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Lewis Falls in the Fall time...

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Extremophiles are extremely small organisms that thrive in conditions we would describe as extreme. In Yellowstone these conditions can include extreme heat and an altered pH—either very low pH (acidic) or very high pH (alkaline), or a combination of heat and altered pH. The environments of extremophiles would kill other organisms. However, if their environment would change to conditions that we would consider more hospitable, the extremophiles would perish. Extremophiles are a type of microorganism, a living thing so small that it cannot be seen without a microscope. However, colonies, or large groups of microorganisms living together, are visible in many of Yellowstone’s thermal features.
Extremophiles in Yellowstone fall into two categories:

  • Thermophiles are microorganisms that live in extremely hot environments..
    • Thermophiles thrive in temperatures higher than 113° F (45° C).
    • Hyperthermophiles thrive in temperatures between 176° F (80° C) and 235° F (113° C).
  • There are also extremophiles that not only require hot conditions, but also thrive in an altered pH environment.
    • Acidophiles thrive in environments with a pH level of 1 to 5. So the acidophiles in Yellowstone are actually thermoacidophiles—they thrive in both hot and acidic environments.
    • Alkaliphiles live and grow in environments with a pH level of 9 to 11.

To review what these measurements mean, pH is a scale of acidity and alkalinity ranging from 1 to 14, with a pH of 7 indicating neutral, neither acidic nor alkaline. Any pH below 7 is considered acidic, anything above is alkaline, or basic. A substance with a pH close to 1 would be an extremely strong acid; a pH close to 14 would be a extremely strong base.

To better appreciate the impact of pH, it may be helpful to compare the conditions in Yellowstone’s thermal features to those tolerated by humans. In the human body the normal pH is 7.35 to 7.45. A pH above 7.8 or below 6.8 is generally fatal. Our pH may vary slightly from the range of 7.35 to 7.45 during the day, but only by very minute amounts.

Other extremophiles found throughout the world includes:

  • Barophiles microorganisms survive under great pressure.
  • Halophiles are aerobic (uses oxygen) microorganism that live and grow in high saline/salty environment.
  • Psychrophiles thrive in extreme cold. They grow best in environments of about 14° to 68° F (-10° to 20° C).
  • Xerophiles live in extremely dry environments.
“Normal” microorganisms are called mesophiles. They grow best in temperatures between 50° to 122° F (10° to 50° C).


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