Glowing blue helps shield this tardigrade from harmful ultraviolet light

When blasted with ultraviolet radiation, a newly discovered species of tardigrade protects itself by glowing blue.

Tardigrades, microscopic animals also known as water bears or moss piglets, are nature’s ultimate survivor. They’re game for temperatures below –270° Celsius and up to 150° C and can withstand the vacuum of space, and some are especially resistant to harmful UV radiation (SN: 7/14/17). One species shields itself from that UV radiation with glowing pigments, a new study suggests. It’s the first experimental evidence of fluorescent molecules protecting animals from radiation, researchers report October 14 in Biology Letters.

“Tardigrades’ tolerance for stress is extraordinary,” says Sandeep Eswarappa, a biochemist at the Indian Institute of Science in Bangalore, India, “but the mechanisms behind their resistance is not known in most [species].”

He and his colleagues investigated these mechanisms in a new tardigrade species from the genus Paramacrobiotus that the scientists identified and then grew in the lab after plucking specimens from a mossy wall on campus. Eswarappa found that like many other tardigrades, these Paramacrobiotus are resistant to ultraviolet radiation. After sitting under a germicidal UV lamp for 15 minutes — ample time to kill most microbes and give humans a skin lesion — all Paramacrobiotus specimens survived, seemingly unfazed by the ordeal.

The secret of how these water bears persisted eluded Eswarappa and his team until one day when the researchers happened to view a tube of the ground-up tardigrades in a UV transilluminator, used to visualize fluorescence in the lab. To the team’s surprise, the tube glowed blue. “It was our mini-eureka moment,” Eswarappa says.

Molecules fluoresce when they absorb higher energy light and release lower energy light. Some biologists have suggested that fluorescent pigments could shield certain animals, such as comb jellies or corals, from UV radiation, though such powers hadn’t been shown in the lab (SN: 11/17/17).

Individual Paramacrobiotus vary in how much they fluoresce, the team found, and more fluorescent tardigrades are more resistant to UV light. After one hour of UV exposure, 60 percent of strongly fluorescent individuals survived more than 30 days, while all less-fluorescent specimens died within 20 days.

To further link fluorescence with protection, the researchers soaked roundworms and individuals from a tardigrade species that isn’t resistant to UV light in a bath of glowing Paramacrobiotus extract. Thus endowed, both animals were more UV tolerant compared with individuals immersed in only water.

The experiments clearly show that the pigments are “a mechanism for UV tolerance in these animals, and that’s a nice step forward,” says Paul Bartels, an invertebrate zoologist and tardigrade expert at Warren Wilson College in Asheville, N.C., who wasn’t involved in the study. “It’s a really cool study.”

Eswarappa was surprised to find that the tardigrades’ glow played a role in UV protection, since “the finding of fluorescence was serendipitous.” He suggests that the fluorescent pigments absorb UV rays, emitting harmless blue light, though the study can’t say precisely how the pigments confer protection. The glow itself, for example, may simply be an ancillary effect of the pigments, and not involved in UV shielding. Eswarappa speculates that the glowing pigments may help these water bears survive in southern India, where summertime UV levels can be extreme.