Tardigrades: evolutionary adventures and the water bear glow up

Scientists discover another superpower of tardigrades – what does this mean for us?

Writer: Elizabeth Jovena Sulistyo
Editor: Lia Bote
Artist: Patrick Marenda

On 10th October 2020, a paper made headlines with the revelation that a species of the tardigrade genus Paramacrobiotus can survive harmful radiation by glowing blue. This discovery, described by Suma and coworkers in the journal Biology Letters, was found while studying the UV radiation tolerance of an unknown species of tardigrade. By lucky chance, the team noticed a tube of them glowing near a UV source, and subsequent experiments revealed that they exhibit a natural fluorescence which acts as a shield to protect them against radiation. Remarkably, during this investigation, the team also managed to transfer the fluorescent extract from this Paramacrobiotus species to both the nematode Caenorhabditis elegans and another tardigrade species Hypsibius exemplaris, protecting them from UV exposure as well. 

Fluorescence is the emission of light after temporary absorption of electromagnetic radiation. Despite pervading the animal kingdom – scorpions, parrots, chameleons and frogs can autofluoresce – its functional significance in nature is unknown. Photoprotection against UV radiation is a suspected purpose for a few organisms like comb jellies and corals, with corals demonstrating a strong correlation between fluorescence and susceptibility to bleaching. However, there has been no direct experimental evidence for this in any organism until now. 

Tardigrades, also known as water bears or moss piglets, are microscopic, invertebrate animals famous for being nearly indestructible. Discovered in 1773, these tough little creatures are strangely adorable, looking like a cross between a caterpillar and a marshmallow. They form the phylum Tardigrada, which comprises about 1,300 known species split in two major clades, Heterotardigrada and Eutardigrada. Most of them grow to be about 0.5 to 1 mm in length and their eight legs terminate in either claws or suction discs. 

But what makes them so special? Tardigrades can exist in a cryptobiotic or inactive ‘tun’ state, which is where they are particularly invulnerable and able to endure extremely harsh conditions. This adaptation allows them to survive dehydration in between their periods of activity, when they must be in an aquatic environment. Therefore, as extremophiles, they have been found to occupy an immense range of niches, including marine, freshwater and terrestrial habitats.

Tardigrades only live for 3 to 24 months in their active state, but can have a total lifespan of up to 30 years. For the majority of their existence, they are cryptobiotic; they reversibly suspend their metabolism through anhydrobiosis (desiccation), anoxybiosis (oxygen depletion), chemobiosis (high toxin concentrations), cryobiosis (freezing temperatures) or osmobiosis (excessive salinity). Remarkably, they are the first animals found to be able to survive exposure to the vacuum and radiation of outer space. The tun form curls up, reducing its surface area for evaporation, with lost water replaced by bioprotectants such as trehalose that protect cellular macromolecules and internal organs.

Recently, it was shown that a unique protein called Dsup (damage suppressor) is partly responsible for tardigrades’ resilience. This protein binds to nucleosomes and protects chromosomal DNA from reactive hydroxyl radical-mediated cleavage, generated by ionising radiation in water and soil. This is just one of many antioxidant defences that set tardigrades apart.

The incredible resilience of tardigrades has various implications for research. In the medical field, studying the stress mechanisms in tardigrades that protect cells and maintain genome integrity after radiation can advance our understanding of cancer. For instance, the DNA-protecting properties of Dsup would be extremely useful in protecting humans against X-ray-induced damage and extending cell longevity. So far, the transfection of Dsup into human embryonic kidney cells has indeed been shown to increase tolerance to radiotherapy and reduce oxidative stress.

In addition, the protective characteristics of trehalose in cryptobiosis has driven developments in dry-state preservation of cells, bioreagents and organs for both medical and research purposes. As similar defensive compounds are being discovered in tardigrades (TDPs, CAHS proteins), much more progress can be expected, especially for pharmaceutical use, since trehalose can be produced at an industrial scale. From another commercial standpoint, even the fluorescent extract produced by Paramacrobiotus could be a great addition in sunscreen if it can be patented and mass produced.

Moreover, the tardigrade’s temperature and radiation tolerance opens up vast prospects for human survival, especially in space. Several programmes, such as TARDIS and Phobos Life Project, have already been launched to study how they react in space, but other possibilities include studying how they react to Martian conditions or travelling cosmic distances via simulations on Earth. In fact, there may already be thousands of tardigrades living on the moon from a spacecraft that crashed in 2019. And who knows? Perhaps, in the far future, we could be living across the universe because of these little water bears.

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