How important is -80C?

[Tom Knight]

Chris Jones <cjones from moravian.edu> writes:

> We're looking into replacing one of two -80 degree freezers, and the 
> question came up as to whether we should bother. Currently most of the 
> contents are bacterial and plasmid stocks as well as some isotopes. It 
> seems to me that these would fare no worse in a -20 degree box, but I 
> wondered what experience others have had. What samples "must" be stored 
> at -80C?

Bacterial stocks are not stable at -20.  Competent cells, in
particular, lose competence quickly at -20, but even glycerol stocks
will suffer over year long periods at -20.  Some restriction enzymes
are not stable at -20 and need to be stored at -80.  I have no idea
why you are storing isotopes at -80, unless they are sensitive
chemicals.  The atoms certainly don't decay any differently depending
on the temperature.  It might be possible that radiation damage of the
chemicals is affected.

You want to be sure that your -20 freezers are not frost-free
freezers, which will subject samples to heat/thaw cycles and destroy
living samples very quickly.