They may have the best view of the heavens, but life in space is not easy. Depending on the length of their missions, astronauts can spend months in the company of only a few crewmates. With no regular rising or setting of the sun, their sleep and wake cycles are completely disrupted. And it is not easy to forget that without the protection of their spacesuit or spacecraft, they would perish in 9 to 12 seconds in the icy vacuum of space.
These conditions are not exactly unique to space. Soldiers deployed on long missions in submarines or researchers stationed in Antarctica face similar stresses from harsh environments, disrupted circadian rhythms, and isolation.
Unlike in the depths of the ocean or the frigid planes of the South Pole, astronauts experience the constant weightlessness and radiation exposure only found in space. Microgravity can especially affect an astronaut’s health. For one, it makes it difficult to sleep.
“You can't lay your head on a pillow, which doesn't sound like a big deal, but actually, when you're doing that all your life and all of a sudden you can't do that anymore, it takes a while to get used to,” explained NASA flight surgeon, Richard Scheuring.
Humans did not evolve to live in a microgravity or high radiation environment. Our cells are firmly adapted to life at 1g, so when humans dared to explore the cosmos, our cells were not quite ready for it.
“When we culture cells in space, they don't respond the way they should,” said Brian Crucian, an immunologist and the technical manager of the Immunology/Virology Laboratory at the NASA Johnson Space Center.
“Most of the stressors associated with flight are impacting the immune system,” Crucian said. For example, “just being weightless, the immune cells don't work very well,” he added.
While having a dysregulated immune system in the closed environment of a spacecraft might not seem like much of a problem, humans are not the only living creatures onboard. Hitching a ride on humans themselves, microorganisms from fungi to protozoa to bacteria find their way to space too.
It turns out that space affects microbes as well by causing them to express different genes that often increase their virulence.
The pairing of a sub-optimal immune system with potentially dangerous pathogens is a particularly worrying combination for long missions to the Moon or Mars, where medicine delivery opportunities are few and radiation levels are high. But the extreme environment of spaceflight also presents scientists with unique opportunities to study immune function, microbial pathogenesis, and to isolate novel compounds only produced by microbes under the unique stressors of space. Understanding how space effects both humans and microbes may lead to the discovery of new therapeutics for illnesses encountered both in space and on Earth.
Weightless immune cells go haywire
Scientists don’t have to go all the way into space to study the effects of weightlessness on human cells. They just need to fly to 32,000 feet and then fall to 24,000 feet in about 20 seconds.
By performing rollercoaster-like climbs and drops in midair, an airplane gives its occupants the sensation of about 25 seconds of weightlessness. NASA’s KC-135 airplane, nicknamed the “Vomit Comet,” helped train astronauts for life in space and served as the setting for numerous science experiments on the effects of weightlessness.
On one of these nausea-inducing parabolic flights, Stephen Chapes, a professor emeritus at Kansas State University, studied how immune cells sense gravity (1). Chapes and his team saw that cultured immune cells responded to the change in gravity within just 8 seconds. The cells spread during the periods of microgravity and returned to their normal size when gravity was restored.
“It was really exciting,” Chapes said. “This was the very first time that people were seeing that cells respond to the change in the gravitational field in seconds.”
Immune cells not only change shape in response to gravity, but a number of experiments with immune cells sent to space on various shuttle missions showed that their functions change as well.
For example, when Chapes sent macrophages and T cells to space, he found that they produced more cytokines than the same cells back on Earth (2). It might seem like a good thing to have more of these immune modulating molecules, but an overproduction of cytokines can recruit too many immune cells, resulting in organ damage.
Microgravity is not the only unique aspect of space; there is damaging radiation too.
Galactic ions blast through cells
The types of radiation we are familiar with on Earth — X rays at the dentist office or gamma rays for cancer treatment — are very different from the types of radiation in space.
Astronauts have to worry about two main types of radiation: galactic cosmic rays, which consist of heavy ions and high energy protons originating outside of our solar system, and high-energy protons released by the Sun during solar flares, called solar particle events.
While in low Earth orbit, like on the International Space Station, astronauts are protected from the majority of these kinds of radiation by the Earth’s magnetosphere, but they can be exposed to secondary radiation.
However, when astronauts leave the protection of Earth’s magnetosphere to embark on future missions to the Moon and Mars, they lose that protection, resulting in damage to their cells.
“The majority of background radiation in deep space is smaller particles like protons, but then you’ve got all these bigger ions from helium all the way up to iron particles,” said Afshin Beheshti, a bioinformatician and principle investigator at KBR/NASA Ames Research Center.
If gamma rays on Earth are the sand grains of radiation, iron particles are the bowling balls. Gamma particles go through the body and scatter, but when heavy particles like iron ions hit cells, they deposit all of their energy there. “It causes all this damage in the cell because it sits in there, in a sense,” said Beheshti.
To study the effects of space radiation on the human immune system on Earth, scientists head to NASA’s Space Radiation Laboratory at Brookhaven National Lab. There, a particle accelerator creates a variety of heavy ion and high energy proton beams that allow scientists to simulate the kinds of radiation exposures astronauts face in space.
In a recent study, Beheshti investigated how the combination of microgravity and space radiation affected mouse immune cells (3). His team found that galactic cosmic rays heavily suppressed T helper and T cytotoxic cell functions, and high doses of protons simulating a solar particle event suppressed T cytotoxic cells as well.
“But what really was surprising,” Beheshti said, was that “gamma [radiation] didn’t do that. It’s really unique to space radiation.”
In addition to studying the acute effects of space radiation on the immune system, NASA also wants to know the effects of long-term exposure.
On a mission to Mars, for example, “You can have a long-term protracted exposure [to] galactic cosmic rays,” said Evagelia Laiakis, a radiation scientist at Georgetown University. “A lot of the studies that we do here on Earth are acute exposures, meaning that you give all of the dose at once, versus a protracted exposure, which will be like a two-year mission.”
Laiakis recently found that exposing mice to high energy particles at 1-, 2-, and 4-month time intervals led to changes in the metabolomes of their spleens (4). By dosing the mice at subsequent time intervals, Laiakis and her team hopes to model the effect of chronic radiation exposure on the important immune organ.
“I was surprised that a low dose of radiation can have such long-term effects on metabolism,” Laiakis said. “We don't know whether it's one specific population of immune cells that is being affected that is driving the changes, or if it’s the collective population of immune cells in the spleen.”
The key question is how the changes caused by long-term radiation exposure affect the immune system as a whole.
“What if there was a challenge with bacterial infections or viral infections?” Laiakis asked. How would the immune system respond?
Chicken pox in space
Along with microgravity and radiation, isolation, altered circadian rhythms, and the general stress of being in space affect astronauts’ immune function.
Crucian has studied the effects of spaceflight on the immune systems of astronauts for decades and has seen numerous changes to astronaut immune systems in space.
One of the clearest indicators of a dysfunctional immune system in space, he said, is viral reactivation.
Typically when we get sick with a virus, we eventually clear the infection and have no trace of viral particles in our body. But rather than getting cleared by the immune system, some viruses such as the chicken pox or herpes virus, integrate their viral DNA into our cells.
T cell are the main immune cells keeping latent viruses in check, but if a virus manages to evade them, it can reactivate. For example, when the chicken pox virus reactivates, it causes shingles. This reactivation may occur in later adulthood in people infected with the virus as a child.
“We see these viruses reactivating in astronauts, not to the point where they have shingles or any disease. But we can detect viruses in their saliva, in their blood, [and] in their urine,” Crucian said. “The immune cells not functioning well is a bonafide, legitimate, in-the-astronaut phenomenon because the astronauts are shedding the virus.”
If astronauts shed chicken pox viral particles in the closed environment of the International Space Station, for example, will their fellow astronauts get infected?
“We haven't seen transfer of latent viruses,” said Scheuring, who has served as a NASA flight surgeon for 17 years. “They're generally not coming in physical contact with the other astronauts, even though they are in close proximity.”
While they may not be getting sick from latent viruses or spreading them, astronauts still get sick in space.
“For ridiculously healthy people that went to a pre-flight quarantine that are now essentially in a quasi-isolation chamber device, [you’d expect] the number [of illnesses] to be zero,” Crucian said. “They’re not. We see some minor infectious disease. There's some atopic dermatitis and skin rashes that persist.”
On long missions lasting years in deep space where astronauts have no rapid return option, reduced ability to exercise, and little to no opportunity for a food resupply mission, their stress will likely magnify, leading to further dysregulation of the immune system.
“For some types of issues, we're worried about it blossoming into something more serious,” Crucian said.
One potential serious outcome? Infection by a pathogen onboard.
Space microbes come along for the ride
From the early Soyuz missions in the 1960s to the missions on the International Space Station today, microbes have hitched a ride on humans to get to space. Like humans, microbes evolved in Earth’s gravity, so when they find themselves relinquished from its grasp, their biology gets pretty interesting.
A bacterial or fungal cell is used to settling on a surface, but “when it is floating, that is where the physiology gets screwed,” said Kasthuri Venkateswaran, an astrobiologist at NASA Jet Propulsion Laboratory.
Faced with microgravity and increased radiation, some bacteria produce more extracellular polysaccharides to help them form biofilms to better adhere to surfaces. Other microbes begin to produce different compounds than they would on Earth as a form of self-defense against other microbes. When researchers flew the bacteria Salmonella typhimurium on shuttle mission STS-115, for example, they saw that it altered its gene expression and killed mice more quickly than ground-based control bacteria (5).
In adapting to the harsh conditions of space by producing biofilms and anti-microbial compounds, microbes can also become more virulent, potentially putting the humans onboard in danger.
Luis Zea, an aerospace engineer focused on bioastronautics at Colorado State University, recently received samples of the bacteria Pseudomonas aeruginosa back from space. He noticed that the bacteria that came back from space produced different colored pigments than the equivalent samples back on Earth.
“There’s a lot of pinkish and brownish and greenish on the samples that flew to space,” Zea said. These pigments often associate with virulence in P. aeruginosa. “Just by the fact of being exposed to the stress of flight, they may have been activating all these pathways that make them more virulent.”
When scientists say that microbes become more virulent in space, they are not suggesting that microbes are evolving. The microbes are simply adapting to a new environment.
“Evolution cannot happen overnight,” Venkateswaran said. The International Space Station has only been around for 22 years. “In 20 plus years, microbes cannot become a new species. It might acquire adaptability for surviving such conditions by changing a few [pieces of] genetic machinery, but it cannot be a new superbug.”
In a recent analysis of the microbiome onboard the European and American sections of the International Space Station, Maximilian Mora, then a PhD student at the Medical University of Graz in Austria, found that while microbes adapted to space by forming more biofilms and interacting with surfaces, they did not evolve to become more dangerous to humans (6).
“If we could have reported, ‘they are all mutating to killer bacteria!’” Mora said, “there would be much more buzz about the paper.”
That’s not to say that the changes to bacteria in space don’t still look like something out of a science fiction movie. “I was alone in the lab, and I had to put this slimy bacteria [from the space station] on a new plate. I said, ‘the overworked PhD student working with a slimy bacterium from space almost coming out of the plate, working alone at night,’ that’s how the horror movies start,” Mora laughed.
The exciting new biology microbes exhibit in space may seem out of this world, but scientists are finding that it has implications for human health back on Earth as a source of new therapeutics.
New medicines for humans on Earth and in space
In stressful environments, fungi and bacteria produce compounds called secondary metabolites, which are not essential for their growth or reproduction, but can be integral for defense. Many secondary metabolites are antibiotics or antifungals that kill off competing microbial species. Penicillin is probably one of the most famous fungal secondary metabolites. These compounds can also take the form of toxins or pigments — whatever may give a microbe an advantage over another species.
Given that space is a very stressful environment, it gives scientists the perfect laboratory to look for new compounds that may benefit human health.
In a recent study, Venkateswaran and his team isolated a new strain of the filamentous fungus, Aspergillus niger, from the International Space Station (7). A. niger is a melanized fungal species often used to produce citric acid and enzymes important in the biotechnology sector.
After performing metabolomic analyses on the new strain, they found that it produced the antioxidant pyranonigrin A, which “can be used as a therapeutic agent for radiation,” Venkateswaran said.
The space station A. niger strain produced higher levels of many other secondary metabolites in space that its corresponding strain on Earth did not, including bicoumanigrin, which has shown some in vitro anti-cancer activity (8).
In addition, some microbes might survive better in space than they do on Earth, and may produce useful compounds in space that would have been difficult or impossible to identify on Earth. Venkateswaran and his team collaborate with pharmaceutical scientists at the University of Southern California to characterize novel compounds that microbes produce in space for their potential use as new therapeutics.
“You don't have a FedEx to send things to Mars,” Venkateswaran said. He imagines that in the future, scientists might send astronauts instructions for isolating certain compounds from particular microbes found on a spacecraft to benefit their health while on a mission.
“But we are not there yet,” he said.
Safer space travel
Studying the human immune system and microbes in space has helped researchers better understand human health and microbial biology here on Earth. These studies also help researchers develop new technologies and interventions to make space travel safer.
In addition to supplying Mars-bound astronauts with a panel of vaccinations, nutritional supplements, and medications, Crucian also anticipates providing them with miniaturized microfluidic devices to help monitor their white blood cell counts or detect a reactivated virus.
“If I'm halfway to Mars, and there's a solar particle event, we might want to monitor these things before we dose with certain medications,” he said. He and his team are also collaborating with scientists at St. Michael’s College in Vermont to develop a virtual reality headset to relieve some of the stress associated with spaceflight.
While space may be a stressful place, understanding how our biology changes because of it will help humans travel further and longer than ever before, exploring new worlds and making new discoveries.
“Space research is exciting,” said Laiakis. “There is so much we still need to learn, but I don't think that is really stopping us from exploring space.”
- Armstrong, J.W. et al. The Effect of Space and Parabolic Flight on Macrophage Hematopoiesis and Function. Experimental Cell Research 216, 160-168 (1995).
- Chapes, S.K. et al. Cytokine secretion by immune cells in space. Journal of Leukocyte Biology 52, 104-110 (1992).
- Paul, A.M. et al. Beyond Low-Earth Orbit: Characterizing Immune and microRNA Differentials following Simulated Deep Spaceflight Conditions in Mice. iScience 23, 101747 (2020).
- Laiakis, E.C. et al. Effects of Low Dose Space Radiation Exposures on the Splenic Metabolome. Int. J. Mol. Sci. 22, 3070 (2021).
- Wilson, J.W. et al. Space flight alters bacterial gene expression and virulence and reveals a role for global regulator Hfq. PNAS 104, 16299-16304 (2007).
- Mora, M. et al. Space Station conditions are selective but do not alter microbial characteristics relevant to human health. Nat Commun 10, 3990 (2019).
- Romsdahl, J. et al. Metabolomic Analysis of Aspergillus niger Isolated From the International Space Station Reveals Enhanced Production Levels of the Antioxidant Pyranonigrin A. Front. Microbiol. 11, 931 (2020).
- Hiort, J. et al. New natural products from the sponge-derived fungus Aspergillus niger. J. Nat. Prod. 67, 1532-1543 (2004).