By: Reagan Smith
Tardigrades, commonly referred to as water bears, are microscopic (0.1-1 mm long) invertebrate organisms that have survived on Earth for the last 600 million years. Their unique abilities and survival mechanisms allow them to thrive in environments too harsh for any other living organism. These creatures can survive a range of temperatures extending from an intense 151°C to a frigid -200°C, six times the pressure of the deepest part of the ocean, the vacuum of deep space, X-ray radiation a thousand times what humans can withstand, exposure to harsh chemicals, changes in salinity, lack of oxygen, and dehydration. They possess incredible adaptations that have built up over millions of years to create an almost alien-like creature, and these adaptations provide scientists with an extraordinary amount of topics to study. The knowledge gained from analyzing tardigrades can be applied to medical advancements and new discoveries.
Tardigrades have two forms of reproduction – asexual and sexual – and the ability to reproduce in either manner gives them a competitive advantage as a species. In asexual reproduction, the tardigrade performs parthenogenesis, where an unfertilized egg from a female develops into a carbon-copy of her. This benefits the female because she does not have to waste time and energy looking for a suitable mate; instead, she can find a stable source of food and a safe shelter for her offspring. In turn, sexual reproduction promotes variation in their gene pool, leading to diversity and potentially beneficial mutations in their populations. A population of organismsthat has access to both asexual and sexual modes of reproduction can greatly boost its rate of growth and the offsprings’ likelihood of survival.
The 900 discovered species of tardigrades cover the Earth in marine, freshwater, and terrestrial environments. While all species need to be surrounded by water for gas exchange and metabolic activities, terrestrial tardigrades require only a thin surrounding layer, such as the environment created by a patch of moss in a forest or a leaf on a tree, to stay active. Marine tardigrades have a lower tolerance for dehydration and as such require an environment with an increased water volume.
One of the key mechanisms a tardigrade possesses for survival is cryptobiosis, a death-like state where metabolic activities cease. There are various kinds of cryptobiosis: anhydrobiosis, cryobiosis, osmobiosis, and anoxybiosis. As the names suggest, anhydrobiosis is a response to no water, cryobiosis protects against lowered temperatures, osmobiosis is induced when there is an increased solute concentration in the external environment (for example, higher salinity), and anoxybiosis occurs when there is no oxygen. Scientists studying these processes have observed that they all involve responding to a lack of water, leading to the conclusion that the response mechanisms are similar. Since tardigrades mainly perform anhydrobiosis, scientists have conducted the most research on this phenomenon.
A particular study led by Thomas Sorenses-Hygum and Robyn Stuart explores the effect of desiccation (removal of water from an organism’s habitat) on a tardigrade’s activity and ability to survive. The group studied Echiniscoides sigismundi, a marine intertidal tardigrade, and looked at the effect of surface substrate, time spent dessicated, and initial amount of water in the environment. This species tends to be highly resistant to freezing, lack of distilled water, and high-salinity solutions.
The study investigated the effects of three substrate materials: filter paper, glass, and hedgehog spines. They found that after 24 hours of dehydration, 98% of the specimens survived on the filter paper substrate, 29% on glass, and 50% on the spines. The standout number from this study is that 82% of tardigrades survived 2 weeks of desiccation on filter paper, an incredibly long time considering these animals typically depend on water to live. Echiniscoides sigismundi has two mechanisms for surviving: they cover themselves in three distinct cuticle layers as protection from the outside environment, and populations will clump together and dehydrate in small groups.
While the ability to form a cuticle layer appears to be unique to the E. sigismundi species, all tardigrades possess the ability to form a dehydrated dry husk, called a tun, when undergoing cryptobiosis. In this state, the organism sheds nearly all of its internal water. Its metabolism is slowed to 0.01%, and it replaces the water lost with a sugar called trehalose, which stabilizes cell proteins and membranes until water is reintroduced into the environment.
Another fascinating trait of tardigrades involves protection of their DNA. A group of scientists, including Carolina Chavez, Grisel Cruz-Becerra, Jia Fei, George A Kassavetis, and James T Kadonaga from the University of California, San Diego have sequenced the genomes of the species Ramazzottius varieornatus and Hypsibius exemplaris. They found the nuclei of R. varieornatus to contain a DNA damage suppressor protein (referred to as Dsup) that protects against hydroxyl radicals. This protein binds to the minor groove in nucleosomes, directly preventing radicals from reacting with exposed hydrogen atoms in that location. A hydroxyl radical is the neutral form of a hydroxide ion, meaning it has no charge; it is extremely volatile and highly reactive. Dsup is a charged, intrinsically disordered protein, meaning it doesn’t have a fixed 3D folded structure and is highly flexible.
The scientists from UCSD began by tagging Dsup with FLAG, a short hydrophilic polypeptide used to study protein-protein interactions, and His6, an amino acid chain containing histidine residues. They then recreated mononucleosomes (made of a strand of DNA wrapped twice around a histone protein) by taking the 5S rDNA sequence, which is a segment of DNA that codes for a subunit of the ribosome protein, from Xenopus borealis, commonly known as the Marsabit clawed frog. The group compared binding of Dsup to a mononucleosome made of 147 nucleotide base pairs (bp) to a 147-bp-long free DNA segment via a technique referred to as gel mobility shift analysis. Free DNA is made of degraded DNA fragments circulating in the blood plasma, coming from degraded tumor cells, mitochondrial DNA, and fetal DNA. The data showed that Dsup was more likely to bond to the nucleosomes than to free DNA. Then, they tested Dsup on mononucleosomes that contained linker DNA (a section of DNA composed of 38-54 bps that connects nucleosomes in conjunction with histone H1) and compared the results to the 147-bp mononucleosome. There was no significant difference in the protein binding interactions, showing that Dsup binds to the nucleosome core instead of the linker DNA that connects the nucleosomes together. The group then shifted its focus to testing Dsup’s effect on DNA cleavage, where the DNA breaks off into fragments due to harmful outside factors. They created hydroxyl radicals and exposed the DNA/Dsup cells to them; without Dsup, the DNA strand degraded to 100-1000 bp fragments. Therefore, Dsup allows tardigrade cells to remain undamaged when exposed to harsh UV rays, or placed in space, and understanding how to express a protein with human cells may be the key to preventing harmful diseases.
Studying and understanding the precise mechanisms of tardigrades’ survival techniques can help us preserve the life span of cells and prevent diseases. Could the sugar trehalose stabilize dehydrated cells? Do human genes code for a protein similar to Dsup that could protect our DNA from hydroxyl radicals? These organisms may be small, but they possess a seemingly infinite amount of information in every one of their cells, and it is up to science to discover their secret to life.
What did you learn?
1. What are some characteristics of tardigrades?
These animals are microscopic organisms that live all across the Earth in a variety of environments. They can reproduce asexually or sexually, giving the species the reproductive benefits of both.
2. What adaptations do tardigrades possess to survive in these harsh environments?
Tardigrades have the ability to be dehydrated and form a tun, existing in a comatose state. Their cells replace water with a special sugar, trehalose, and they stay like this until reintroduced to water. They also have a special protein that binds to histones, composed of DNA, to prevent hydroxyl radicals from binding and damaging the DNA. This enables them to withstand exposure to UV and X-rays, as well as harsh chemicals.
Citations:
Bordenstein, Sarah. “Tardigrades.” Microbial Life Educational Resources, Marine Biological Laboratory, 9 Jan. 2021, serc.carleton.edu/microbelife/topics/tardigrade/index.html#:~:text=Tardigrades%20can%20survive%20dry%20periods,less%20than%200.01%25%20of%20normal.
Chavez, Carolina, et al. “The Tardigrade Damage Suppressor Protein Binds to Nucleosomes and Protects DNA from Hydroxyl Radicals.” ELife, vol. 8, 2019, doi:10.7554/elife.47682.
Clausen, Lykke K. B., et al. “First Record of Cysts in the Tidal Tardigrade Echiniscoides Sigismundi.” Helgoland Marine Research, vol. 68, no. 4, 2014, pp. 531–537., doi:10.1007/s10152-014-0409-0.
Johnson, Sheila. “What Are the Benefits of Parthenogenesis?” Sciencing, 2 Mar. 2019, sciencing.com/benefits-parthenogenesis-13770.html.
Sørensen-Hygum, Thomas L., et al. “Modelling Extreme Desiccation Tolerance in a Marine Tardigrade.” Scientific Reports, vol. 8, no. 1, 2018, doi:10.1038/s41598-018-29824-6.
Image Credit:
No changes were made: Tardigrade | Incursion dans l'univers microscopique. | Philippe Garcelon | Flickr, Creative Commons Legal Code
Reagan is a junior from Sage Hill School in Newport Coast, California. She is passionate about all things related to science, especially biology and medicine. She also loves being able to use her talents and knowledge to help others and make a difference in the world.
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