Suma et al., Biology Letters (2020) 20200391
Tardigrades, small aquatic creatures known as water bears, can survive extreme heat, radiation, and even the vacuum of outer space—conditions that would kill most animals. Now, scientists have discovered a new species of tardigrade that can endure ultraviolet (UV) light so lethal, it is regularly used to get rid of hard-to-kill viruses and bacteria.
The discovery was made by chance: Researchers at the Indian Institute of Science scoured their campus for water bears, and then exposed them to extreme conditions. They happened to have a germicidal UV lamp in the lab, so they hit their specimens with it. The dose of 1 kilojoule per square meter, which killed bacteria and roundworms after just 5 minutes, was lethal to Hypsibius exemplaris tardigrades at 15 minutes; most died after 24 hours. But when they hit a strange, reddish brown species with the same dose, all survived. What’s more, when the researchers upped the dose four times, about 60% of the reddish brown bears lived for more than 30 days.
The researchers realized they had found a new species of tardigrade, part of the Paramacrobiotus genus. To figure out how the new species—which was found living in moss on a concrete wall in Bengaluru, India—survived, the scientists examined it with an inverted fluorescence microscope. To their surprise, under the UV light, the reddish tardigrades became blue (above). Fluorescent pigments, likely located under the tardigrades’ skin, transformed the UV light into harmless blue light, the team reports today in Biology Letters. In contrast, Paramacrobiotus with less pigment died about 20 days after exposure.
Next, the researchers extracted the fluorescent pigments and used them to coat H. exemplaris and several Caenorhabditis elegans earthworms. Animals with the jury-rigged shields survived at almost twice the rate of animals without the shields. It’s likely, scientists say, that the tardigrades evolved fluorescence as a means to tolerate the high doses of UV typical for hot summer days in southern India.