Why Tardigrades Are Not Considered as Extremophiles

 by Rukhsar Naguman

Intense heat, freezing cold, high atmospheric pressure, radioactivity, and high acidity. These environments do not seem very welcoming for life but for some micro-organisms, these environments are their ideal conditions to not only survive but to thrive. Such micro-organisms are called extremophiles – from Latin, extremus meaning “extreme” and Greek philia meaning “love.” They can occur in all the domains of life, bacteria, archaea, and eukaryotes, and their abilities range from one environment such as thermophiles (heat), psychrophiles (cold), acidophiles (acidic), etc., to multiple such as thermoacidophiles which are adapted to both intense heat and high acidity. Each type of extremophile has evolved making proteins and enzymes (biological catalysts) which perform efficiently under harsh conditions. Thermophiles thrive in high temperatures ranging from 41 to 122 °C. Psychrophiles or cryophiles on the other hand prefer temperatures as low as -20 °C. But the most extreme living things on Earth are Tardigrades.

 

 

Tardigrade under a microscope. Credits: Robert Picket, Papilio / Alamy Stock Photo

Tardigrades, also known as water bears, are eight-legged micro-organisms occupying almost every place on Earth. Although being aquatic, they do not swim and unlike macro-organisms, they are not closely related to anything else on Earth. Despite the ongoing dispute between taxonomists, the leading system places tardigrades inside a clade called Tactopoda which includes two groups (Tardigrades and Euarthropoda). There are over 1300 described species for tardigrades composed of three main classes: Eutardigrada, Mesotardigrada, and Heterotardigrada. In terms of size, they are usually under 500 microns, but they do range widely from 50 to 1200. Tardigrades can be found in 3 major aquatic environments. Some live in marine/brackish water, others in freshwaters (ponds, lakes, and rivers), and finally some in terrestrial habitats (films of water on land, soil, leaf litter, mosses, etc.). These terrestrial habitats can dry up in the daytime which is why tardigrades can survive difficult situations so well. In freshwater, they live among vegetation and mainly feed on the cell contents of plants. Some species of tardigrades have eyes. These are not the typical set of eyes as they are not organs (a group of tissues working together to perform a certain function). Each tardigrade eye is in fact, a single photoreceptive cell.

Many of the tardigrade species have abilities to survive harsh environments such as temperatures as low as -272 °C and as high as 150 °C, high radiation, and extreme atmospheric pressures. In September 2007, the European Space Agency (ESA) sent a colony of tardigrades for 12-days aboard the uncrewed FOTON-M3 spacecraft. After the 12-days trip, most of the colony survived the pressure in the vacuum of space and the exposure to cosmic radiation. Previous studies done on Earth showed that tardigrades may produce more antioxidants when exposed to high radiation which slows down any cell damage. A new spaceflight experiment from NASA called Cell Science-04 or CS-04 will research the effects of microgravity on Hypsibius exemplaris, a species of tardigrades, testing whether the same happens under microgravity. A tardigrade’s life span can vary from a couple of weeks to more than 100 years. A museum specimen of a dried moss that was in an herbarium for 120 years revealed active tardigrades when soaked in water. Tardigrades are capable of surviving in such harsh environments by undergoing cryptobiosis, a process in which the metabolism of the tardigrade can lower to less than 0.01% of normal and they are in a “tun” state until conditions get better.

One of the biggest misconceptions about tardigrades is either that they are immortal, or they are extremophiles. Despite having the ability to survive the extremely hostile conditions, they are only resistant when they are in a “tun” state. Otherwise, they are quite delicate and due to their small size, tardigrades can easily be hurt or killed by bigger single-celled organisms. Tardigrades are not considered extremophiles as they do not “love” living in extreme conditions. They simply undergo processes to survive them. Exposing tardigrades to harsh environments for enough time can kill them. They prefer to be in aquatic environments rich in food, not faced with cosmic radiation in the vacuum of space. Despite them not being extremophiles, they are equally intriguing microscopic organisms serving their crucial part in experimental science for a better future.

 Sources:

https://www.nasa.gov/ames/research/space-biosciences/cell-science-04-spacex-22

https://www.youtube.com/watch?v=kux1j1ccsgg