The Resilient Tardigrade - How Does It Survive Our Experiments?

There are numerous dynamic, complex and interconnected ecosystems living on our planet. We have seen beautifully biodiverse rainforests populating the land, with endless expanses of aquatic lifeforms residing in the oceans, seas and rivers. The variety is mindboggling - but it only becomes more perplexing when you realise that even the harshest zones in the world can be inhabited by life. From the boiling hot spewing geysers in Yellowstone Park to the frozen tundras of the Arctic, zoologists have observed an astonishing number of different species climatised to the conditions of the most extreme areas in our planet. Suitably, we call such organisms extremophiles  - from the Latin word 'extremus', meaning 'extreme', and the Greek word 'philia', which stands for 'love'. They are literally the risk-lovers of the natural world.

Don't let the semantics fool you, however. Though extremophiles are known for their ability to resist extreme temperatures, pressures and even air shortages in the wild, most are not as adaptable as you might think. Just like if we took an immortal jellyfish from its natural habitat and dropped it in the middle of acid water, forcing an extremophile out of its typical environmental conditions will in most cases kill them. Such is true for the Antarctic krill that lives in the North Pole's subglacial lakes, for example, which has steadily been decreasing in numbers the last few years from the rise of global warming. Like every other living creature, extremophiles have their boundaries...or most of them do, anyway.

One peculiar exception to the rule is the famous water bear, otherwise known as the moss piglet or (more properly) as the tardigrade. You might have heard of these from your friends sometime, or perhaps found out about them from articles explaining the crazy experiments we've thought out for them - 'cause they really make for some ludicrously great stories. Given their microscopic size, tardigrades are very manneuverable. As such, researchers have decided thrown them in lava, sucked all of the water from their bodies and even shot them into outer space - all of which they survived, miraculously enough. Water bears are tough - but what makes them like that, anyway?

Experimenting With Water Bears

To more thoroughly explain the concept behind their adaptive response mechanisms, we should understand the details of experiments analysing the survivability of tardigrades (which are also quite fun to read about, I think!). It should be mentioned that there are multiple varieties of tardigrades, too - over 900 species of them, to be exact. This is because, in reality, tardigrades are animals that belong in the taxonomic phylum Tardigrada. Since a lot of them are quite similar, people don't usually mention their species names that much - but we will. (Not to mention that, like any other phylum, the Tardigrada still display an amazing amount of morphological diversity between species. Some are skinny, some are more segmented, and yet others even have tank-like armour on their backs!) In this case, the first tardigrade I will talk about is the Ramazzottius varieornatus, which, of course, still looks appropriately like a moss piglet.

Normally found living in moving freshwater environments (such as rivers), the R. varieornatus has been repeatedly analysed before for its strange tolerance toward high temperatures, surviving temperatures of over 80ºC. One recent study from 2020 analysed this by placing live specimens in different temperature conditions - and the results were indicative of their internal metabolism. At first, the researchers observed active tardigrades that were either acclimated or non-acclimated (meaning that their surrounding conditions were changed suddenly) to record the temperature at which half of their population died out. Both populations had 50% mortality rates when faced with temperatures over 37ºC. Not overly high. However, tardigrades have a peculiar mechanism to protect themselves from being completely desiccated by the heat (explored below), and populations that had undergone this process were observed to resist the temperatures much more comfortably. Nonetheless, this trait is considered relatively tame in comparison to the tardigrade's other resilient properties. So much so, in fact, that high temperatures have even been referred to as the tardigrades' "Achilles heel" by the team's researchers.

Comparatively, low temperatures seem to be just fine with them. Since the 1990s, tardigrade species like the Adorybiotus cornifer have been known to even survive conditions down to -196ºC - which is, of course, the freezing point of liquid nitrogen. Now, that is not to say that the typical tardigrade will be happier frozen, especially since they tend to die out quite steadily after a few minutes in the condition. The important point, however, is that they can even survive like that in the first place. If they are taken back to their original state before they die, tardigrades can further continue living a perfectly normal life. They quickly get back in shape, scavenging for food again to resupply their energy reserves, and they eventually even start reproducing again (they do live a wild life).

The same is true if we take them into the stratosphere and bring them back to the planet's surface, as NASA's 2007 FOTON-M3 mission so lovingly proved to us. Once they had sent their rocket into outer space, it arrived at an orbiting satellite, inside of which some zealous microbiologists investigated how the tardigrade Macrobiotus richtersi reacted to being exposed to the unhindered radiation of our sun and the microgravity and airless vaccuum  of our galaxy (correction: they live very dangerous lives). Incredibly, the tardigrades all escaped the conditions as if they had never experienced them before, carrying neither damage to their DNA nor any significant down-regulation in their genetic expression. After which, of course, they simply went back to their everyday lives unfazed. In fact, the female tardigrades involved in the mission were apparently so relaxed that they actually laid eggs during the space flight. Consequently, the newborn individuals were then observed to carry no abnormalities, leading perfectly normal water bear lives. (Seriously, though - imagine being born in space!)

We have talked of the amazingly abstract experiments done to analyse tardigrades, so now comes the kicker. How in the world do tardigrades, with their peaceful and preyed-on existence, possess the abnormal resilience that they do? Additionally - and perhaps more confusingly - why do they display such? Let's see.

The Reasons Behind The Tardigrade's Resilience

As you may recall, tardigrades are much more capable of tolerating desiccation at high temperatures when they are allowed time to acclimate to the change. For a long time, researchers believed this change in their metabolism to be a result of a disaccharide (a sugar) known as trehalose, which has been observed in other organisms tolerant to desiccation. The majority of previous studies, however, have not detected significant trehalose concentrations in desiccated tardigrades - and so a new theory was proposed.

When tardigrades are exposed to high temperatures too abruptly, they tend to die out quickly; when they are slowly accustomed to it over a period of minutes (raising the temperature slowly over time), they are known to withstand it. These observations indicated the necessity for the up-regulation of protectant molecules to respond to the changing environment. With this in mind, researchers began looking into their specific DNA sections more closely, eventually finding a set of genes that displayed higher expression rates in desiccated tardigrades than in hydrated ones. The corresponding proteins produced by these genes are known as intrinsically disordered proteins (IDBs), and their effects are truly quite miraculous. Though there is still much to discover about them, IDBs are now believed to protect their encompassing tardigrade via vitrification, which is when molecules condense/partially solidify into an amorphous solid (as covered by my post on glass).

As the water in a tardigrade slowly evaporates, the decrease in humidity in its cells triggers metabolic pathways that stimulate the activation of its IDB genes. When these form, they then proceed to surround heat-sensitive compartments and molecules - including other proteins - until they eventually vitrify around them. This acts as a sort of insulation barrier to the heat, allowing fragile cell structures to retain their shape and continue functioning as normal once the heat has dissipated (at which point the IDBs get broken down again). Other genes expressed simultaneously cause the tardigrade to enter an inactive, low-energy state to preserve energy for the future, allowing it to survive for years without food. This entire process is known as cryptobiosis, and it is also this mechanism that allows tardigrades to survive the freezing temperatures aforementioned - along with massive hydrostatic pressures and oxygen shortages. Finally, tardigrades further contain a unique protein known as the Dsup protein that defends their DNA from radiation damage - a feature that scientists are very intruiged by.

I should note that the research behind these strange animals is not done purely for fun (even if flinging them into space might be rather entertaining). Advances on identifying cryptobiotic genes and mechanisms have seen much interest in the field of genetic engineering. With enough time, scientists could even duplicate their effects in tardigrades inside agricultural crops, preventing them from desiccating. And, with the looming threat of global warming, this potential has become particularly relevant in equatorial regions like Africa and south-east Asia these days, offering local civilisation a way to preserve their food in times of drought. Genetic modification of the Dsup gene has also been proven to function on human cells, and could have the possible application of protecting us from UV-caused skin cancers and the like. Not to mention the tardigrade's potential as a model organism -  it poses no threat to humans, it is practically universal on Earth (existing in mud volcanoes, crashing rivers and even your eyelashes!), and it is superbly durable. No wonder scientists find the tardigrade so fascinating.

References

• Neves, R. C. et al (2020). Thermotolerance experiments on active and desiccated states of Ramazzottius varieornatus emphasize that tardigrades are sensitive to high temperatures. Scientific Reports 10:94. Retrieved from https://doi.org/10.1038/s41598-019-56965-z
• Ramløv, H. & Westh, P. (1992). Survival of the cryptobiotic eutardigrade Adorybiotus coronifer during cooling to −196 °C: Effect of cooling rate, trehalose level, and short-term acclimation. Cryobiology 29(1):125-130. Retrieved from https://doi.org/10.1016/0011-2240(92)90012-Q
• Rebecchi, L. et al (2009). Tardigrade Resistance to Space Effects: first results of experiments on the LIFE-TARSE mission on FOTON-M3. Astrobiology 9(6):581-91. Retrieved from https://doi.org/10.1089/ast.2008.0305
• Jönsson, K. I. & Holm, I. & Tassidis, H. (2019). Cell Biology of the Tardigrades: Current Knowledge and Perspectives. Results and Problems in Cell Differentiation 68:231-249. Retrieved from https://doi.org/10.1007/978-3-030-23459-1_10
• Boothby, T. C. et al (2018). Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation. Molecular Cell 65(6):975-984. Retrieved from https://doi.org/10.1016/j.molcel.2017.02.018
• Hashimoto, T. et al (2016). Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein. Nature Communications 7. Retrieved from https://doi.org/10.1038/ncomms12808