Bacterial profiles in hydrothermally active deep sediment layers from Middle Valley (NE Pacific), Sites 857 and 858

Cragg, Barry Andrew and Parkes, Ronald John 1994. Bacterial profiles in hydrothermally active deep sediment layers from Middle Valley (NE Pacific), Sites 857 and 858. Proceedings of the Ocean Drilling Program Scientific Results 139 , pp. 509-516. 10.2973/odp.proc.sr.139.236.1994

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Abstract

Sediment samples (2 cm3 each) were obtained from Ocean Drilling Program Leg 139 Sites 857 and 858 in the Middle Valley of the northern Juan de Fuca Ridge for direct microscopic determination of bacterial depth distributions in a region influenced by hydrothermal activity. Site 857 (Holes 857A and 857B) was 1.6 km away from Site 858, a hydrothermal venting area, but had a similar regional structural setting and thus provided a reference site. Hydrothermal samples were obtained from four holes drilled in a transect into the active vent field at Site 858. Temperatures increased from Hole 858A to 858C to 858B to 858D with approximate temperatures at 10 meters below seafloor (mbsf) of 20°, 33°, 100°, and 105°C, respectively, compared with approximately 8°C at 10 mbsf in Holes 857 A and B. At Site 857 substantial bacteria populations, including dividing cells, were present in all samples (deepest sample 107.5 mbsf) and there were no significant differences between data from the two holes approximately 10 m apart. This implies that there is considerable microbial homogeneity between these spatially close areas of the seafloor, and thus the data were combined. Bacterial numbers were highest, 1.73 × 107 cells/cm3, in the near-surface samples at 1.51 mbsf and decreased linearly with increasing depth to 1.07 × 106 cells/cm3 at 107.5 mbsf, a 16-fold decrease. This distribution is similar to those at other sites previously studied, despite the low organic carbon concentration (0.56%) compared to these other sites. In contrast, bacterial distributions at the four holes of the hydrothermal upwelling site differed significantly and could not be combined. Although near-surface bacterial populations were similar to those at the reference site at 1.51 mbsf, 2.90 × 106 cells/cm3 to 1.58 × 10 cells/cm3, bacterial populations decreased rapidly with depth; the deepest sample with significant populations was 64 mbsf. The higher temperature gradients at this site may limit the bacterial depth distribution to a much greater extent than at Site 857, although both sites have a similar low organic carbon content. Average total organic carbon in the top 5 m was 0.56% at Site 857 and 0.48% at Site 858. However, due to the steep thermal gradients encountered at Site 858 (1.7 to 1 l°C/m), bacteria even at shallow depths appear to be adapted to high temperatures and hence may be considered thermophilic. Depth profiles of bacterial populations at the vent sites are related to chlorinity changes, which reflect the influence of vent fluid. At the two "hot" holes (858B and 858D) which were influenced by near-surface lateral fluid flow, there was a significant (P < 0.001) subsurface peak in the bacterial population associated with increases in chlorinity and a temperature increase to 169°C. The presence of these bacterial populations was difficult to attribute to contamination during drilling from above, or cooling artifacts, and thus may represent uniquely high-temperature-adapted (hyperthermophilic) bacteria.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Subjects:	Q Science > QE Geology Q Science > QR Microbiology
Uncontrolled Keywords:	deep microbiology, high temperature, hyperthermophiles
Publisher:	Ocean Drilling Program
ISSN:	1096-7451
Last Modified:	12 Jun 2019 02:19
URI:	https://orca.cardiff.ac.uk/id/eprint/8690

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