Extremophiles

While we here in the US honor our workers today on Labor Day, I thought it would be appropriate to honor some of biology's hardest workers. These organisms didn't take the easy way out, signing up for a life at pH 7 and a nice cozy 30°C with two weeks of paid vacation a year, but instead took the path less traveled and choose a life at the extremes of environmental possibility on Earth. We call them extremophiles, and today I'll list a few of the currently known record holders.

Temperature: As humans we can probably sympathize with the difficulties of surviving in temperature extremes better than any other environmental condition.

Category: High temperature
Term: Hyperthermophile
Species (domain): Pyrolobus fumarii (Archaea)
Habitat: Undersea hydrothermal vents
Minimum tolerated: 90°C (194°F)
Optimum conditions: 106°C (223°F)
Maximum tolerated: 113°C (235°F)
Notes: This archaean's optimum temperature for growth is above the boiling point of water at sea level. However, since this critter lives at the bottom of the ocean, the high water pressures prevent water from boiling, and thus the archaean has liquid water to live in. Next time your neighbor says that the 86°F day (30°C) is "roasting", remind them that Pyrolobus fumarii can survive, and actually requires, temperatures 60°C warmer. I've read (but can't find the source now) that an hour in a typical autoclave will not kill this archaean (and actually, wouldn't an hour outside of an autoclave be worse?)

Category: Low temperature
Term: Psychrophile
Species (domain): Polaromonas vacuolata (Bacteria)
Habitat: Sea ice
Minimum tolerated: 0°C (32°F)
Optimum conditions: 4°C (39°F)
Maximum tolerated: 12°C (54°F)
Notes: I suspect that the minimum temperature for this bacterium may be lower, as I have seen references to polar fish that can survive in water temperatures down to -1.86°C. If polar fish can do it, bacteria can probably do it too.

pH:Acidity and alkalinity do horrible things to biological molecules. Human extracellular fluids (e.g. blood) are typically maintained at pH 7.4, and death usually results if they vary by more than a pH unit ("The pH range of 6.8 to 7.8 in the extracellular fluid is generally compatible with life," reports Berne et al.'s human physiology text).

Category: Low pH
Term: Acidophile
Species (domain): Picrophilus oshimae (Archaea)
Habitat: Acidic hotsprings
Minimum tolerated: -0.06 (I didn't even know pH could be negative)
Optimum conditions: 0.7
Maximum tolerated: 4
Notes: This archaean also grows optimally at 60°C, so it's a thermophile too. In case you've forgotten your pH scale, this arcahean's optimum pH is approximately one million times more acidic than our body's minimum survivable pH.

Category: High pH
Term: Alkaliphile
Species (domain): Natronobacterium gregoryi (Archaea)
Habitat: Soda lakes (no, they're not the oft-dreamed-of lakes full of Mountain Dew)
Minimum tolerated: 8.5
Optimum conditions: 10
Maximum tolerated: 12
Notes: Not happy to be just the record-holding alkaliphile, Natronobacterium gregoryi prefers to grow in 20% NaCl, making it a halophile.

Other:

Category: Pressure
Term: Barophile
Species (domain): MT41 (Mariana Trench-41; Bacteria)
Habitat: Deep ocean sediments
Minimum tolerated: 500 atm
Optimum conditions: 700 atm
Maximum tolerated: >1000 atm
Notes: According to my source this species does not have a formal name yet; it also likes to grow at 10°C, so it's a psychrophile too (it does get awfully cold at the bottom of the ocean).

Category: Salt (NaCl)
Term: Halophile
Species (domain): Halobacterium salinarum (Archaea)
Habitat: Salterns (salt-producing facilities)
Minimum tolerated: 15%
Optimum conditions: 25%
Maximum tolerated: 32% (a saturated solution)
Notes: This organism's maximum is unbeatable, since it's hard to get saltier than a saturated solution. For a comparison, ocean saltwater is approximately 3% NaCl, so Halobacterium salinarum's optimum growth conditions are eight times saltier than the ocean. Humans can't even drink seawater for an extended period of time and live to tell the tale.

It's interesting to note that all these record holders are prokaryotes (Bacteria or Archaea); we eukaryotes are just not very good at this extremophile thing.

References:

Berne, R., M. Levy, B. Koeppen, and B. Stanton. 1998. Physiology. 4th edition. Mosby, MO.

Madigan, M., J. Martinko, and J. Parker. 2003. Brock Biology of Microorganisms. 10th edition. Prentice Hall, NJ.