What's the opposite of a thermophile?
The term you are looking for is psychrophile or cryophile. There are examples of microbes adapted to very cold conditions in Bacteria, Archaea, and Eukarya. There are also fairly complex animals capable of surviving very low temperatures -  Tardigrades, for example. There are multiple issues that organisms face in withstanding extremely low temperatures - protein denaturation, membrane fluidity, intracellular ice-formation, just to name a few.

There is this review that might be useful to you.


D'Amico, S., Collins, T., Marx, J.-C., Feller, G., Gerday, C. Psychrophilic microorganisms: Challenges for life (2006) EMBO Reports, 7 (4), pp. 385-389. link