At work we have two different freezers: one for regular stuff and one for stuff that needs to be at -70C (human plasma, lyophilized bacteria). The question I have is what changes between -20 and -70? I mean, frozen solid is frozen solid, right?
Freezing doesn’t stop chemical processes - like proteins degrading - it only slows them down. The colder it is, the slower the chemical reactions take place. My WAG is that certain things at your work must be kept at -70 in order to be kept from degrading within a reasonable time frame.
I do not know the answer here; a medical professional or biologist is going to have to give the Straight Dope. But I’ll hazard the guess that breakdown processes will occur, slowly, at -20 and be all but stopped at -70. There are certain foods that will spoil at a 0 F (~-18 C) freezer if left there, frozen, too long. So biological and histological specimens are probably in much the same state.
Just to add a bit of argument to blow away the idea that “sub-zero = frozen” I would note that 0 degress celcius is the point at which water freezes. If you want to freeze mercury, I believe you need to get things a bit colder. So even beyond the biological realm, the difference between two temperatures can make a lot of difference dependent on the affected materials. Water-centrism is a human invention and should not be viewed as having any particular magic which makes the laws of water have to apply to everything else.
Even for water, there are differences in the solid state depending on temperature and pressure, as indicated in this phase diagram. Admittedly, this really matters most at significantly higher pressures than I’d expect occur in the OP’s freezers, but still…
Nucleic acids are much more stable at -70 than they are at -20. We have the same setup in our lab.
Well, as a rule of thumb reactions occur 2x faster for every 10 degrees up in temperature.
I’m not sure if this rule holds even reasonably true for sub-zero zero conditions but 1/2^5 = 32x slower then a -20 freezer.