Mosses thrive in the most extreme environments on Earth, from the peaks of the Himalayas to the sands of Death Valley, the Antarctic tundra to the lava fields of active volcanoes. Inspired by moss's resilience, researchers sent moss sporophytes—reproductive structures that encase spores—to the most extreme environment yet: space. Publishing in the Cell Press journal iScience on November 20, their results show that over 80% of the spores survived 9 months outside of the International Space Station (ISS) and made it back to Earth still capable of reproducing, demonstrating for the first time that an early land plant can survive long-term exposure to the elements of space.
"Most living organisms, including humans, cannot survive even briefly in the vacuum of space," says lead author Tomomichi Fujita of Hokkaido University. "However, the moss spores retained their vitality after nine months of direct exposure. This provides striking evidence that the life that has evolved on Earth possesses, at the cellular level, intrinsic mechanisms to endure the conditions of space."
The concept of space moss occurred to Fujita while studying plant evolution and development. He was struck by moss's ability to colonize even the harshest environments on Earth. "I began to wonder: could this small yet remarkably robust plant also survive in space?"
To find out, Fujita's team subjected Physcomitrium patens, a well-studied moss commonly known as spreading earthmoss, to a simulated a space environment, including high levels of UV radiation, extreme high and low temperatures, and vacuum conditions.
They tested three different structures from the moss—protenemata, or juvenile moss; brood cells, or specialized stem cells that emerge under stress conditions; and sporophytes, or encapsulated spores—to find out which had the best chance of surviving in space.
"We anticipated that the combined stresses of space, including vacuum, cosmic radiation, extreme temperature fluctuations, and microgravity, would cause far greater damage than any single stress alone," says Fujita.
The researchers found that UV radiation was the toughest element to survive, and the sporophytes were by far the most resilient of the three moss parts. None of the juvenile moss survived high UV levels or extreme temperatures. The brood cells had a higher rate of survival, but the encased spores exhibited ~1,000x more tolerance to UV radiation. The spores were also able to survive and germinate after being exposed to −196°C for over a week, as well as after living in 55°C heat for a month.
The team suggested that the structure surrounding the spore serves as a protective barrier, absorbing UV radiation and blanketing the inner spore both physically and chemically to prevent damage. The researchers note that this is likely an evolutionary adaptation that allowed bryophytes—the group of plants to which mosses belong—to transition from aquatic to terrestrial plants 500 million years ago and survive several mass extinction events since then.
To see if this adaptation could make the sporophytes fit for the actual conditions of space, the team sent the spores beyond the stratosphere.
In March 2022, the researchers sent hundreds of sporophytes to the ISS aboard the Cygnus NG-17 spacecraft. Once they arrived, the astronauts attached the sporophyte samples to the outside of the ISS, where they were exposed to space for a total of 283 days. The moss then hitched a ride back to Earth on SpaceX CRS-16 in January 2023 and was returned to the lab for testing.
"We expected almost zero survival, but the result was the opposite: most of the spores survived," says Fujita. "We were genuinely astonished by the extraordinary durability of these tiny plant cells."
Over 80% of the spores survived their intergalactic journey, and all but 11% of the remaining spores were able to germinate back in the lab. The team also tested the chlorophyll levels of the spores and found normal levels for all types, with the exception of a 20% reduction in chlorphyll a—a compound which is particularly sensitive to changes in visual light, but this change didn't seem to impact the health of the spores.
"This study demonstrates the astonishing resilience of life that originated on Earth," says Fujita.
Curious how much longer the spores could have survived in space, Fujita's team used the data from before and after the moss's expedition to create a mathematical model. They predicted that the encased spores could have survived for up to 5,600 days—approximately 15 years—under space conditions. However, they emphasize that this number is just a rough estimate, and that a larger data set is needed to make more realistic predictions for how long moss could survive in space.
The researchers hope that their work helps advance research on the potential of extraterrestrial soils for facilitating plant growth and inspires exploration into using mosses to develop agricultural systems in space.
"Ultimately, we hope this work opens a new frontier toward constructing ecosystems in extraterrestrial environments such as the Moon and Mars," says Fujita. "I hope that our moss research will serve as a starting point."
