*School of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA
†Chesapeake Bay Institute, Johns Hopkins University, 4800 Atwell Road, Shadyside, Maryland 20764, USA
Complex communities of thermophilic bacteria have been cultured from the 350 °C waters emanating from sulphide chimneys, or ‘black smokers’, at 21 °N along the East Pacific Rise1. Several of the bacterial communities were shown to grow rapidly at 100 °C and atmospheric pressure, producing methane, hydrogen gas and carbon monoxide. These gases are found in superheated vent water, having previously been attributed to abiogenic reactions. Before concluding that these ‘black smoker’ bacteria actually contribute to the chemistry of the superheated hydrothermal fluids, it was necessary to test their ability to grow and produce gases at in situ vent temperatures and pressures. Here we report that a bacterial community originally cultured from 306 °C water is capable of chemolithotrophic growth in a titanium growth chamber under in situ vent pressure of 265 atm and at temperatures of at least 250 °C. (At 265 atm, seawater remains liquid at temperatures of at least 460 °C2.) Transmission electron microscopy of thin sections of bacteria cultured at 250 °C has revealed the presence of at least two morphologically distinct organisms.

Abstract
The upper temperature at which a living system can exist is limited by the hydrolytic breakdown rate of its chemical constituents. The peptide bonds of proteins, the phosphodiester and N-glycosyl bonds in RNA and DNA, and the pyrophosphate and N-glycosyl bonds in nucleotides such as ATP and NAD are among the more important bonds that will undergo hydrolysis. The decomposition of biomolecules via non-hydrolytic pathways such as decarboxylations and dehydrations may also be critical factors in determining this upper temperature limit. Baross and Deming recently reported 'black smoker' bacteria, which they isolated from deep-sea hydrothermal vents, growing at 250 degrees C. Here I have attempted to establish the rates for the hydrolysis and/or decomposition of critical biomolecules to determine their ability to exist at this temperature. My results clearly indicate that if these organisms exist, and if their metabolic reactions occur in an aqueous environment, they could not survive at this temperature if they were composed of biomolecules such as proteins and nucleic acids, due to the very rapid rate of decomposition of such molecules.

Abstract
Baross and Deming reported that thermophilic marine bacteria isolated from the vicinity of a submarine hot-spring grow at temperatures up to at least 250 degrees C. They did not, however, conduct the appropriate control experiments to eliminate the possibilities of chemical artefacts or contamination. Here, in experiments using the same growth medium, the same temperature and pressure apparatus and the same sampling and analytical procedures, we report results nearly identical to theirs. We conclude that their evidence indicating bacterial growth at 250 degrees C may be due to artefacts produced in the medium and to contaminants introduced during sample processing.