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T H E RMOP H ILE S
Thermophiles, or heat-loving microscopic organisms, are nourished by the extreme habitat at hydrothermal
features in Yellowstone National Park. They also color hydrothermal features shown here at Clepsydra Geyser.
Life in Extreme Heat
The hydrothermal features of Yellowstone are
magnificent evidence of Earth’s volcanic activity.
Amazingly, they are also habitats in which microscopic organisms called thermophiles—“thermo” for
heat, “phile” for lover—survive and thrive.
Grand Prismatic Spring at Midway Geyser Basin
is an outstanding example of this dual characteristic.
Visitors marvel at its size and brilliant colors. The
boardwalk crosses a vast habitat for thermophiles.
Nourished by energy and chemical building blocks
available in the hot springs, microbes construct
vividly colored communities. Living with these
microscopic life forms are larger examples of life in
extreme environments, such as mites, flies, spiders,
and plants.
For thousands of years, people have likely wondered about these extreme habitats. The color of
Yellowstone’s superheated environments certainly
caused geologist Walter Harvey Weed to pause, think,
and even question scientists who preceded him. In
1889, he wrote:
There is good reason to believe that the existence
of algae of other colors, particularly the pink, yellow and red forms so common in the Yellowstone
waters, have been overlooked or mistaken for
deposits of purely mineral matter.
However, he could not have imagined what a
fantastic world exists in these waters of brimstone.
Species, unseen to the human eye, thrive in waters
as acidic as the liquid in your car battery and hot
enough to blister your skin. Some create layers that
look like molten wax on the surface of steaming
alkaline pools. Still others, apparent to us through
the odors they create, exist only in murky, sulfuric
caldrons that stink worse than rotten eggs.
Today, many scientists study Yellowstone’s thermophiles. Some of these microbes are similar to the
Words to Know
Extremophile: A microorganism living in extreme
conditions such as heat and acid, that cannot survive
without these conditions.
Thermophile: Heat-loving extremophile.
Microorganism: Single- or multi-celled organism of
microscopic or submicroscopic size. Also called a microbe.
Microbes in Yellowstone: In addition to the thermophilic
microorganisms, millions of other microbes thrive in
Yellowstone’s soils, streams, rivers, lakes, vegetation, and
animals. Some of them are discussed in other chapters of
this book.
Bacteria (Bacterium): Single-celled microorganisms
without nuclei, varying in shape, metabolism, and ability
to move.
Archaea (Archaeum): Single-celled microorganisms
without nuclei and with membranes different from all
other organisms. Once thought to be bacteria.
Viruses: Non-living parasitic microorganisms consisting of
a piece of DNA or RNA coated by protein.
Eukarya (Eukaryote): Single- or multi-celled organisms
whose cells contain a distinct membrane-bound nucleus.
Life in Extreme Heat
131
T H E RMOP H I LE S
Thermophiles in the Tree of Life
Bacteria
Archaea
Green
nonsulfur
bacteria
Mitochondrian
Proteobacteria
Grampositive
bacteria
Eukarya
Myxomycota
Crenarchaeota
Euryarchaeota
Entamoebae
Fungi
Thaumarchaeota
Plantae
Chloroblast
Cyanobacteria
Animalia
Ciliates
Korarchaeota
Nanoarchaeota
Flavobacteria
Flagellates
Trichomonads
Thermotoga
Microsporidia
Thermodesulfobacterium
Aquifex
Diplomonads
DRAWING BY MARY ANN FRANKE
Yellowstone’s hot springs contain
species from the circled groups on this
Tree of Life. Jack Farmer conceived of
this version of the tree of life, which first
appeared in GSA Today, July 2000 (used
with permission).
In the last few decades, microbial
research has led to a revised tree of
life, far different from the one taught
before. The new tree combines animal,
plant, and fungi in one branch. The
other two branches consist solely of
microorganisms, including an entire
branch of microorganisms not known
until the 1970s—the Archaea.
Dr. Carl Woese first proposed this
“tree” in the 1970s. He also proposed
the new branch, Archaea, which
includes many microorganisms formerly
considered bacteria. The red line links
the earliest organisms that evolved
from a common ancestor. These are
all hyperthermophiles, which thrive in
water above 176°F (80°C), indicating
life may have arisen in hot environments
on the young Earth.
first life forms capable of photosynthesis—the process
of using sunlight to convert water and carbon dioxide
to oxygen, sugars, and other by-products. These life
forms, called cyanobacteria, began to create an atmosphere that would eventually support human life.
Cyanobacteria are found in some of the colorful mats
and streamers of Yellowstone’s hot springs.
About Microbes
Other life forms—the Archaea—predated cyanobacteria and other photosynthesizers. Archaea can live
in the hottest, most acidic conditions in Yellowstone;
their relatives are considered among the very earliest
life forms on Earth.
Yellowstone’s thermophiles and their environments
provide a living laboratory for scientists, who continue
to explore these extraordinary organisms. Researchers
know that many mysteries of Yellowstone’s extreme
environments remain to be revealed.
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Yellowstone Resources and Issues Handbook, 2019
Relevance to Yellowstone
Among the earliest organisms to
evolve on Earth were microorganisms
whose descendants are found today
in extreme high-temperature, and in
some cases acidic, environments, such
as those in Yellowstone. Their history
exhibits principles of ecology and ways
in which geologic processes might have
influenced biological evolution.
Regardless of scientific advances, visitors and
explorers in Yellowstone can still relate to something
else Weed wrote about Yellowstone, more than a
century ago:
The vegetation of the acid waters is seldom a
conspicuous feature of the springs. But in the
alkaline waters that characterize the geyser
basins, and in the carbonated, calcareous waters
of the Mammoth Hot Springs, the case is otherwise, and the red and yellow tinges of the algae
combine with the weird whiteness of the sinter
and the varied blue and green of the hot water
to form a scene that is, without doubt, one of
the most beautiful as well as one of the strangest
sights in the world.
American Society for Microbiology: www.microbeworld.org
Allen, E. T., Arthur L. Day, and H.E. Merwin. 1935. Hot
springs of the Yellowstone National Park. [Washington]:
Carnegie institution of Washington.
Brock, T.D. 1994. Life at High Temperatures. Yellowstone
Association/Mammoth, WY.
Brock, Thomas D. 1995. The road to Yellowstone and beyond. Annual Review of Microbiology. 49
Dyer, B.D. 2003. A field guide to bacteria. Ithaca, NY:
Cornell University Press.
Franke, M.A, et. al. 2013. Genetic Diversity in Yellowstone
Lake: The Hot and Cold Spots. Yellowstone Science 21
(1): 6-22.
Fouke. B.W. 2011 . Hot-spring Systems Geobiology: abiotic and biotic influences on travertine formation at
Mammoth Hot Springs, Yellowstone National Park,
USA. Sedimentology. 58: 170-219.
Hamilton, T.L. et. al. 2012. Environmental constraints defining the distribution, composition, and evolution of
chlorophototrophs in thermal features of Yellowstone
National Park. Geobiology. (10) 3: 236-249.
Inskeep WP , et. al. 2013. The YNP metagenome project:
environmental parameters responsible for microbial
distribution in Yellowstone. Frontiers in Microbiology.
00067.
Klatt, C. G., et. al. 2011. Community ecology of hot spring
cyanobacterial mats: predominant populations and their
functional potential. ISME Journal. 5(8): 1262–1278.
Marquez, Luis et al. 2007. A virus in a fungus in a plant:
3-way symbiosis required for thermal tolerance. Science
315 (5811): 513–515.
Qin, J., C.R. Lehr, C. Yuan, X. C. Le, T. R. McDermott, and B.
P. Rosen. Biotransformation of arsenic by a Yellowstone
thermoacidophilic eukaryotic alga. Proceedings of the
National Academy of Sciences of the United States of
America. 106 (13): 5213.
Reysenbach, A.L., and Shock, E. L. 2002 . Merging Genomes
with Geochemistry in Hydrothermal Ecosystems.
Science. 296: 1077-1082.
Sheehan, K.B. et al. 2005. Seen and unseen: discovering the
microbes of Yellowstone. Guilford, Conn: Falcon.
Snyder, J.C. et. al. 2013. Functional interplay between a
virus and the ESCRT machinery in Archaea. Proceedings
of the National Academy of Sciences. 110 (26)
10783-10787.
Spear, J. R. et. al. 2005. Hydrogen and bioenergetics in the
Yellowstone geothermal ecosystem. Proceedings of the
National Academy of Sciences. 102 (7) 2555-2560.
Steunou A.S., et. al. 2008. Regulation of nif gene expression
and the energetics of N2 fixation over the diel cycle in a
hot spring microbial mat. ISME Journal. (4):364-78.
Takacs-Vesbach, C., et al. 2013. Metagenome sequence
analysis of filamentous microbial communities obtained
from geochemically distinct geothermal channels reveals
specialization of three aquificales lineages. Frontiers
Research Foundation.
Thermal Biology Institute of Montana State University: www.
tbi.montana.edu
Ward, D.M., Castenholz, D.W., and Miller, S.R. 2012.
Cyanobacteria in Geothermal Habitats. In Brian A.
Whitton, ed. Ecology of cyanobacteria II: their diversity
in space and time. Dordrecht: Springer
Reviewers
Dr. Eric Boyd, Montana State University Department of
Microbiology and Immunology
Annie Carlson, Research Permit Coordinator
Life in Extreme Heat
133
T H E RMOP H ILE S
More Information
T H E RMOP H I LE S
The travertine terraces in Mammoth Hot Springs host thermophilic cyanobacteria.
Thermophilic Bacteria
The word “bacteria” is often associated with disease,
but only a few kinds of bacteria cause problems for
humans. The other thousands of bacteria, although
all simple organisms, play a complex role in Earth’s
ecosystems. In fact, cyanobacteria made our oxygenrich atmosphere possible. They were the first photosynthesizers, converting light energy into oxygen
more than 3 billion years ago. Without bacteria, we
would not be here.
Most bacteria photosynthesize, providing oxygen
which can then used by other thermophiles. Some
use chemical sources of energy, such as hydrogen or
sulfur, to convert carbon dioxide into biomass other
thermophiles can use (chemosynthesize). All of the
cyanobacteria and green nonsulfur bacteria photosynthesize. Some fulfill both roles. For example,
Thermus sp.—which are photosynthetic— may also
be able to oxidize arsenic into a less toxic form.
Individual bacteria may be rod or sphere shaped,
but they often join end to end to form long strands
called filaments. These strands help bind thermophilic mats, forming a vast community or miniecosystem. Other groups of bacteria form layered
structures resembling tiny towers, which can trap
sand and other organic materials.
Thermophilic Bacteria in Yellowstone National Park
pH and Temperature
Description
Location
Cyanobacteria
Calothrix
Name
pH 6–9
30–45°C (86–113°F)
Color: dark brown mats
Metabolism: photosynthesis by day;
fermentation by night
•
•
Mammoth Hot Springs
Upper, Midway, and
Lower geyser basins
Phormidium
pH 6–8
35–57°C (95–135°F)
Color: orange mats
Metabolism: photosynthesis
•
•
Mammoth Hot Springs
Upper, Midway, and
Lower geyser basins
Oscillatoria
pH 6–8
36–45°C (96–113°F)
Color: orange mats
Metabolism: photosynthesis; oscillating moves
it closer to or away from light sources.
•
•
Mammoth Hot Springs
Chocolate Pots
Synechococcus
pH 7–9
52–74°C (126–165°F)
Color: green mats
Metabolism: photosynthesis by day;
fermentation by night
•
•
Mammoth Hot Springs
Upper, Midway, and
Lower geyser basins
Green Sulfur
Chlorobium
pH 6–9
32–52°C (90–126°F)
Color: dense, dark green mats
Metabolism: anoxygenic photosynthesis—
produces sulfate and sulfur, not oxygen.
•
•
Mammoth Hot springs
Calcite Springs
Green nonsulfur
Chloroflexus
pH 7–9
35–85°C (95–185°F)
Color: green mats
Metabolism: anoxygenic photosynthesis
•
•
Mammoth Hot Springs
Upper, Midway, and
Lower geyser basins
Aquifex
Hydrogeno
baculum
pH 3–5.5
55–72°C (131–162°F)
Color: yellow and white streamers
Metabolism: uses hydrogen, hydrogen sulfide
and carbon dioxide as energy sources; can use
arsenic in place of hydrogen sulfide.
•
•
Norris Geyser Basin
Amphitheater Springs
DeinococcusThermus
Thermus
pH 5–9
40–79°C (104–174°F)
Color: bright red or orange streamers;
contains carotenoid pigments that act as
sunscreen.
•
Lower Geyser Basin
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Yellowstone Resources and Issues Handbook, 2019
T H E RMOP H ILE S
Archaea are the most extreme of all extremophiles, and some scientists think they have not changed much from
their ancestors. Grand Prismatic Spring at Midway Geyser Basin contains archaea.
Thermophilic Archaea
Archaea are the most extreme of all extremophiles—some kinds live in the frigid environments of
Antarctica; others live in the boiling acidic springs of
Yellowstone. These single-celled organisms have no
nucleus but have a unique, tough outer cell wall. This
tough wall contains molecules and enzymes that may
keep acid out of the organism, allowing it to live in
environments of pH 3 or less. (For example, Vinegar
has a pH of less than 3.) Archaea also have protective
enzymes within their cells to keep them from becoming too acidic.
Some scientists think present-day archaea have not
changed much from their ancestors. This may be due
to the extreme environments in which they live, which
would allow little chance for successful changes to
occur. If this is so, modern archaea may not be much
different from the original forms—and thus may provide an important link with Earth’s earliest life forms.
Once thought to be bacteria, organisms in the
domain Archaea actually may be more closely related
to eukarya—which includes plants and animals.
Many kinds of archaea live in the hydrothermal
waters of Yellowstone. For example, Grand Prismatic
Spring at Midway Geyser Basin contains archaea.
They are most well known in the superheated acidic
features of Norris Geyser Basin and in the muddy
roiling springs of the Mud Volcano area.
Whenever you see a hot, muddy, acidic spring, you
are probably seeing the results of a thriving community of archaeal cells called Sulfolobus. This is the
archaea most often isolated and most well known by
scientists. In sulfuric hydrothermal areas, it oxidizes sulfur into sulfuric acid, which helps dissolve
the rocks into mud. The Sulfolobus community in
Congress Pool (Norris) is providing interesting new
research directions for scientists: It is parasitized by
viruses never before known on Earth.
Archaea can be found in the Mud Volcano area,
among other places in Yellowstone National Park.
Thermophilic Archea found in Yellowstone National Park
pH and Temperature
Description
Location
Domain Archaea
Name
pH 0.9–9.8
upper temp.: 92°C
(197.6°F)
Color: cream or yellow-colored
Metabolism: chemosynthesis, using
hydrogen, sulfur, carbon dioxide
Form: unicellular, tough cell
membrane
•
In many of Yellowstone’s
hydrothermal features
Sulfolobus
is the genus
most often
isolated
pH 0–4,
40–55°C (104–131°F)
Color: green
Metabolism: chemosynthesis
Phylum: red algae
•
•
•
Norris Geyser Basin
Lemonade Creek
Nymph Creek
Life in Extreme Heat
135
T H E RMOP H I LE S
Microscopic eukarya live unseen in the extreme environments of Yellowstone. Norris Geyser Basin is one of the
best places in Yellowstone to see thermophilic algae.
Thermophilic Eukarya
Plants, animals, and mushrooms are the eukarya
most of us know. However, millions of unseen, microscopic members of this kingdom exist throughout
our world, including in the extreme environments of
Yellowstone.
Norris Geyser Basin is one of the best
places to see thermophilic algae. Bright green
Cyanidioschyzon grows on top of orange-red iron
deposits around Whirligig and Echinus geysers
and their runoff channels. Waving streamers of
Zygogonium are especially easy to see in Porcelain
Basin, where their dark colors contrast with the white
surface.
From the boardwalk crossing Porcelain Basin, you
can also see larger eukarya, such as ephydrid flies.
They live among the thermophilic mats and streamers, and eat algae, among other things. The species
that lives in the waters of Geyser Hill, in the Upper
Geyser Basin, lays its eggs in pink-orange mounds,
sometimes on the firm surfaces of the mats. Part of
the thermophilic food chain, ephydrid flies become
prey for spiders, beetles, and birds.
Some microscopic eukarya consume other thermophiles. A predatory protozoan called Vorticella
thrives in the warm, acidic waters of Obsidian Creek,
which flows north toward Mammoth Hot Springs,
where it consumes thermophilic bacteria.
Thermophilic eukarya include one form that is
dangerous to humans: Naegleria, a type of amoeba
that can cause disease and death in humans if inhaled
through the nose.
Although they aren’t visible like mushrooms,
several thermophilic fungi thrive in Yellowstone.
THERMAL BIOLOGY INSTITUTE, MONTANA STATE
UNIVERSITY
Counterclockwise from top: Ephydrid flies lay eggs in pink-orange mounds, sometimes on the firm surfaces of
the mats; Waving streamers of Zygogonium are easy to see in Porcelain Basin; The fungi Curvularia protuberata
lives in the roots of hot springs panic grass.
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Yellowstone Resources and Issues Handbook, 2019
Of all the thousands, if not millions, of thermophilic species thriving in Yellowstone’s extreme
environments, the eukarya are the group that bridges
the world of thermophilic microbes with the larger
life forms—such as geese, elk, and bison—that thrive
in ecological communities beyond the hot springs.
Thermophilic Eukarya found in Yellowstone National Park
Name
Red algae
Cyanidio
schyzon
Green algae
Zygogonium
pH and
Temperature
Description
Location
pH 0–4
40–55°C
(104–131°F)
Color: bright green
Metabolism: photosynthetic
Form: coating on top of
formations; mats
•
•
•
Norris Geyser Basin
Lemonade Creek
Nymph Creek
pH 0–4
32–55°C
(90–131°F)
Color: appears black or dark
purple in sunlight
Metabolism: photosynthetic
Form: filaments and mats
Warm
Alkaline
Predator of Bacteria; can infect
humans when ingested through
nose
•
•
Huckleberry Hot
Springs
Boiling River
Consumer; single-celled ciliate
(feathery appendages swirl
water, bringing prey)
•
Obsidian Creek
•
Norris, especially
Porcelain Basin
Upper Geyser Basin,
especially Geyser Hill
Mammoth Hot Springs
Protozoa
Naegleria
(amoeba)
Vorticella
(ciliate)
Euglenids
Mutablis
pH 1–2
<43°C (109°F)
Single-celled; photosynthetic;
moves by waving one or two
strands called flagella
Fungi
(Curvularia
protuberata)
≤65°C (149°F)
with panic grass
<55°C (131°F)
without
Grows in roots of hot springs
panic grass (Dichanthelium
lanuginosum), enabling both to
survive high temperatures; the
plant also produces sugars that
the fungus feeds on
Ephydrid fly
(Ephydra sp.)
>pH 2
<43°C (109°)
Nonbiting insect that eats
microscopic algae as larvae and
adult; prey for spiders, beetles,
dragonflies, killdeer
•
•
Ross’s bentgrass
(Agrostis
rossiae)
38°C (100°F)
Warm springs
spike rush, with
some Tweedy’s
rush
Warm
Acidic
One of Yellowstone’s three
endemic plant species; may
bloom in winter; dries out in
summer’s hot air temperatures
•
•
Banks of Firehole
River
Near Shoshone Lake
Forms thick floating mats,
which also provide habitat for
thermophilic algae and other
thermophiles
•
Obsidian Creek
Note: Algae and Warm springs spike rush photos by Carolyn Duckworth. Fungi photo by Russell Rodriguez and Joan Henson. Ross’s bentgrass
photo by NPS/Jennifer Whipple.
Life in Extreme Heat
137
T H E RMOP H ILE S
Curvularia protuberata lives in the roots of hot
springs panic grass. This association helps both organisms survive higher temperatures than each could
alone. In addition, researchers have recently discovered a virus inside the fungus that is also essential to
the grass’s ability to grow on hot ground.
T H E RMOP H I LE S
A virus was discovered in Congress Pool, shown here, at Norris Geyser Basin. It was infecting the archaean
Sulfolobus.
THERMAL BIOLOGY INSTITUTE, MONTANA STATE UNIVERSITY
Thermophilic Viruses
Like bacteria, the word “virus” often conjures up
images of sickness and death. However, relatively
few of the many types of viruses cause problems
for humans. None of the thermophilic viruses in
Yellowstone should cause problems for human
health—our bodies are too cold, for one thing.
Unlike microorganisms in the three domains,
viruses are not considered to be alive, yet they are still
called “life forms.” They have no cell structure, only
a protein “envelope” that encloses a piece of genetic material. They cannot reproduce on their own.
Instead, a virus inserts itself into a host cell and uses
that cell’s nutrients and metabolism to produce more
viruses.
Scientists suspect many viruses exist in
Yellowstone’s hydrothermal features because they
would be a logical part of the thermophilic ecosystem. One kind was discovered in Congress Pool, at
Norris Geyser Basin. It was infecting the archaeum
Sulfolobus. Another kind of virus has been identified
in pools near Midway Geyser Basin.
This virus parasitizes Sulfolobus.
Thermophilic Viruses found in Yellowstone National Park
Name
Viruses (not in a
domain)
pH and Temperature
Description
Location
pH 0.9–5.8; optimum
2–3
55–80°C (131–176°F)
optimum 70–75°C
•
Protein coats a core of genetic
material
Cannot reproduce by itself
Reproduces by using the host
cell’s metabolism
Not considered living
Predators of other microbes
•
In many of Yellowstone’s
hydrothermal features
•
•
•
•
Unnamed
Acidic
Boiling
•
Shape very similar to viruses
that infect bacteria and
animals, which could mean this
group of viruses existed early
in the development of life on
Earth
•
Unnamed pool near Midway
Geyser Basin
Unnamed
Acidic
Boiling
•
Parasitizes the archaean,
Sulfolobus
•
Norris, Congress Pool
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Yellowstone Resources and Issues Handbook, 2019
T H E RMOP H ILE S
Channels formed with water runoff from geysers create bacterial columns such as this one, located where the
runoff channels from Pinwheel and Whirligig geysers meet.
Thermophilic Communities
Thermophilic communities are as diverse as the communities that humans live in. Community formations,
colors, and locations vary depending on the types
of microbes, the pH, and the temperature of their
environments. Here, we discuss the microbe communities most easily seen in Yellowstone.
Millions of individual microbes can connect into
long strands called filaments. Some bacteria and algae
form thin and delicate structures in fast moving water
such as the runoff channels of hot springs and
geysers. Other microbes form thick, sturdy structures in slower water or where chemical precipitates
quickly coat their filaments.
A bacterium called Thermocrinis forms structures
and is descended from ancient bacteria that metabolized hydrogen and oxygen. Its filaments entwine,
forming mats. Flowing water carries other microbes,
organic matter, and minerals that become caught in
the streamers and add to the mat.
Photosynthetic activity of cyanobacteria such as
Lyptolyngbya form columns or pedestals. Oxygen
bubbles rise in the mat, forcing the microbes upward.
The higher formations capture more organic matter
and sediment than the lower mats, which help build
the columns. Called stromatolites or microbialites,
these structures are similar to ancient microbial communities preserved in formations around the world.
Mats can be as thin as tissue paper or as thick as
lasagna. Multiple layers of microorganisms make up
inch-thick mats. Dozens of types of microbes from all
three domains can exist in these layers. Each layer is
a community, and each layer interacts with the other
layers, forming a complex, larger community full of
millions of microorganisms and their life processes.
Changes in Communities
Visible and invisible changes occur in thermophile
communities as light,
temperature, and
chemical concentrations
change—both short term
(within one day) and
long term (seasonally).
As day brightens to noon,
cyanobacteria sensitive to
light may move away from
the surface; microbes
less sensitive to light may
move to the top layers of
the mat. When light levels
cause shifts in organisms, the community
is responding to a light
gradient.
Temperature and
chemical gradients most
often affect thermophilic
communities in runoff
channels of geysers and
in shallow outflows from
hot springs. The runoff
channels from Pinwheel
and Whirligig geysers
meet. The outer edges
of both are too hot for
visible thermophile communities to develop. But
as Pinwheel’s water cools
in the shallower channel edge, Cyanidium (an
alga) can grow, forming
a bright green community. Whirligig’s runoff
is hotter, which prevents Cyanidium from
199°F (93°C)
Archaea
163°F (73°C)
Cyanobacteria
144°F (62°C)
Fungi
140°F (60°C)
Algae
133°F (56°C)
Protozoa
122°F (50°C)
Mosses,
crustaceans,
and insects
80°F (27°C)
Trout
Thermophilic community
inhabitants are
controlled, in part, by
water temperature and
pH. The chart provides
general guidelines for
maximum for each type.
Life in Extreme Heat
139
T H E RMOP H I LE S
COURTESY CAROLYN DUCKWORTH
growing, but another type of thermophile thrives by
oxidizing the abundant iron in the water, thereby
forming the orange community.
At the Chocolate Pots, which you can see from
pullouts along the Gibbon River just north of
Gibbon Meadows, iron-rich water flows from the
vents. Cyanobacteria—such as Synechococcus, and
Oscillatoria—thrive in this feature. The bacterial
filaments form mats, in which orange-red iron oxide
(rust) is captured. The iron may also be caught on
the bacteria as the microbes move about within the
mat. An olive green color indicates where the orange iron and green bacteria are enmeshed. Darker
streaks indicate the presence of manganese. Scientists
think that mixing of iron-rich waters with shallow
Bison and elk find food and warmth on the less
extreme edges of thermophilic environments in
winter.
Thermophiles by Place and Color in Yellowstone National Park
Location
Characteristics
Thermophiles by Temperature
Thermophiles by Color
Upper, Middle, and Lower Geyser Basins and West Thumb Geyser Basin
•
•
•
pH 7–11
(alkaline)
underlain by
rhyolitic rock
water rich
in silica,
which forms
sinter and
geyserite
deposits
•
•
•
>75°C (167°F), Bacteria and
Archaea
>75°C (167°F), Thermocrinis and
other bacteria form streamers of
pink, yellow, orange, or gray
<75°C (167°F), Synechococcus,
Lyptolyngbya, and Calothrix
(cyanobacteria) plus Roseiflexus
(filamentous green bacterium)
form mats that line cooler hot
springs and runoff channels
•
>75°C (167°F), Sulfolobus, an
archaean, and viruses that
parasitize Sulfolobus
>60°C (140°F), filamentous bacteria
in yellowish streamers and mats
<60°C (140°F), filamentous bacteria
and archaea form red brown mats
<56°C (133°F), Zygogonium, other
algae, and fungi form mats in
runoff channels
•
•
•
Pink, yellow, orange, gray
filaments—Thermocrinis
bacteria
Orange mats—cyanobacteria,
especially on sunny summer
days (carotenoids protect the
organisms from the bright sun)
Olive-green mats—
cyanobacteria mixed with iron
Norris Geyser Basin and Mud Volcano Area
•
•
pH 0–5
(acidic)
underlain by
rhyolite rock
•
•
•
•
•
•
•
Pink–pinkish-orange mats and
streamers—Thermus aquaticus
and other Thermus sp.
Green streamers and mats—
Cyanidium
Orange—iron and/or
arsenic, perhaps oxidized by
thermophiles
Gray, muddy pools—Sulfolobus
Mammoth Hot Springs
•
•
•
pH 6–8
(neutral
to slightly
acidic)
underlain
by ancient
limestone
deposits
water rich
in calcium
carbonate
and sulfur
•
•
•
•
66–75°C (151–167°F), Aquificales
(bacteria) filaments near hot
springs vents
<66°C (151°F), Chloroflexus
(green nonsulfur bacteria)
and cyanobacteria mats, and
filamentous bacteria streamers
<58°C (136°F), Chromatium
(bacteria) form dark mats
(uncommon)
25–54°C (77–129°F), Chlorobium
(bacteria) mats; Calothrix
streamers; Synechococcus
Note: Photos by Carolyn Duckworth.
140
Yellowstone Resources and Issues Handbook, 2019
•
•
•
Orange—Chloroflexus and
cyanobacteria in summer
Green—Chloroflexus and
cyanobacteria in winter;
Chlorobium in cooler water
Cream—filamentous bacteria
Thermophiles in Time and Space
To Mars—and Beyond?
The hydrothermal features of Yellowstone and their
associated thermophilic communities are studied
by scientists searching for evidence of life on other
planets. The connection is extreme environments.
If life began in the extreme conditions thought to
have been widespread on ancient Earth, it may well
have developed on other planets—and might still
exist today.
The chemosynthetic microbes that thrive in some
of Yellowstone’s hot springs do so by metabolizing
inorganic chemicals, a source of energy that does not
require sunlight. Such chemical energy sources provide the most likely habitable niches for life on Mars
or on the moons of Jupiter—Ganymede, Europa, and
Callisto—where uninhabitable surface conditions
preclude photosynthesis. Chemical energy sources,
precipitation-derived waters creates the unique conditions that give rise to the large iron and manganese
oxide mounds at Chocolate Pots. These formations
are not commonly encountered in the park.
Communities formed by thermophilic microbes
sustain communities of larger organisms within
Yellowstone’s hydrothermal areas. These communities in turn affect even larger communities of the
park’s mammals. For example, bison and elk find
food and warmth on the less extreme edges of thermophilic environments in winter. In turn, coyotes,
wolves, and bears seek prey in these areas—especially
in late winter and early spring when bison and elk are
weaker than at any other time of year.
Whether it’s the strike of a grizzly’s paw or a
What’s the Connection?
•
Yellowstone’s hydrothermal features contain modern
examples of Earth’s earliest life forms, both chemoand photosynthetic, and thus provide a window into
Earth’s ancient past.
•
Yellowstone’s hydrothermal communities reveal
the extremes life can endure, providing clues to
environments that might harbor life on other worlds.
•
Yellowstone’s environments show how mineralization
preserves biosignatures of thermophilic communities,
which could help scientists recognize similar
signatures elsewhere.
•
Based on life on Earth, the search for life on other
planets seems more likely to encounter evidence of
microorganisms than of more complex life.
Earth formed
Era of chemical
evolution
4
Age of dinosaurs
Oxygen atmosphere forming
Earth’s crust stabilizing
4.5
Age of mammals
Age of microorganisms
3.5
3
2.5
Billions of years ago
2
1.5
1
0.5
Life in Extreme Heat
0
141
T H E RMOP H ILE S
THERMAL BIOLOGY INSTITUTE, MONTANA STATE UNIVERSITY
Top: Structures formed by the bacteria Thermocrinis.
Bottom: Millions of individual microbes can connect
into long strands called filaments, shown here with the
help of a microscope.
shift in heat beneath the Earth, these communities
change through both common and strange processes.
Biologists continue to discover more about the individuals involved in thermophilic communities, and
ecologists follow the threads of these intricate webs.
T H E RMOP H I LE S
NASA/JPL
along with extensive groundwater systems (such as
on Mars) or oceans beneath icy crusts (such as on
Jupiter’s moons) could provide habitats for life.
Similar Signatures
Thermophile communities leave behind evidence of
their shapes as biological “signatures.” For example,
at Mammoth Hot Springs, rapidly depositing minerals entomb thermophile communities. Scientists
compare these modern signatures to those of ancient deposits elsewhere, such as sinter deposits in
Australia that are 350 million years old. These comparisons help scientists better understand the environment and evolution on early Earth, and give them
an idea of what t