The Hot Water Cascade Spring flows down over algae on the rocks into a pool at Hot Springs National Park.
Blue Green Algae, (Cyanobacteria), is one of four thermophiles that has adapted to thrive in the extreme temperatures of our Hot Springs, (Average 143° F, 62° C). Pictured above is the Hot Water Cascade near the Arlington lawn at the end of Bathhouse Row.

NPS Photo by: Mitch Smith

Gray and White photo using scanning electron microscopy of 2 tardigrades, (also know as water bears), floating in space. The have 4 legs with 3 pointy thin claws on each one. Their body sections are bulbous and their mouths look like suction tubes.
Tardigrades, (Sometimes called "Water Bears") are known for being one of the MOST EXTREME of all the extremophiles. They can withstand both temperature extremes, dehydration, pressure extremes, radiation extremes and even the vacuum of space.

CC Photo of Tardigrade Hypsibiusdujardini using SEM by Willow Gabriel of Goldstein Labs


Scientific knowledge of the environmental limits of microbial life on Earth expanded dramatically in the early 1990s as microbiologists applied new techniques of molecular biology over a broad range of environmental extremes. New discoveries have revolutionized scientific understanding of Earth's biosphere, opened up new views of the history of land-based life, and increased the possibilities that life could exist elsewhere in the cosmos. Scientists found tiny species living in places that were previously unimagined.

Extremophiles (literally "extreme-loving") are defined as organisms that occupy environments judged by human standards as harsh. These encompass both physical and chemical extremes. Different classes of extremophiles have been defined based on the nature of the environments where they are found.


Though many think the very hot temperatures of our thermal springs would be inhospitable to life. In fact, they are very complex ecosystems. Species that can specifically withstand extreme heat are called Thermophiles.

Most thermophiles live at temperatures between 60 and 80 ° C (140 to 176 ° F). Thermophiles are capable of growing, carrying out metabolic processes, and reproducing at these extreme temperatures. One thermophile, Pyrolobus fumarii, has been found at temperatures of 113° C /235° F, which is hotter than the boiling point of water!

Thermophiles have adapted enzymes and protein molecules that can conduct functions like photosynthesis and digestion within a cell at extreme temperatures. In most organisms, enzymes commonly break apart at temperatures above 47° C/116° F, but the enzymes in thermophiles are more resilient because they are packed very tightly and held by especially strong links.The DNA of thermophiles is adapted to life at high temperatures as well. This attribute is being utilized in innovative ways to evolve the food, clothing, paper and biotechnology industries.

Life in Extreme Heat

The admin fountain covered in blue-green algae shoots up a spray of hot thermal water into the air with magnolia trees in the distance. The admin fountain covered in blue-green algae shoots up a spray of hot thermal water into the air with magnolia trees in the distance.

Left image
Admin Fountain covered in blue-green algae, (cyanobacteria)
Credit: NPS Photo/Mitch Smith

Right image
Same Admin Fountain photographed with a FLIR thermography camera.
Credit: NPS Photo

This photo comparison shows a regular photo of the admin thermal springs fountain covered in cyanobacteria, (blue-green algae), juxtaposed next to a similar picture taken with a FLIR Thermography camera so you can witness the different heat gradients. 

A blue circle graphic with 4 thermophiles around it. 3 varied green blobs of blue-green algae, a red bean-shaped bacteria, a spiral blue shell for the ostracod, and a microscope representing nanobacteria.
A graphic representation of the four thermophilic organisms that reside in the thermal springs ecosystem at Hot Springs National Park. Design by Thomas Waymouth

Thermophilic Dwellers of Hot Springs National Park

There are 47 springs still flowing at our park and their average temperature is 143°F, (62°C). Of those 47 springs there are 35 that are actively tested by our hydrologist weekly.

There are 4 general types of Thermophilic residents that live in the thermal springs at Hot Springs National Park. These are Cyanobacteria, (Blue-Green Algae), Thermophilic Bacteria, Ostracods, and Nanobacteria, (It is strongly debated amongst scientists as to whether nanobacteria or "nanobes" are actually living organisms).

Blue-Green Algae - Cyanobacteria & Algae

Close up of blue, green and yellow algae drying out of hot water on jagged tufa rock near the hot water cascade. Close up of blue, green and yellow algae drying out of hot water on jagged tufa rock near the hot water cascade.

Left image
Photo of Cyanobacteria, (blue-green algae) out of water near the hot water cascade
Credit: NPS Photo/Lissa Allen

Right image
Photo taken using Scanning Electron Microscopy, (SEM)
Credit: "blue-green algae of genus Synechocystis" by BASF licensed under CC BY-NC-ND 2.0

Algal Communities

A 2013 study found 45 species of algae living in various temperature gradients of the thermal springs at our park. 29 of these were cyanobacteria, (blue-green algae), and 16 of these were diatoms, (algae). The diatoms were only found near the lower temperature springs.

Diatoms are unicellular algae and they produce 20-50% of the air we breathe! They are the only organism on the planet with cell walls made of transparent, ornately patterned silica. Diatoms remove carbon dioxide (CO2) from the atmosphere and convert it into organic carbon in the form of sugar and then release oxygen. These are the only plants that can survive in our thermal springs.

Blue-green algae or cyanobacteria is sometimes considered part plant and part animal. This is because they are not true algae. They have no nucleus, no chloroplast, and no structures that are evident in photosynthetic true algae. In fact blue-green algae is more akin to bacteria which have similar biochemical and structural characteristics. The blue-greens are widely distributed over land and water, often in environments where no other vegetation can exist. They can tolerate temperatures in the range of 158 to 163 degrees F (70 to 73 degrees C). One type of cyanobacteria found in our park, Phormidium treleasei, is so rare that it has only been recorded from five other places in the world. 

The blue-greens are microscopic life forms that exhibit several different types of organization. Some grow as single cells enclosed in a sheath of slime-like material. The cells of other cyanobacterias, including Phormidium treleasei aggregate into colonies or slimy mats of tangled filaments attached deep under water which can detach and float to the surface.

Edinburgh's rocky coastline with both the sea and the land completely covered in bright blue and green cyanobacteria, (algae).

CC Rosser1954 - Cyanobacteria covered water and shoreline, St Margaret's Loch, Holyrood Park, Edinburgh.

We have found fossils of cyanobacteria that are over 3 billion years old!

Though these strange species make look strange or gross to some, we may owe them a debt. Many scientists suspect that when primitive Earth was much hotter, the Blue-Green Algaes photosynthetic evolution made them instrumental in creating the atmosphere. This allowed a major evolutionary transformation leading to the development of aerobic metabolism and to the subsequent rise of higher plant and animal forms.


The next featured residents of our thermal springs are the Ostracods. The arthropods of the class Ostracoda are one of the most successful crustacean groups with over 8000 living species. They have adapted to all kinds of wet environments including marine, freshwater, thermal and even some terrestrial habitats such as moss.
8 small photos in a collage of various kinds of tiny thermophilic crustaceans found in our hot springs. They are tan, black, orange textured creatures that look like shrimp, snails and/or beetles.
Photos of organisms taken with a photography microscope from an ostracod survey in 2017.

Photography by Geoscientist-in-the-Park (GIP) interns Meg O’Connor and Paul Solis and supervised by Shelley Todd, Natural Resource Manager for Hot Springs National Park

Ostracods have been noticed in our thermal springs ecosystem as early as 1804 when scientists William Dunbar and Dr. George Hunter were sent to explore "the hot springs on the washita" by President Thomas Jefferson right after the Louisiana Purchase. They mentioned in their journal seeing a small "animalcule" in the hot springs pools.
Ostracods are no longer called "animalcules" but they are sometimes called “seed shrimp” and most of them are about the size of a poppy seed. They can range in size from 0.1 to 32 mm. Though Ostracods are not quite as resilient to high heat as some of the other residents of our waters, they can survive temperatures up to 120 degrees F (49 degrees C).

These Crustaceans have 5 to 7 pairs of appendages which are specialized for different tasks and they live inside a shell made of two valves which they shed several times during their life, growing a new larger one each time. Ostracods are sexually dimorphic meaning that the males and females are different shapes.These tiny thermophilic animals have a wide range of diets, and the group includes carnivores, herbivores, scavengers and filter feeders.

Thermophilic Bacteria

Thermophilic bacteria are those that thrive within high temperatures, usually between 45 and 80 C (113 and 176F) and are found in environments such as hot springs, peat bogs, and near deep-sea hydrothermal vents. One type of "hyperthermophile", Methanopyrus kandleri, can even survive temperatures up to 122°C. (251.6°F)' which is far above the boiling point of water. Hot Springs National park’s geothermal hot springs are an ideal environment for some species of these heat loving bacteria.

Rods of rose-colored thermophilic bacteria rise up in a curved cluster in this photo taken with a microscope.
Rods and spheres of thermophilic bacteria taken with using electron microscopy.


We are still learning so much about these fascinating organisms. They "eat" chemical elements such as sulfur and hydrogen and they can fix their own carbon from carbon dioxide.

Scientists studying Hot Springs National Park’s thermal waters have currently identified two different species of thermophilic bacteria inhabiting the springs, Thermoanaerobaculum aquaticum and Fontimonas thermophile.The world is filled with many different types of bacteria and though they are traditionally seen as agents of disease, the amount of bacteria that harm humans is very low compared to the thousands of types of bacteria that surround us. The thermophilic bacteria found within Hot Springs National Park belong to the latter group and pose no risk to human health.


Close up of a jagged brown, gray and black meteorite sitting on a table behind a ruler and small die with the letter E on it. The meteorite measures 9 centimeters. It is labeled ALH 84001,0 Close up of a jagged brown, gray and black meteorite sitting on a table behind a ruler and small die with the letter E on it. The meteorite measures 9 centimeters. It is labeled ALH 84001,0

Left image
This 4.5 billion-year-old rock, labeled meteorite ALH84001 is from Mars. Researchers found fossilized organic carbon compounds on it's surface. In 1997 NASA scientists came to Hot Springs National Park to search the thermal springs for similar findings.
Credit: NASA

Right image
A close-up look at so-called "nanobacteria" found in our thermal springs.
Credit: NASA 1997



Nanobacteria, (right slide), are the smallest cell-walled organisms on Earth, the existence of which is the center of great controversy. A nanobacterium is around one billionth of a meter in diameter (1/10 the size of bacteria), leaving some to question whether or not they are big enough to hold all the necessary cell components such as DNA, RNA, and plasmids.

The rock pictured above was a portion of meteorite ALH84001 that was dislodged from Mars by a huge impact about 16 million years ago and fell to earth about 13,000 years ago. The meteorite was discovered in Antartica in 1984.

Scientists from NASA found organic carbon compounds, (what appeared to be impossibly tiny fossilized bacterium), on the surface of the meteorite. The presence of these preserved microscopic physical biomarkers supported a strategy of searching for evidence of life in hot spring deposits on Mars.

Because of this they chose 5 thermal springs sites around the world to study and look for "nanobacteria". Two of those were Yellowstone National Park and Hot Springs National Park. When they studied our thermal springs they found what appeared to be over 700 unique nanobes, several of which seemed very similar to those found on the surface of the Mars meteorite.

A lavender colored flower with tendril like petals.
Unique Plants

Many unique plants have adapted to life near the thermal springs. Learn more.

Thermal water cascades down a mossy hillside into two concrete collection pools.
Hot Springs Geology

The story of the Park's thermal springs begins with rocks that formed over 400 million years ago...

A white-spotted brown doe deer nibbles on some green foliage.

Learn about the wildlife in the Park.

Last updated: October 7, 2021

Park footer

Contact Info

Mailing Address:

101 Reserve Street
Hot Springs, AR 71901


501 620-6715

Contact Us