Many organisms, from bacteria to plants and animals, produce natural fluorescence. For example, a recent Massive Science lab note illustrates how bananas fluoresce when they ripen. But many scientists are interested in the biological role of such natural fluorescence. Now, scientists report in Biology Letters that some tardigrades (also called water bears) use fluorescence to thwart lethal doses of UV that would otherwise kill many viruses or bacteria. 

Scientists serendipitously identified this new reddish-brown species — Paramacrobiotus — when they were gauging different tardigrades collected from moss samples grown on a wall. When they exposed all the collected tardigrades with germicidal doses of UV, most of the Hypsibius tardigrade species died in 24 hours, but the Paramacrobiotus species survived. They also figured that 60 percent of Paramacrobiotus survived even after 30 days of exposure to four times more UV. 

Surprised, they investigated further into what supports Paramacrobiotus survival. When they looked at all the tardigrades under a fluorescence microscope, they found that Paramacrobiotus species fluoresce more than Hypsibius, and this fluorescence increased with exposure to UV. They then extracted the fluorescing material from Paramacrobiotus to coat the Hypsibius and several other worms (Caenorhabditis elegans). They found that the fluorescent coat protects the Hypsibius species and other worms from lethal doses of UV at levels twice that of their uncoated peers. 

Tardigrades can withstand extreme environmental stress and can even survive radiations in outer space. Such environmental stresses (like UV) can damage tardigrade DNA. Looks like this new species of tardigrades could have evolved to exploit fluorescence as their natural defense mechanism.