Bacterial profiles in a sulfide mound (Site 1035) and an area of active fluid venting (Site 1036) in hot hydrothermal sediments from Middle Valley (northwest Pacific)

Sediment samples (1 cm3 each) were obtained from two sites (Ocean Drilling Program [ODP] Sites 1035 and 1036) 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. These data were compared to data gathered during Leg 139, Site 858, at the same location. Site 1035 was cored to 170 meters below seafloor (mbsf), and significant numbers of bacterial cells were detected in most samples with 4 x 105 cells/cm3 at the base of the hole. Dividing and divided cells were only found above 64 mbsf. The temperature at the base of the hole was estimated at ~113°C—the current estimated upper temperature limit for bacteria. When the data were divided according to growth-temperature ranges of bacteria (mesophile = 10° -45°C, thermophile = 45° -80°C, and hyperthermophile = >80°C), the bacterial profile clearly displayed three bands of bacterial populations. Only populations in the upper mesophilic section of the hole agreed with a general population profile obtained from many other ODP legs. At higher temperatures bacterial populations were markedly lower than this general profile. Samples from Holes 1036A, 1036B, and 1036C all showed reduced populations when compared to the general profile. The deepest reliable enumeration was at 30 mbsf in Hole 1036B with 5 x 105 cells/cm3. Bacteriological sampling from Hole 1036C stopped before very high temperatures were encountered in the borehole; however, at Holes 1036A and 1034B, samples from temperatures apparently >200°C were obtained. Bacterial populations decreased rapidly from the surface and became nondetectable at ~110°C, but, at ~155° -185°C, intact cells were observed. This was similar to data from Site 858, where intact bacterial cells were also detected in this temperature range. Analysis of geochemical data suggested, however, that the reasons for the presence of these cells may be different. For Site 858, bacterial cells could be explained by a constrained lateral flow of entrained seawater carrying cells from shallower sediments down into the hot sediments containing hydrothermal fluids. This was not the case at Site 1036, where rapid seawater recharge occurred throughout the depths where bacteria were detected, which may have distorted, and significantly reduced, the assumed temperature profile. These populations appear to exist in a thermophilic/hyperthermophilic environment at the edge of hydrothermal sediment layers. There was some chemical evidence of in situ bacterial activity and they may be utilizing the products of hydrothermal alteration (e.g., methane) rising up from deeper sediment layers