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Tardigrades

August 14, 2017

There's a common conception that cockroaches will inherit the Earth. Humans, genus Homo, have existed since the appearance of Homo habilis, about two or three million years ago. Cockroaches, however, date back about 300 million years, to the Carboniferous period.

Cockroaches are a hardy insects that are able to survive for a month without food. They can survive without air for more than half an hour, and they're immune to below freezing temperatures for hours at a time. What gives them a definite survival advantage is their radiation resistance, leading to their supposed resilience from a nuclear war. While the direct radiation from a nearby gamma ray burst, shielded by the atmosphere, would not reach Earth's surface, humans might not survive the environmental effects of such a radiation burst, but cockroaches might.

Cockroach, Ectobius vittiventris

One of many species of the cockroach.

This is the Ectobius vittiventris, first described by A. Costa in 1847.

(Wikimedia Commons image by Amada44.)


One interesting part of cockroach history was the first birth in space of a terrestrial organism, the 33 offspring of a Russian cockroach named Nadezhda. This occurred in 2007 on the Foton-M 3 bio-satellite. Back on Earth, the offspring of Nadezhda's offspring were found to develop normally.

Recent research has shown that cockroaches might not be the front-runners for terrestrial succession. The new contenders are the tardigrades, also called "water bears" because of the superficial resemblance (see figure). Tardigrades are eight-legged, segmented, water-dwelling micro-animals first discovered by the German zoologist, Johann August Ephraim Goeze, in 1773. They are just a half millimeter long, and tardigrade fossils have been found in Cambrian period deposits, dating to 530 million years ago. Tardigrades are ubiquitous, being found in environments ranging from the deep sea, to tropical rainforests, to Antarctica.

Tardigrades

Animal, vegetable, or mineral? an scanning electron microscope micrograph of a Milnesium tardigradum Schokraie (left), and a photo of a generic "water bear" (right). (Left image from E. Schokraie, U. Warnken, A. Hotz-Wagenblatt, M.A. Grohme, S. Hengherr, et al., "Comparative proteome analysis of Milnesium tardigradum in early embryonic state versus adults in active and anhydrobiotic state," PLoS ONE, vol. 7, no. 9 (September 27, 2012), article n. e45682, doi:10.1371/journal.pone.0045682, and right image by Aditya Sainiarya, both via Wikimedia Commons.)


Tardigrades have been found to be the most resilient animals known, with some tardigrade species surviving at liquid helium temperatures (4 kelvin), and others surviving for a few minutes at 150 °C. The deep ocean tardigrade species survive at pressures greater than those found in deep ocean tranches, more than a thousand atmospheres of pressure. They are also resistant to ionizing radiation at hundreds of times the lethal dose for humans, and they survive the vacuum of outer space. Tardigrades can survive without food or water for more than 30 years, after which time they can forage and reproduce after hydration.

Scientists at the University of North Carolina (Chapel Hill, NC), the University of California (Berkeley, California), and the University of Modena and Reggio Emilia (Modena, Italy) investigated the mechanism by which tardigrades are able to survive desiccation for many years.[1-2] They found that intrinsically disordered proteins are enriched during tardigrade desiccation, and these proteins, which vitrify, increase their desiccation tolerance.[1] says Thomas Boothby, a postdoctoral fellow at the University of North Carolina, and the study's first author,
"The big takeaway from our study is that tardigrades have evolved unique genes that allow them to survive drying out... In addition, the proteins that these genes encode can be used to protect other biological material--like bacteria, yeast, and certain enzymes--from desiccation."[2]

It had always been assumed that this ability to survive desiccation came from trehalose, a sugar that aids desiccation survival in yeast, brine shrimp, and some nematodes.[2] Trehalose works by forming glass-like solids, rather than crystals, when they dry.[2] DNA sequencing, however, showed that tardigrades do not produce trehelose, so these researchers looked for another mechanism, which turned out to be intrinsically disordered proteins, which have no fixed three-dimensional structure.[2]

Structural diagram of trehalose

Structural diagram of trehalose.

(Wikimedia Commons image by Fvasconcellos.)


To verify this mechanism, the research team inserted protein-encoding genes into yeast and other bacteria, and these organisms were also protected from desiccation.[2] There's great utility in this finding, since the same mechanism might be used to make crops drought-resistant. It might also allow stabilization of sensitive pharmaceuticals in a dry state, so they would not need to be refrigerated.[2]

Such proteins that resist crystallization are likely responsible for the cold endurance of tardigrades. In a 2016 study, Japanese scientists from the National Institute of Polar Research (NIPR, Tokyo, Japan) and Sokendai (The Graduate University for Advanced Studies, Tokyo, Japan) examined the survival of revived tardigrades of the species, Acutuncus antarcticus, retrieved from an Antarctic frozen moss sample in 1983 and stored at −20 °C for 30.5 years.[3] One of the two resuscitated tardigrades successfully reproduced, as did a hatchling from a recovered egg, after this long-term cryptobiosis.[3-4] The extant records for organism survival under cryptobiosis is 39 years for nematodes, and 8 years for tardigrades in dried storage under a frozen condition.[4]

Frozen, revived, tardigrade Acutuncus antarcticus with three eggs.

Acutuncus antarcticus tardigrade with three eggs, frozen for more than 30 years, then revived.

(Still image from a NIPR video by Megumu Tsujimoto.)


The frozen tardigrades did not recover instantly. They were reared on agar plates with algae provided as food. Although one of the tardigrades slightly moved its fourth pair of legs on the first day, it took two weeks to crawl and eat.[4] It appears that this time was required to repair damage experienced over its thirty year cryptobiosis.[3] This individual went on to lay nineteen eggs, fourteen of which hatched successfully.[4] Another tardigrade showed movement at first, but it died twenty days after rehydration.[4] The tardigrade that hatched from the revived egg ate, grew, and reproduced without any obvious abnormality, and it laid fifteen eggs, of which seven successfully hatched.[4]

Meteors have caused mass extinction events, such as the demise of the dinosaurs, in Earth's past. Probability-wise, it might just be a matter of time before such an event happens again (see figure). Humans have developed enough technology to deflect such threats; but, perhaps, our technological civilization is just a tiny blip in the history of our species. What are the chances that all all life on Earth might be extinguished?

Meteor impact rates as a function of asteroid mass.

Meteor impact rates as a function of asteroid mass. The dashed vertical lines show the minimum masses needed for complete sterilization of the Earth, the lower one producing enough environmental impact, while the higher one would render a planet-sized impact crater. (Fig. 1 of ref. 5, licensed under the Creative Commons Attribution 4.0 International License.[5])


That's the question asked by astrophysicists and astronomers at the University of Oxford (Oxford, UK), andHarvard University (Cambridge, Massachusetts). Their conclusion, as reported in an open access paper in Scientific Reports, is that the likelihood of complete sterilization is lower than one chance in 100,000 over the lifetime of our planet, and less than 10-7 per billion years from other causes.[5-6] Although humans are a fragile species, the lowly tardigrade is quite resistant to astrophysical cataclysms, such as meteor impact.[5]

The research team restates the tardigrade ability to survive extreme temperature and pressure, and also radiation levels of ∼5000-6200 grays (Gy).[5] For comparison, a whole body exposure of 5 grays will kill a human within 14 days. There are implications of such survivability of life beyond Earth in the persistence of life on other planets.[6] Even if tardigrades do inherit the Earth, the environment would need to recover to a more normal state after a few decades.[6]

References:

  1. Thomas C. Boothby, Hugo Tapia, Alexandra H. Brozena, Samantha Piszkiewicz, Austin E. Smith, Ilaria Giovannini, Lorena Rebecchi, Gary J. Pielak, Doug Koshland, and Bob Goldstein, "Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation," Cell, col. 65, no. 6 (March 16, 2017), pp. 975-984, DOI: http://dx.doi.org/10.1016/j.molcel.2017.02.018.
  2. Tardigrades use unique protein to protect themselves from desiccation, Cell Press Press Release, March 16, 2017.
  3. Megumu Tsujimoto, Satoshi Imura, and Hiroshi Kanda, "Recovery and reproduction of an Antarctic tardigrade retrieved from a moss sample frozen for over 30 years," Cryobiology, vol. 72, no. 1 (February 2016), pp. 78-81, https://doi.org/10.1016/j.cryobiol.2015.12.003.
  4. Animals revived after being in a frozen state for over 30 years, Research Organization of Information and Systems Press Release, February 16, 2016.
  5. David Sloan, Rafael Alves Batista, and Abraham Loeb, "The Resilience of Life to Astrophysical Events," Scientific Reports, vol. 7 (July 14, 2017), article no. 5419, doi:10.1038/s41598-017-05796-x. This is an open access article with a PDF file at the same URL.
  6. Nicola Davis, "Tardigrades: Earth’s unlikely beacon of life that can survive a cosmic cataclysm," Guardian (UK), July 14, 2017.

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