Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.

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Glycerol dialkyl glycerol tetraethers (GDGTs) are core membrane lipids of the Crenarchaeota. The structurally unusual GDGT crenarchaeol has been proposed as a taxonomically specific biomarker for the marine planktonic group I archaea. It is found ubiquitously in the marine water column and in sediments. In this work, samples of microbial community biomass were obtained from several alkaline and neutral-pH hot springs in Nevada, United States. Lipid extracts of these samples were analyzed by high-performance liquid chromatography-mass spectrometry and by gas chromatography-mass spectrometry. Each sample contained GDGTs, and among these compounds was crenarchaeol. The distribution of archaeal lipids in Nevada hot springs did not appear to correlate with temperature, as has been observed in the marine environment. Instead, a significant correlation with the concentration of bicarbonate was observed. Archaeal DNA was analyzed by denaturing gradient gel electrophoresis. All samples contained 16S rRNA gene sequences which were more strongly related to thermophilic crenarchaeota than to Cenarchaeum symbiosum, a marine nonthermophilic crenarchaeon. The occurrence of crenarchaeol in environments containing sequences affiliated with thermophilic crenarchaeota suggests a wide phenotypic distribution of this compound. The results also indicate that crenarchaeol can no longer be considered an exclusive biomarker for marine species.The western United States has a long geological history of volcanic activity caused by the collision of the Pacific and North American tectonic plates. Associated with this volcanism are convective hydrothermal systems, which are manifested as surficial hot springs (19). Meteoric water enters the subsurface hydrothermal system along boundary faults and reaches about 200 to 280°C (6, 17) before coming up again along vertical fractures. Temperatures in these springs range from 20°C to Ͼ100°C at depth; the majority, however, have temperatures below 100°C (14). Typical features of the hot springs include dense communities of floating microbial mats; biodiversity within the mats is high, and in particular, the archaeal communities appear to be dominated by members of the crenarchaeota.Cultivated and uncharacterized crenarchaeota synthesize a wide variety of glycerol dialkyl glycerol tetraether (GDGT) core membrane lipids (11,12,39). In addition to the stability afforded by the ether linkages and by the tetraether (monolayer) structure, the presence of cyclopentane rings in the C 40 isoprenoid backbone has been shown to increase the thermal tolerance of membranes (16). Indeed, both in culture studies and in the marine system, the average number of rings correlates positively with temperature (40, 42, 50).The molecular structure of crenarchaeol represents an unusual exception to this relationship because crenarchaeol contains five rings. However, unique among archaeal lipids, it has one C 40 isoprenoid containing two cyclopentane rings (C 40:2 ) and one C 40 isoprenoid containing two cyclopentane rings pl...