Microbes fit for space exploration

Written by Rashmi Shivni

As scientists begin to find water and other trace elements necessary for life in our solar system, the concern about microbial contamination between Earth and space grows. Researchers have collected evidence of the potential for bacterial transfer during space missions and are now using tiny terrestrials when exploring the final frontier.

Landing on the moon

Apollo 12 astronaut, Pete Conrad, harvests Surveyor 3’s parts to bring back to Earth for scientific analysis. Weeks later, scientist found something even more surprising – a colony of Streptococcus mitis (1).

Credit: NASA

On November 19, 1969, Apollo 12 astronauts touched the lunar surface. Several yards away sat the lifeless Surveyor 3, a probe that made its permanent home on the moon 2 years earlier. One objective of the Apollo 12 mission was to fetch some of the probe’s components for scientific examination. A few weeks after the astronauts’ homecoming, scientists discovered a Streptococcus mitis stowaway colony inside the harvested television camera (1).

For decades, debate ensued over the bacteria’s survival on the near-vacuum lunar surface. Many scientists believed the Surveyor 3 camera was contaminated after returning to Earth due to poor handling protocols. Regardless of when contamination took place, the incident forced astrobiologists to become more cognizant of microorganisms when planning future space exploration missions.

“Microbes are extremely resilient,” said Rika Anderson, an assistant biology professor at Carleton College in Minnesota. “They rapidly adapt to almost any environment they’re thrown into. They teach us about our past and could help us understand life in our future or outside our planet.”

Adapting with No Bounds

Most modern organisms require specific conditions to survive: liquid water, oxygen, moderate temperatures, stable aboveground air pressure, and low-level radiation exposure. However, not all terrestrial organisms live harmoniously under such moderate conditions; in fact, there are extremophiles living in generally life-threatening environments, and several “ordinary” microbes are capable of readily adapting to extreme conditions.

“Studying microbial evolution, especially in extreme environments, can teach us how life originated and spread on this planet. It can also teach us about the potential for life to survive on other worlds in our solar system,” Anderson said.

For example, ancient sulfur-oxidizing bacteria once lived in Earth’s oxygen-poor oceans, which may be analogous to Venus’s high-pressure, sulfuric atmosphere. Microbial communities from deep-ocean hydrothermal vents might thrive well in chemically active subsurface oceans on Saturn’s moon Enceladus or Jupiter’s moon Europa. And archaea frozen within frigid Antarctic ice could potentially fare well inside Pluto’s icy heart.

These hypothetical scenarios suggest that terrestrial microbes could be used as models for foreign biomes. But could microbes adapt to space or near-vacuum environments not naturally occurring on Earth?

According to a meta-analysis by the International Space Station’s (ISS’s) microbial research teams, humans bring large microbiomes on board even after going through pathogen-removal quarantine for 10 days before entering the spacecraft. Lingering microbes sparked several ideas in astrobiologists wanting to see how microorganisms change in the ISS and space shuttle’s closed-system microgravity environments.

Between 2006 and 2008, researchers sent several Salmonella typhimurium samples into low-Earth orbit on the space shuttles Atlantis and Endeavour. Once returned to Earth, the S. typhimurium was injected into mice and found to be more virulent than usual (2).

“The results of the study were dramatic. [Mice] infected with bacteria grown in-flight displayed a decreased time to death, increased percent mortality, and decrease in the lethal dose,” researchers reported in the ISS study.

Alongside microgravity and long-term indoor conditions, astrobiologists inquired if microorganisms could prosper in radiation-rich conditions, such as the exterior of space stations or under Mars’ thin atmosphere. In 2007, researchers simulated Martian terrains using desiccated Atacama Desert soil irradiated with ultraviolet lights. They introduced Bacillus to this manufactured environment and observed that the species was able to adapt to both deadly conditions. The researchers also speculate that this species could withstand the interplanetary trip to Mars (3).

Fear of Spreading Germs

These findings left scientists wondering if the ability of microbes to adapt to new environments poses greater contamination threats than originally thought.

“This is a huge concern, especially with plans to send people to Mars in the near future and potential places for colonization in the far future,” said Angela Zalucha, a research scientist at the SETI Institute. “With the presence of liquid water, carbon, and other elements needed for life, we have to be careful when approaching Mars.”

The Surveyor 3 incident was actually the first in a series of possible contamination events. In 2014, 2 years after the Curiosity rover was launched, scientists revealed that 377 strains of 65 microbial species were on the vehicle prior to launch. Researchers gathered this information while swabbing the rover during assembly and tested the samples under simulated UV radiation, desiccation, cold, and pH extremes. Eleven percent of the 377 strains survived the simulation, and now scientists believe microbes could have survived the ride through space and settled on the Martian surface.

According to Anderson and Zalucha, there’s an entire wing at NASA called the Office of Planetary Protection dedicated to contamination prevention. For the next rover mission—Mars 2020, which is a collect and cache mission for future sample return—scientists on Earth will thoroughly investigate samples for signs of ancient Martian microbial life. Planetary Protection committees are currently setting forth stringent microbe scrubbing protocols to prevent similar outcomes that came to light from the Curiosity rover studies.

“Humans are walking bags of microbes,” Zalucha said. “We take microbes everywhere we go. With that said, we also need to be careful of what we bring back to Earth. Contamination works both ways, so it’s of utmost importance that we know how to manage microbes in space.”