Newly Discovered Glowing Tardigrade Is Weirdly Resistant to Lethal Doses of UV Radiation

Newly Discovered Glowing Tardigrade Is Weirdly Resistant to Lethal Doses of UV Radiation
A newly described tardigrade glows in blue under UV light. (Image: H R. Suma et al., 2020/Biology Letters)

A newly discovered species of tardigrade that glows blue when exposed to ultraviolet light uses the powers of fluorescence as a protective shield, according to new research.

Tardigrades, nicknamed water bears or moss piglets, are microscopic animals capable of tolerating some unbelievably tough conditions, such as freezing temperatures, radiation, dehydration, and even the vacuum of space. In 2016, scientists in Japan even managed to revive a tardigrade that had been frozen for more than 30 years. Around 1,300 different species of these eight-legged creatures are known to exist, and they’re found all around the world.

Scientists from India have uncovered yet another tardigrade superpower, at least in one particular species. These previously unknown tardigrades, assigned to the Paramacrobiotus genus, exhibit natural fluorescence, casting an eerie blue glow when exposed to ultraviolet light. What’s more, and as the authors of the new study argue, this fluorescence protects the tardigrades from levels of UV radiation known to kill other microorganisms, such as bacteria and viruses. The new paper, published today in Biology Letters, was co-authored by biochemist Sandeep Eswarappa from the Indian Institute of Science in Bangalore.

Eswarappa and his colleagues pulled the new tardigrade and others from moss growing on a concrete wall in Bangalore. Using a germicidal lamp, the scientists blasted the specimens with ultraviolet light, which was done to test the creatures’ tolerances. Fifteen-minute doses delivered at 1 kilojoule per square metre wiped out most individuals from a tardigrade species known as Hypsibius exemplaris, and all were dead after 24 hours.

Oddly, however, a mysterious group of tardigrades featuring reddish-brown spots all managed to survive for 30 days following a dosage that kills bacteria and nematode worms (Caenorhabditis elegans) in just five minutes. In a follow-up test, Eswarappa and his colleagues cranked the dose to 4 kilojoules per square metre — and this time for a full hour. Incredibly, 60% of the strange tardigrades managed to survive for 30 days after this intense exposure. At this point, it was clear that the scientists had stumbled onto a new species, which they provisionally assigned as Paramacrobiotus BLR.

“After UV radiation treatment, tardigrades were observed daily for signs of life — active movement and egg laying,” wrote the authors in the study. “There was no significant change in the number of eggs laid, their hatchability and the hatching time, between UV-treated and untreated Paramacrobiotus BLR specimens.”

The next step involved an investigation of the tardigrades with an inverted fluorescence microscope, which caused the reddish-brown tardigrades to cast a blue light. The scientists, thinking the tardigrades’ fluorescent skin pigments might be linked to the UV resistance, performed an interesting experiment: They covered H. exemplaris specimens, along with some nematode worms, with the pigments, and once again exposed them to the UV lamp. The resulting fluorescent compound formed a “shield” that helped these organisms to survive at nearly twice the rates of unprotected cohorts.

Accordingly, the new study shows that “it is possible to transfer the UV tolerance property from Paramacrobiotus BLR strain to the UV sensitive H. exemplaris and C. elegans using the fluorescent extract,” wrote the authors, adding that this provides a “direct experimental demonstration of photoprotection by fluorescence.”

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Organisms have all sorts of strategies to protect themselves against UV radiation, such as DNA repair mechanisms and UV-absorbing compounds (melanin in mammals being a good example). Scientists have suspected that fluorescence might confer a similar effect among corals and comb jellies, but there was no experimental proof.

The exact mechanism of protection remains unknown, but Eswarappa’s team suspects the fluorescent shield absorbs harmful UV radiation and emits harmless blue light. Paramacrobiotus BLR likely evolved this special trick to protect itself against the high UV radiation found in tropical southern India, where the UV index can reach as high as 10, according to the authors.