all increased. Below 100 mbsf, bacterial processes occurred at very low rates. However, bacterial activity increased sharply around 450 mbsf, associated with the base of the inferred hydrate zone and the free-gas zone beneath; anaerobic methane oxidation, methanogenesis from both acetate and H (sub 2) :CO (sub 2) , acetate oxidation, sulfate reduction, and bacterial productivity were all stimulated (from 1.5 to 15 times), demonstrating that the sediments near and below the BSR form a biogeochemically dynamic zone, with carbon cycling occurring through methane, acetate, and carbon dioxide. At Site 995, pore-water acetate was present in surprisingly high concentrations, reaching approximately 15 mM at 691 mbsf, approximately 1000 times higher than "typical" near-surface concentrations (2-20 mu M). Potential rates of acetate metabolism were extremely high and could not be sustained without influx of organic carbon into the sediment; hence in situ rates are likely to be lower than these potential rate measurements. However, there is evidence for upward migration of high concentrations of dissolved organic carbon into the sediments at these sites. Rates of acetate methanogenesis below the BSR were 2-3 orders of magnitude higher than H (sub 2) :CO (sub 2) methanogenesis and were associated with extremely high quantities of free gas. Methane oxidation rates at the base of the hydrate zone at Site 995 were 10 times greater than H (sub 2) :CO (sub 2) methanogenesis. However, acetate methanogenesis at Site 995 exceeded methane oxidation through and below the BSR, potentially providing an unexpected source of methane gas for the formation of hydrates. These results confirm and extend previous results from Cascadia Margin, demonstrating that gas hydrate-containing sediments provide a unique deep bacterial habitat in marine sediments.