Cosmic explosions thousands of light-years away could shut down photosynthesis in the ocean at depths of up to 260 feet, a new study suggests. The calculations add to a growing body of research linking these great blasts, called gamma-ray bursts, with biological damage and even mass extinctions on Earth.

Gamma-ray bursts are tremendous explosions detonated during a massive star's death throes. When stars eight times the mass of the sun or larger reach the end of their lives, they die in spectacular supernova explosions that can temporarily outshine entire galaxies.

Under certain conditions – astronomers aren't exactly sure what – all that energy can be concentrated into a tight beam extending like a spotlight away from the star. These bright beams, known as gamma-ray bursts, can last up to 10 seconds, and carry energies equivalent to billions of nuclear bombs going off at once.

And according to the new research, phytoplankton would not enjoy them. In a paper published on the astronomy preprint site arXiv.org, biologist Liuba Penate of the Universidad Central de Las Villas in Cuba and colleagues model the marine food web from plankton up if a gamma-ray burst were to strike.

Earlier research showed that the burst would trigger an ultraviolet flash at the Earth's surface. Incoming gamma rays would smack electrons off of atoms in the atmosphere, which would knock secondary electrons off nearby atoms and trigger a cascade of bright light.

"You'd actually see a pretty bright flash," said astronomer Brian Thomas of Washburn University in Kansas, who was involved in the earlier work. "If you're standing outside looking up at the sky, you might well be blinded by this."

Phytoplankton don’t have to worry about going blind, but they would feel the flash on a molecular level, Penate's team argues. The radiation could damage simple organisms' DNA, and even shut off photosynthesis entirely for at least the 10 second duration of the flash.

The team calculated how deeply the radiation would penetrate the ocean if a gamma-ray burst went off 6,000 light-years from Earth. In clear waters, photosynthesis would be totally suppressed down to about 260 feet in the ocean. In more turbid waters, photosynthesis would still suffer in the first 65 feet of water.

As the first level of the oceanic food chain, phytoplankton support a variety of more complicated organisms. These tiny plants are also responsible for releasing oxygen into the atmosphere. One species, Prochlorococcus marinus, is responsible for 20 percent of the total oxygen released by the biosphere all by itself, the researchers write. Whatever messes with phytoplankton messes with the whole biosphere.

On the other hand, photosynthesis could turn back on after the burst calms down.

"If you turn off photosynthesis for 10 seconds, that's no big deal," Thomas said – plants are used to not being able to produce for hours at a time at night. The real worry is if plankton DNA ends up too severely damaged to be repaired quickly.

The study is "definitely something new, and it's worth doing," Thomas said. But he was worried that the authors didn't include enough detail on how they made their calculations. "It throws into question somewhat the accuracy of the results," he said. The qualitative results are reasonable, he said, "but I wouldn't necessarily take seriously the exact numbers."

The effects of a gamma-ray burst itself could linger well after the 10-second flash dies down, he added. Thomas and his colleagues' earlier research found that a nearby gamma-ray burst would deplete the ozone layer, which would increase the amount of ultraviolet radiation reaching Earth from the sun and ultimately do catastrophic damage to the DNA of organisms on Earth. A study in the International Journal of Astrobiology in 2004 suggests the Ordovician mass extinction 440 million years ago could have been at least partially caused by a nearby gamma-ray burst.

But gamma-ray bursts are so rare, astronomers expect one to detonate within 6,000 light-years of Earth only once every 15 billion years – longer than the age of the universe. That means we are probably safe from gamma-ray bursts, says astronomer Ethan Siegel.

"That may not be as exciting as 'We're all gonna die!'" Siegel said. "But the statistics definitely work in favor of our survival."

Image: NASA/Swift/Mary Pat Hrybyk-Keith and John Jones

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