Life at high temperatures

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Obsidian Pool
Obsidian Pool, Yellowstone National Park : James W. Brown

How are these organisms capable of growth at such high temperatures? What is the upper limit of life? The answers to these questions aren't entirely clear, and high temperature may not really be an extreme condition except from our anthropocentric point of view. Nevertheless, here are some of the issues of life an very high temps:

Membrane fluidity and integrity

As temperature increases, the fluidity of the cells lipoprotein membranes increases. This fluidity must be balanced. In general, the membrane lipids of thermophiles have a higher melting point that those of mesophiles, so the the fluidity of the membranes of these organisms is appropriate at their optimum growth temperature. Organisms can also change their mix of membrane lipids in response to changes in temperature.

DNA structure

The two strands of typical DNA in solution separate at about 70°C. Although increasing the fraction of G=C pairs increases this melting point somewhat, there is no correlation between the growth temperature of an organism and the G+C content if its DNA. It turns out that in the cell, the DNA is inherently resistant to denaturation because of the high ionic strength and low water activity (i.e. most of the water is already tied up in hydration shells) of the cytoplasm. Most organisms negatively-supercoil their DNA, which makes it more easily denatured, but extremely thermophilic Archaea & Bacteria positively supercoil their DNA. DNA-binding proteins such as histones or histone-like proteins also stabilize DNA to thermal denaturation.

RNA structure

The folded structure of non-mRNAs (e.g. rRNA, tRNA, RNase P RNAs, snRNAs, snoRNAs, tmRNA, &c) can be denatured just like the strands of DNA. However, modest changes in the sequences and structures of RNAs can stabilize the structure. Most thermophilic RNAs are rich in G-C basepairs, and more importantly very low in G-U pairs, mismatches, bulges, and other irregularities that, in mesophiles, lead to flexibility in the RNA. The RNAs are also usually short, with no extra sequences; shorter sequences have fewer nonfunctional folding possibilities. In addition, base modifications and changes in protein binding can stabilize RNAs.

Protein structure

The denaturation of proteins from mesophiles at high temperature is dramatic - that's what happens in an egg when you boil it. However, stabilizing a protein for function at high temperatures seems to be relatively easy by organisms in evolution, although our understanding of these changes is poor at best.

Enzymatic function

The function of an enzyme is tuned to the organisms growth temperature. Mesophilic enzymes work best at 20-40°C and denature at higher temperatures, but enzymes from thermophiles work best at their growth temperature and denature when heated further. Thermophilic enzymes work slowly if at all at mesophilic temperatures; they are too rigid at these temps and are essentially 'frozen'. In other words, enzymes are tuned for optimal flexibility at the temperature they need to function at.

enzyme curves

A more complex issue is balancing catalytic function in the cell. Different reactions will increase in speed at different rates as the temperature goes up, and these must be held in balance by homeostatic mechanisms.

Small molecule stability

This may be a more difficult problem for thermophiles. The half-life of GTP at 100°C is measured in seconds, and yet even organisms that grow at this temperature use GTP for translation, RNA synthesis, and many other processes. Many other small molecules are not very heat resistant - ATP, UTP, NAD, FAD, &c. These may be synthesized on a just-in-time basis so that their degradation isn't too great a loss for the cell; another way to put this is that the flux of reactions through these intermediates may be very high even though their steady-state concentration may be low.

How high can it go?

The highest temperature of cultivated species is about 118°C, but there is good evidence for life growing at up to about 135-140°C in hydrothermal environments. This may represent the upper limit for life, because at this temperature amino acids become racemized (flip from L to D) at significant rates. This is independent of any of the normally stabilizing mechanisms, and the flipping of an amino-acid in a protein would potentially lead to its irreversible denaturation. However, one temperature after another has been suggested to be the limit for life, for seemingly very good reasons, and been proven wrong by biology. So who knows?