Prokaryotic microorganisms are now known to exist in many hot environments. Some of these hot microbial habitats are associated with volcanic sources of heat, such as terrestrial (Brock, 1985) and shallow marine hot springs (Hafenbradl et al., 1996) along with deep-sea hydrothermal vents (Baross and Deming, 1995), and some are hot because of the deep nature of the environment, such as deep terrestrial geological formations (Boone et al., 1995) and oil wells (Grassia et al., 1996). The proposed upper temperature limit for life varies from 120° to 150°C or higher (Stetter et al., 1990; Daniel, 1992; Segerer et al., 1993), and the extent of the deep biosphere is often drawn along that nebulous temperature contour under the assumption that life will exist in all locations within its physical limits (e.g., Whitman et al., 1998).
In contrast to terrestrial hot springs where microbes are obviously thriving, in many hot subsurface environments it is difficult to determine the level of microbial activity (Kieft and Phelps, 1997). For instance, whereas microorganisms have certainly been identified in deep, hot marine sediments (Cragg and Parkes, 1994), it is not possible to determine from the direct microscopic counts whether the microbes are active, dormant, or no longer viable, as fluorescent stains can still bind to intact dead cells (Kepner and Pratt, 1994). It is possible that, though the upper temperature limit of life is currently 120°C or above, the practical limit of activity for a buried microbial community is significantly lower.
If buried microbial communities remain active at high temperatures, the community composition must change with temperature. Buried microbial communities are subjected to ever-increasing temperatures, but the temperature range of any given microorganism is finite. In general, microbes have a temperature range of 30°-50°C, and thus each spans only a subset of the temperatures possible for life; this range appears to be dictated by the underlying biochemistry of the organism (Brock et al., 1994). High-temperature microbial communities studied to date are therefore composed of very different organisms than their low-temperature counterparts; this generalization should hold true in deep environments as well. As most sedimentary geological formations are emplaced at 2°-30°C and can heat up to over 150°C, if the community of microbes seen at a temperature of 80°C (well within the limits of life as we know it) is to be active, it cannot consist of the same microbes that were most active soon after the time of deposition. Microbial succession must take place, replacing a low-temperature microbial community with one that is adapted to high temperatures.
This process of burial and heating is occurring at Middle Valley, Juan de Fuca Ridge, but on a more accessible spatial scale than in most geological environments. Middle Valley is a sedimented ridge off the coast of British Columbia, Canada; young, hot oceanic crust is covered by 200-1000 m of clays and turbiditic silts (Shipboard Scientific Party, 1998). Seawater that has been hydrothermally heated and altered penetrates up through the sediments, discharging locally with temperatures around 270°C (Butterfield et al., 1994). Near these venting locations thermal gradients within the sediments are extremely steep (3°-12°C/m) (Shipboard Scientific Party, 1998), much greater than average geothermal gradients in deep-sea sediments or continental systems that are closer to 0.01°-0.1°C/m (Garland, 1971). Previous sampling at Middle Valley showed evidence of microbial communities within these hot sediments throughout the known temperature range of life and beyond (Cragg and Parkes, 1994), but the species composition of these communities is unknown.
We sampled sediment cores taken on Ocean Drilling Program Leg 169 from Site 1036, the Dead Dog vent field in Middle Valley, to confirm the report of microorganisms in very hot sediments and to determine the composition of the microbial communities inhabiting these hot sediments. Samples were taken from warm (50°-70°C) and hot (90°-130°C) sections of core, and phospholipid analyses were performed as well as standard Acridine orange direct counts to estimate microbial biomass, community composition, and temperature-driven microbial succession.