Extremophiles, organisms who experience optimum growth in conditions lethal to most others, and extremotolerant organisms who may not necessarily be extremophiles yet can natheless thrive in such conditions, are of extreme interest to astrobiologists.

Acidophiles & Hyperacidotolerant, Near-Acidophilic Neutriphiles
This group includes organisms capable of thriving in hyperacidic conditions, most of which have an optimal growth pH of 5 or lower (OGpH≤5), although a few have an OGpH slightly above this (e.g., Nanoarchaeum equitans, OGpH=6) but can nonetheless be found flourishing in hyperacidic environments (pH=0.5-1.5).

Alkaliphiles
This group includes organisms with an optimal growth pH of 10 or higher (OGpH≥10)

Barophiles
Also known as piezophiles, this group includes organisms with an optimal growth pressure of 10 Mpa (~100 bar) or higher (≥98.6 atmospheres).

Halophiles
This group includes organisms which require sodium chloride for their growth and reproduction...

Hyperthermophiles
This group includes organisms with an optimum growth temperature of ≥80°C.

Radioresistant Organisms

Xerophiles
This group includes organisms who can thrive and reproduce at water activity lower than that typical for most life on Earth (a_w≤0.8)

Cryophiles
Also known as psychrophiles, this group includes organisms with an optimal growth temperature equal to or less than ten degrees celsius (OGT≤10°C).

Hypobarophiles

Haloalkaliphiles

Thermoacidophiles
This group includes polyextremophiles who are simultaneously both thermophilic and acidophilic.

Thermobarophiles
This group includes polyextremophiles who are simultaneously both thermophilic and barophilic.