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Several strains of a strictly anaerobic, vibrio-shaped or sigmoid, sulfate-reducing bacterium were isolated from deep marine sediments (depth, 80 and 500 m) obtained from the Japan Sea (Ocean Drilling Program Leg 128, site 798B). This bacterium was identified as a member of the genus Desulfovibrio on the basis of the presence of desulfoviridin and characteristic phospholipid fatty acids (is0 17:lw7 and is0 15:0), the small number of growth substrates utilized (lactate, pyruvate, and hydrogen), and 16s rRNA gene sequence analysis data. Based on data for 16s rRNA sequences (1,369 bp), all of the Japan Sea strains were identical to each other and were most closely related to Desulfovibrio salexigens and less closely related to Desulfuvibrio desulfuricans (levels of similarity, 91 and 89.6%, respectively). There were, however, considerable phenotypic differences (in temperatures, pressures, and salinities tolerated, growth substrates, and electron donors) between the Japan Sea isolates and the type strains of previously described desulfovibrios, as well as important differences among the Japan Sea isolates. The Japan Sea isolates were active (with sulfide production) over a wide temperature range (15 to 65°C) and a wide sodium chloride concentration range (0.2 to 10%) (moderate halophile), and they were barophiles that were active at pressures up to about 40 MPa (400 atm). The optimum pressures for activity corresponded to the calculated pressures at the depths from which the organisms were isolated (for isolates obtained at depths of 80 and 500 m the optimum activities occurred at 10 and 15 MPa, respectively [lo0 and 150 atm, respectively]). This confirms that the organisms came from deep sediments and indicates that they are well-adapted for deep sediment conditions, which is consistent with other characteristics (utilization of hydrogen, fermentation, and utilization of ferric iron and organic sulfonates as electron acceptors). We propose that Japan Sea isolate 500-1 is the type strain of a new species, Desulfovibrio profundus.Bacterial sulfate reduction is the dominant anaerobic terminal oxidation process in marine sediments, and in coastal sediments this process can be responsible for more than 50% of the organic matter degradation (21). The importance of sulfate reduction decreases in deeper-water sediments, where the diagenetic zones are greatly extended (16,22), and as a result sulfate can be present to a depth of several hundred meters in some pelagic sediments (4). Thus, there is the potential for active sulfate reduction to continue at great depth within marine sediments. Recently, this was demonstrated for Japan Sea sediments, where sulfate reduction was shown to be present to a depth of at least 425 m together with significant bacterial populations (30). Viable sulfate-reducing bacteria were also present to a depth of 80 m, and other viable bacterial types were present at even greater depths. These findings significantly extend the environment for sulfate-reducing bacteria and are consistent with report...

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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 welldeveloped BSR existed. In near-surface sediments (top ~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 (~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 ~100 m. Even at this depth, the population was substantial, at 1.8 × 10 6 cells mL -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 (~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 2 :CO 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 ~15 mM at 691 mbsf, ~1000 times higher than "typical" near-surface concentrations (2-20 µM). 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-...