Limits of Microbial Existence: Temperature and p H

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Abstract
A microscopic survey made to detect the presence of bacteria in hot springs of varying temperature and pH characteristics revealed that in neutral and alkaline hot springs bacteria are found at temperatures up to the boiling point of water (92° to 100°C, depending on the altitude). In hot springs of increasing acidity the upper temperature limit at which bacteria are found decreases; at pH 2 to 3 the upper temperature limit is 75° to 80°C. Bacteria have thus been able to evolve with the ability to grow at either high temperature or high acidity, but not at both high temperature and high acidity. These results suggest that there are physicochemical limitations of the environment beyond which life is impossible.