Hyperthermophilic microbes often have close relatives which live at "normal temperatures." The evolutionary transition has been made in both directions by multiple families of archaea and bacteria. There is now enough structural information about closely related proteins to make generalizations about what is needed to keep functioning in kochwasser.
In this week's PNAS has an open access article about the physics and evolution of individual proteins retaining their ancestral functions in a new high temperature environment.What is perhaps not suprising is that different proteins manage in different ways. The two most commonly observed answers-- taking a page from my dad's repairs around the farm-- could be classified as "staple gun" or "ten penny nail." That is, either hot proteins are much more tightly compacted, with no single substitution apparently critical; or they are stabilized, without compaction, by a few major changes in the protein sequence that evidently nail everything together. What is interesting from the paper is that these strategies occur in isolation.
The scientists were able to see a trend in the strategies used and relate it to the evolutionary history of hyperthermophiles. Bacterial hyperthermophile proteins tend to use the sequence change (ten-penny nail) method, wherease archaea used the structural(staple gun) method. To quote from the summary:
We attribute such differences to the vastly different phylogenetic histories of these organisms: The primordial habitat for archaea is believed to be a hot environment. When archaea evolved in such a habitat, its proteins were "de novo" designed in a hot environment that necessarily biased both structural repertoire (as explained in more detail below) and sequences that had to be found to fold and be stable in such structures. On the other hand, T. maritima (the bacterium) is likely to have initially evolved as a mesophilic organism that later recolonized a hot environment. Its thermophilic adaptation required the enhancement of the thermostability of already existing proteins.
Based on their observations I'd predict that the tube worms and crabs living near hot vents are using the ten-penny nail method to hold their proteins together. What I did not grasp from this study is the physics of the reverse case- is it somehow less stressful to get cold, so that cold-dwelling archaea don't need whatever the opposite of a ten-penny nail would be to unblock their hot-adapted proteins? This is of some interest because of speculation that life originated at high temperatures (the present authors disagree with this idea), so that the epochal evolutionary event would be in managing in the cold.