Psychrophiles

(Polaromonas vacuolata)

On the opposite end of the spectrum from the thermophiles, psychrophiles
burgeon in extreme cold. Life cannot grow where liquid water can’t exist,
but it does grow at or slightly below freezing temperature.

The most studied psychrophilic environment is Antarctic sea ice. This area is mainly populated with algae, diatoms, and bacteria. Certain species in this ecosystem are incapable of reproduction at temperatures above 2 degrees centigrade. Psychrophiles also inhabit the ocean’s freezing cold floor.
Among the most well known psychrophiles are the worms that dwell in methane
ice at the bottom of the Sea of Cortez. Psychrophiles are probably the
least studied of extremophiles, so obviously little is known about them.

Colored photo:  Methane worms living underwater. 

Black/white photo:  Methane worm up-close.