Bacterial profiles in sediments of the eastern flank of the Juan de Fuca Ridge, Sites 1026 and 1027

Bacterial depth profiles were obtained from 45 sediment samples taken from two sites ~100 km east of the crest of the Juan de Fuca Ridge and roughly 80 km east of the nearest extensive basement outcrop using the Acridine Orange direct count (AODC) technique. At Site 1026 bacteria were present at all depths. Total bacterial numbers followed the same general depth trend found at other previously analysed Ocean Drilling Program (ODP) sites. Total counts decreased from near-surface concentrations of 1.47 x 108 to 9.93 x 106 cells cm-3 by 8 mbsf. Below this depth, bacterial numbers remained approximately constant to the deepest depth of ~70 mbsf at 1.07 x 107 cells cm-3. Dividing cells were present in all samples and generally paralleled total cell numbers, averaging 12% of the total population. At Site 1027 significant bacterial cells were present in 29 of the 33 samples. Total bacterial numbers were highest at the near-surface (1.67 x 108 cells cm-3) but decreased rapidly with depth reaching 2.16 x 106 cells cm-3 in the deepest sample at 565.10 mbsf. The bacterial profile in the top ~300 mbsf followed the same general trend observed at other ODP sites. Between ~374 and ~509 mbsf bacterial populations were lower than expected. Dividing cells were present in 21 of the 33 samples analyzed and were highest at the near-surface (1.97 x 107 cells cm-3) where they represented 12% of the total population, below this the numbers paralleled the total count. Dividing cells, however, were absent below 429 mbsf. Two depth zones showed consistently elevated bacterial numbers. The first of these (~190 to ~255 mbsf) was associated with an increase in pore-water ammonia and alkalinity, the second (~528 to ~565 mbsf) was associated with the diffusion of sulfate from the underlying bedrock. As organic carbon concentrations are essentially consistent throughout the bottom 300 m of the hole, the stimulation of bacterial populations in this deeper zone depth is probably due to the sulfate providing a more efficient electron acceptor at depth and this coincided with removal of CH4 and thus anaerobic methane oxidation, which has previously been associated with deep elevation of bacterial populations at a number of ODP sites. In addtion, aspects of the bacterial distributions seemed to indicate the sequential presence of different bacterial temperature groups from the near-surface psychrophilic bacteria to the deeper thermophilic populations