Copyright © 2020-2021 C.E. by D. Jon Scott

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.

Note: Currently all of the organisms listed here are prokaryotes. Later this area will be expanded to include eukaryotes.

Permission is hereby granted to show and distribute this material for educational purposes with proper attribution.

Even where fair use does not necessitate the use of proper attribution, it would be appreciated, as this project required an enormous amount of work.

Heat X-Philes: Thermophiles & Hyperthermotolerant, Near-Thermophilic Mesophiles
This group includes not only “simple” thermophiles, with an optimal growth temperature of anywhere from 50°C to 80°C (OGT=65°C±15°C), or 122°F to 176°F (149°F±27°F), but also hyperthermophiles, who have an optimal growth temperature over 80°C (OGT≥80°C) or 176°F (OGT≥176°F).

Heat X-Philes: Archaea — Thermophilic Archaea & Hyperthermotolerant, Near-Thermophilic Archaeal Mesophiles

Heat X-Philes: Bacteria — Thermophilic Bacteria & Hyperthermotolerant, Near-Thermophilic Bacterial Mesophiles

Cold X-Philes: Cryophiles & Hyperpsychrotrophic, Near-Cryophilic Mesophiles
Also known as psychrophiles, this group includes organisms with an optimal growth temperature equal to or less than ten degrees celsius (OGT≤10°C).

Cold X-Philes: Archaea — Cryophilic Archaea & Hyperpsychrotrophic, Near-Cryophilic Archaeal Mesophiles

Cold X-Philes: Bacteria — Cryophilic Bacteria & Hyperpsychrotrophic, Near-Cryophilic Bacterial Mesophiles

Pressure X-Philes: Barophiles & Hyperbarophilic, Near-Barophilic Mesobarophiles
Also known as piezophiles, this group includes organisms with an optimal growth pressure of 10 Mpa (~100 bar) or higher (≥98.6 atmospheres).

Pressure X-Philes: Archaea — Barophilic Archaea & Hyperbarotolerant, Near-Barophilic Archaeal Mesobarophiles

Pressure X-Philes: Bacteria — Barophilic Bacteria & Hyperbarotolerant, Near-Barophilic Bacterial Mesobarophiles

Vacuum X-Philes: Hypobarophiles & Hypobarotolerant, Near-Barophilic Mesobarophiles

Vacuum X-Philes: Archaea — Hypobarophilic Archaea & Hypobarotolerant, Near-Hypobarophilic Archaeal Mesobarophiles

Vacuum X-Philes: Bacteria — Hypobarophilic Bacteria & Hypobarotolerant, Near-Hypobarophilic Bacterial Mesobarophiles

Radio X-Philes: Radiophilic & Radioresistant Organisms
Ionizing-radiation-resistant organisms (IRROs) are organisms for whom more than 1,000 Gy (100,000 rd or CGS) are needed

Radio X-Philes: Archaea — Radiotolerant Archaea & Archaeal Radiophiles

Radio X-Philes: Bacteria — Radiotolerant Bacteria & Bacterial Radiophiles

Acid X-Philes: 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).

Acid X-Philes: Archaea — Acidophilic Archaea & Hyperacidotolerant, Near-Acidophilic Archaeal Neutriphiles

Acid X-Philes: Bacteria — Acidophilic Bacteria & Hyperacidotolerant, Near-Acidophilic Bacterial Neutriphiles

Base X-Philes: Alkaliphiles & Hyperalkalitolerant, Near-Alkaliphilic Neutriphiles
This group includes organisms with an optimal growth pH of 10 or higher (OGpH≥10)

Base X-Philes: Archaea — Alkaliphilic Archaea & Hyperalkalitolerant, Near-Alkaliphilic Archaeal Neutriphiles

Base X-Philes: Bacteria — Alkaliphilic Bacteria & Hyperalkalitolerant, Near-Alkaliphilic Bacterial Neutriphiles

Salt X-Philes: Halophiles & Hyperhalotolerant, Near-Halophilic Organisms
This group includes organisms which require sodium chloride for their growth and reproduction...

Salt X-Philes: Archaea — Halophilic Archaea

Salt X-Philes: Bacteria — Halophilic Bacteria

Drouth X-Philes: Xerophiles and Hyperxerotolerant Organisms
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)

Drouth X-Philes: Archaea — Xerophilic Archaea

Drouth X-Philes: Bacteria — Xerophilic Bacteria

Time X-Philes: Chronotolerant Organisms
This group includes organisms who can survive exceedingly long periods of time; in some cases even the passage of hundreds of millions of years. For example, Virgibacillus marismortui (formerly Bacillus marismortui and Bacillus strain 2-9-3) was revived by scientists from 250-million-year-old spores (Vreeland, 2000); for comparison, the first dinosaurs began appearing about 220 MYA — 30 million years later.

Time X-Philes: Archaea — Chronotolerant Archaea

Time X-Philes: Bacteria — Chronotolerant Bacteria

Salt & Base X-Philes: Haloalkaliphiles
This group includes polyextremophiles who are simultaneously both halophilic and alkaliphilic.

Salt & Base X-Philes: Archaea — Haloalkaliphilic Archaea

Salt & Base X-Philes: Bacteria — Haloalkaliphilic Bacteria

Heat & Acid X-Philes: Thermoacidophiles
This group includes polyextremophiles who are simultaneously both thermophilic and acidophilic.

Heat & Acid X-Philes: Archaea — Thermoacidophilic Archaea

Heat & Acid X-Philes: Bacteria — Thermoacidophilic Bacteria

Heat & Pressure X-Philes: Thermobarophiles
This group includes polyextremophiles who are simultaneously both thermophilic and barophilic.

Heat & Pressure X-Philes: Archaea — Thermobarophilic Archaea

Heat & Pressure X-Philes: Bacteria — Thermobarophilic Bacteria

Cold & Pressure X-Philes: Cryobarophiles
This group includes polyextremophiles who are simultaneously both cryophilic and barophilic.

Cold & Pressure X-Philes: Archaea — Cryobarophilic Archaea

Cold & Pressure X-Philes: Bacteria — Cryobarophilic Bacteria

Cold & Vacuum X-Philes: Cryohypobarophiles
This group includes polyextremophiles who are simultaneously both cryophilic and hypobarophilic.

Cold & Vacuum X-Philes: Archaea — Cryohypobarophilic Archaea

Cold & Vacuum X-Philes: Bacteria — Cryohypobarophilic Bacteria

Cold & Drouth X-Philes: Cryoxerophiles
This group includes polyextremophiles who are simultaneously both cryophilic and xerophilic.

Cold & Drouth X-Philes: Archaea — Cryoxerophilic Archaea

Cold & Drouth X-Philes: Bacteria — Cryoxerophilic Bacteria

Cold & Radio X-Philes: Radiocryophiles
This group includes polyextremotolerant who are simultaneously both cryophilic (or at least cryotrophic) and radiotolerant.

Cold & Radio X-Philes: Archaea — Radiocryophilic Archaea

Cold & Radio X-Philes: Bacteria — Radiocryophilic Bacteria

Heat & Radio X-Philes: Radiothermophiles
This group includes polyextremotolerant who are simultaneously both thermophilic (or at least cryotrophic) and radiotolerant.

Heat & Radio X-Philes: Archaea — Radiothermophilic Archaea

Heat & Radio X-Philes: Bacteria — Radiothermophilic Bacteria

Sporing X-Philes: Endospore-Forming Bacteria
Bacterial endospores tend to give the bacteria capable of producing them polyextremotolerance. Endospores are known for being able to survive extremely high doses of radiation, vacuum dessication, extreme heat and cold, lack of nutrients, and in some cases even the passage of hundreds of millions of years (for example, Virgibacillus marismortui (formerly Bacillus marismortui and Bacillus strain 2-9-3) was revived by scientists from 250-million-year-old spores (Vreeland, 2000); for comparison, the first dinosaurs began appearing about 220 MYA — 30 million years later).

X-Philes: Master List
Master list of all extremotolerant prokaryotes.

Taxonomic tree of extremophilic and extremotolerant prokaryotes

Table of extremophilic and extremotolerant prokaryotes

X-Philes: Archaea — Extremotolerant Archaea

Taxonomic tree of extremophilic and extremotolerant archaea

Alphabetic table of extremophilic and extremotolerant archaea

X-Philes: Bacteria — Extremotolerant Bacteria

Taxonomic tree of extremophilic and extremotolerant bacteria

Alphabetic table of extremophilic and extremotolerant bacteria

Space X-Philes: Astrotolerant Organisms
This group includes polyextremophiles capable of surviving space-like extremes.

Taxonomic tree of astrotolerant prokaryotes

Alphabetical table of astrotolerant prokaryotes

Space X-Philes: Archaea — Astrotolerant Archaea

Taxonomic tree of astrotolerant archaea

Table of astrotolerant archaea

Space X-Philes: Bacteria — Astrotolerant Bacteria

Taxonomic tree of astrotolerant bacteria

Table of astrotolerant bacteria

Planet X-Philes: Hypergeophilic & Hypergeotolerant Organisms
This group includes polyextremophiles capable of surviving extremes that are planetary in nature, being decidedly unlike conditions found in space.

Taxonomic tree of hypergeophilic and hypergeotolerant prokaryotes

Alphabetic table of hypergeophilic and hypergeotolerant prokaryotes

Planet X-Philes: Archaea — Hypergeophilic & hypergeotolerant Archaea

Taxonomic tree of hypergeophilic and hypergeotolerant archaea

Alphabetic table of hypergeophilic and hypergeotolerant archaea

Planet X-Philes: Bacteria — Hypergeophilic & hypergeotolerant Bacteria

Taxonomic tree of hypergeophilic and hypergeotolerant bacteria

Table of hypergeophilic and hypergeotolerant bacteria

Note #1: “Instances of hypergeophily” and “instances of hypergeotolerance” currently differ only in that the former includes instances of true thermophily but not hyperthermotolerance, while the latter includes instances of hyperthermotolerance.

Note #2: “Total instances of extremotolerance” was calculated here hastily as hypergeotolerance plus astrotolerance.