The optimum temperature for cell growth is related to the temperature stability of critical macromolecules, such as nucleic acids and proteins, and to how temperature variations affect enzymatic actions necessary to synthesize new cells. Most cell types and organisms are mesophiles: suited to 25–45 °C.
Multicellular animals and plants cannot tolerate an ambient temperature exceeding 50 °C. Above that, only microbes can thrive.
A number of bacteria are happy in hot water, at 85 °C, and some even grow in boiling water, as hot as 100 °C. These high-temperature-tolerant microbes are thermophiles. The distinctive feature of thermophiles is that their enzymes remain stable at high temperatures.
Enzymes commonly become sluggish when it gets cold. But psychrophilic microbes like the chill. (Psychros is the Greek word for “cold.”)
Microbes are alive and well at the poles, in regions permanently frozen, and at near-freezing ocean depths: 0–2 °C. The algae Phormidium frigidum call the bottom of ice-covered lakes in Antarctica home.
Lousy cellular osmotic pressure is a killer. To stay alive, cell walls exchange gases and fluids for energy intake and waste disposal.
If the internal pressure inside a cell is too low, water will seep in, causing swelling, until the cell bloats to death by bursting. Conversely, dehydration results if water can’t get in because osmotic pressure is too high.
Because of this, most microbes make a living in locations with low concentrations of salts and nutrient molecules. Whence the practice of preserving foods by salting or adding copious amounts of sugar.
Some microbes can take the pressure: osmophiles. There are microbial communities at the bottom of the Mariana Trench: 11 kilometers down; the deepest seafloor on Earth. The pressure there is 1,000 times that of sea level.
Some microbes like salt. Halophiles (halos is Greek for salt) require a salty home, needing at least at a 10% salt concentration, with some archaea tolerating up to 37%. There are both archaea and bacterial halophiles.
The Dead Sea in Israel is too salty for fish, and long thought devoid of all life, but haloarchaea love it there. If one finds itself in a liquid less than 10% salt, it disintegrates, as its cell wall falls apart.
Deep Lake in Antarctica is so salty that it stays liquid at minus 20 °C, remaining ice-free throughout the year. In the frigid salt sludge thrive 4 dominant genera of archaea that maintain their distinctiveness while sharing genes among themselves. These 4 comprise 72% of the population in this low-diversity lake ecosystem.
Liberating hydrogen atoms in solution can be a serious problem. Few life forms tolerate the acidic environment that results. Most microbes prefer a pH of 6 to 8 and cannot stand pH as low as 3 or 4. Acidification is the basis for pickling food to preserve it.
Acidophiles can easily take 1.5 pH. The archaeon Picrophilus oshimae grows best at 0.7 pH. Acidophiles maintain an internal pH of 6–7 by constantly pumping out hydrogen ions from their cell envelopes.
Thermoacidophiles prosper at scorching temperature and low pH. The archaeon Thermoplasma acidophilum lives in self-heating coal refuse piles and other such nasty places.
At the other end of the pH scale, some microbes keep the best life sign when the situation is highly alkaline. Alkaliphilic photosynthetic bacteria inhabit alkaline soda lakes in East Africa that have a pH of 11.