Sub-sea-floor sediments may contain two-thirds of Earth's total
prokaryotic biomass. However, this has its basis in data extrapolation
from ~500-meter to 4-kilometer depths, whereas the deepest documented
prokaryotes are from only 842 meters. Here, we provide evidence
for low concentrations of living prokaryotic cells in the deepest
(1626 meters below the sea floor), oldest (111 million years
old), and potentially hottest (~100°C) marine sediments investigated.
These Newfoundland margin sediments also have DNA sequences
related to thermophilic and/or hyperthermophilic Archaea. These
form two unique clusters within Pyrococcus and Thermococcus
genera, suggesting unknown, uncultured groups are present in
deep, hot, marine sediments (~54° to 100°C). Sequences of anaerobic
methane-oxidizing Archaea were also present, suggesting a deep
biosphere partly supported by methane. These findings demonstrate
that the sub-sea-floor biosphere extends to at least 1600 meters
below the sea floor and probably deeper, given an upper temperature
limit for prokaryotic life of at least 113°C and increasing
thermogenic energy supply with depth.
