How water bears survive lethal UV radiation

Paramacrobiotus has a fluorescent shield which helps it survive UV radiation

October 31, 2020 07:28 pm | Updated November 01, 2020 09:47 am IST

Tough bug:  Paramacrobiotus  sp. survived beyond 30 days after 15 minutes exposure to germicidal UV radiation

Tough bug: Paramacrobiotus sp. survived beyond 30 days after 15 minutes exposure to germicidal UV radiation

“I know an animal that can live in boiling water or in solid ice. It can go 10 years without a drop of water - the tardigrade or water bear. They have survived all five mass extinctions. They’ve been in business for a half a billion years,” says Dr. Neil deGrasse Tyson in an episode of Cosmos: A Spacetime Odyssey.

Awestruck by this episode, Sandeep M. Eswarappa from The Indian Institute of Science, Bengaluru, started exploring these tiny creatures that are less than 1 millimetre long. His new paper shows that a tardigrade called Paramacrobiotus BLR strain (BLR for Bengaluru) has a protective fluorescent shield which helps it survive harmful ultraviolet (UV) radiation.

Hunting for tardigrades

It was an exploratory study, and the team hunted for tardigrades within the institute campus. About 1,300 species have been reported worldwide from various habitats. “We knew that we could find them on algae and similar vegetation, so we scraped off moss from the campus’ concrete walls and trees and tried to isolate the tardigrades. Though we collected nearly 100 samples, we finally succeeded in finding just one species,” says Harikumar R. Suma, who was a junior research fellow at the institute during the study. He is the first author of the paper published in the journal Biology Letters.

The study found that Paramacrobiotus sp. survived beyond 30 days after 15 minutes exposure to germicidal UV radiation (one kilojoule per square metre). At this radiation dose, another species of tardigrade called Hypsibius exemplaris died within a day, and most of the roundworms were found dead within a week.

When a four times higher radiation was given, about 60% of the Paramacrobiotus sp. survived beyond 30 days.

To further confirm their findings, the team coated the H. exemplaris and roundworms with the fluorescent extract from Paramacrobiotus sp. Animals with the fluorescent coat were found to survive longer than the control animals without.

But why does this protective coat show fluorescence? “In addition to the UV protective function, we speculate that the fluorescence might serve as a signal to other organisms around — like an indication: ‘Hey the tardigrade is glowing, there is UV radiation here, let’s get away’. It provides a survival advantage to these tardigrades by reducing competition,” explains Dr. Eswarappa.

Evolved fluorescence

The team writes that Paramacrobiotus could have probably evolved this fluorescence mechanism to counter the high UV radiation of tropical southern India. The UV dose in Bengaluru, where it was isolated, could go up to 4 kilojoule per square meter on a typical summer day.

The paper adds, the “small fraction of non-fluorescent variants that co-exist in the same moss habitat may have other mechanisms to escape from UV radiation, for example, living deeper inside the moss where UV radiation cannot reach.”

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