No, don’t be ridiculous
Well, eggs don’t. I am 3 minute man (I said eggs, not ….), so just enough time to prepare the toast “soldiers” and shout at whoever is on the radio. Distracted by the announcement of yet another example of mendacity and duplicity by the EU, I trip: the scalding water just misses the dog, but not my bare feet. Goodness what language. Boiling water is for eggs and sterilizing medical instruments, and any living cell incautious enough to enter this lethal zone faces a foregone conclusion.
Yes, of course
“Comfortable, sir? The temperature will increase gradually and just give a wave if you begin to fry. Off we go!”. Well, let’s stick to bacteria and even if they cannot wave, surely they will show signs of distress? Watch the thermometer climb, now it is passing 80oC. This is far past the thermal limit of any eukaryote (and the scalding blast of the bombardier beetle is the exception that proves the rule). How curious, the bacteria don’t seem to have noticed. If anything they are a trifle sluggish. OK! Let’s show them. Cranking the thermostat to boiling, we wait for the familiar smell of over-done bacteria. Nope. The bacteria show no ill-effects at all. To reach temperatures above 100oC you have to pressurize the system, using an inert gas like argon. In these extreme conditions ultimately life must fail, but even at an eye-watering 122oC a methanogenic bacteria is metabolizing and capable of cell-division. Welcome to the hyperthermophiles.
It all depends on the question
How on earth can you flourish in such conditions? At such temperatures normally proteins denature, after all that’s the point of boiling my egg. Not so with the hyperthermophiles. When it comes to clever tricks to stabilize proteins at what to us would be searing temperatures, then as the biochemist Gregory Petsko remarked “There is more than one way to skin a cat”. With no single principle and a litter of exceptions, one might suspect one is dealing more with the Cheshire Cat, but some guide-lines exist. Proteins are composed of amino-acids that are intricately folded. So one approach is to build linkages, such as so-called salt bridges (notably between glutamic acid and lysine, where hydrogen bonding and electrostatic attraction combine) or disulphide bonds (where sulphur-bearing cysteins interact). Alternatively stability may be engendered by opting for hydrophobic amino acids (such as leucine). Collectively these strategies allow the cell to bask in what would otherwise be killer temperatures.
Hyperthermophiles provoke us to wonder what the real limits of life might be. Temperature is only one factor, but other extremophiles show remarkable resilience to salinity, acidity (and in the oppose direction alkalinity), pressure and even radioactivity. As often as not the challenges come in multiples, such as living several kilometers deep in the Earth’s crust where pressure and the geothermal gradient combine to make a distinctly challenging environment. Such life forms are a focus for astrobiology (cynically defined as the study of things that do not exist) because environments on most alien biospheres are probably far more robust than dear old Earth. On the other hand one can make a case that terrestrial extremophiles have reached the absolute limits of existence of life anywhere in the Universe. Or have they? A common protein (albumin) can be persuaded to act as an enzyme (if you really want to know, it is an esterase) to at least 160oC, although whether any cell could function at this extreme is doubtful.
Or again, maybe not. Consider the “shadow biosphere”, that is the tantalizing prospect of completely alternative life forms, here on Earth. Perhaps surviving at temperatures and pressures beyond our imagination or finding niches such as super-arid deserts, it is an intriguing, but unproven, concept. Yet even if the shadow biosphere remains silent, when it comes to history hyperthermophiles have extremely long memories. Early Earth was extremely hot, not least when colossal impacts released so much kinetic energy as to evaporate most of the ocean. A tough time for early life, even if repopulation was from Mars, and it is probably no coincidence that even though thermophily has been invented several times, the most primitive bacteria seem to have been very heat-tolerant. As the planet cooled so these bacteria retreated to hydrothermal vents and hot springs. But in the end they will have the last laugh. Let’s fast forward a billion years. Hanging in the sky, immense and red is our Sun, bloated with spent nuclear fuels. Eukaryotes have long gone, doomed when surface temperatures climbed towards 60oC. But the Earth is caught in an irreversible thermal trap, and in the ever-shrinking oceans the last hyperthermophiles bring to a muted finale the six billion year experiment that we called the history of life.
Text copyright © 2015 Simon Conway Morris. All rights reserved.
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