“I am very sure that there is life up there, somewhere in our solar system,” says Christine Moissl-Eichinger, a microbiologist at the Medical University of Graz in Austria. But like any scientist, Moissl-Eichinger knows full well that substantial proof is needed for such a substantial claim. So she and others are working to find that proof—both here on Earth and on Mars.
On the Red Planet, NASA’s Mars rover Perseverance is searching for fossils and traces of alien biochemistry in Jezero Crater, an ancient lake bed thought to have once offered habitable conditions for microbial life. Back home, microbiologists are investigating oxygen-poor environments that may mimic the habitat of early Mars. This two-pronged approach of grounding scientists’ extraterrestrial extrapolations with studies of Earthly analogues could help clarify the bedrock limits for life on rocky planets, greatly aiding the development and execution of future extraterrestrial missions.
The Mars Analogues for Space Exploration project (MASE) was a four-year-long effort that used Earth to understand Mars by analyzing five types of harsh but habitable terrestrial environments that may resemble those that once—or even now—exist on our neighboring planet. Its funding concluded in 2017, but MASE researchers continue to publish results about the habitability of Mars. The study sites included a sulfidic spring, a briny mine, an acidic lake and river, and permafrost. Because of the extreme conditions in these environments, organisms that live here are called extremophiles.
Extremophile research was pioneered by the late Thomas Brock, a microbiologist at the University of Wisconsin–Madison. He found, against all expectations, that certain hardy microbes could thrive in geothermal springs hot enough to poach an egg. The microbiologist’s curiosity led to the isolation of a molecule—from a heat-loving bacterium—that is now used in labs across the world to amplify and sequence DNA. Brock passed away in April, but his legacy lives on.
Brock published his extremophile findings in April 1969, mere months before humans first walked on the moon. This paved the way for astrobiology, the study of life in all its forms on this planet and elsewhere in the universe. Astrobiology is not about making money off of space travel, says Luke McKay, a researcher at Montana State University, who was not involved with that study or Moissl-Eichinger’s recent research. It is about basic science and answering a single, timeless question: Does life exist beyond Earth?
This is a question so profound that so far scientists have only managed to chip away at its edges, with each hard-won revelation usually accompanied by a host of newfound mysteries. Moissl-Eichinger and her team’s chief contribution has been their attempt at cultivating extremophiles from MASE’s five environments, but even this straightforward task has been devilishly difficult. Out of more than 1,000 different extremophile species gathered from those sites, the team managed to grow just 31 in the lab. This is a common struggle in environmental microbiology. Because these microbes live in extreme places, it is difficult for researchers to re-create the exact conditions they require to thrive. To capture more of the diversity, the team’s scientists used genetic sequencing, which allowed i to look at all the microbial DNA in their samples. They specifically searched for genes that may help microbes survive hostile conditions, such as extreme temperatures or the absence of oxygen.
“Cultured [microbial] isolates are not representative of the environment, and that’s why it’s really cool what they did. By using isolates and sequencing, I think they really tried to cover all the bases,” McKay says.
Despite their trouble culturing their extremophile samples, the researchers discovered a vast diversity of microorganisms in all five locations. Even in the most extreme Earthly environments, it seems, life indeed finds a way. Most remarkably, the team’s DNA sequencing revealed 34 unique microbial sequences that were conserved in all MASE sites, which is evidence of microbes surviving a combination of extreme environments. According to Moissl-Eichinger, while many microbes are adapted to live in certain conditions such as intense cold or scant oxygen, it is novel to find a group of microbes adapted to survive a combination of these extreme stressors. This ability to survive many types of environments strengthens the researchers’ claims that similar microbes could exist on Mars—not only in the deep past but even today.
“Microbes are everywhere. They can live in places where we’d expect they could not thrive, but somehow they do,” Moissl-Eichinger says. “Of course, on Mars, we do not know if these [extremophiles] are the types of microorganisms we expect to see. They may just be very adapted to life on Earth.”
One way these microbes may be particularly adapted to life on Earth is their dependence on carbon-based compounds, or organic matter. These are the molecular building blocks of life on Earth and may be rare in some otherwise-habitable extraterrestrial environments. Some microbes in environments with scarce organic matter can instead get nutrients from inorganic substances, such as ammonia and certain sulfur compounds. Yet all the microbes cultured in the MASE studies relied on organic carbon to survive—even those that could survive without oxygen. According to Moissl-Eichinger, this could be because microbes that consume organic matter grow faster. So with more time, she and her colleagues might successfully culture microbes that get their nutrients from other chemical sources, potentially revealing new biochemical pathways and ecological niches to consider when searching for life on Mars.
“We are far away from understanding what microbes could look like on Mars and how we can find them. But of course, research always gives a little piece by piece, and at some point, the picture gets fuller,” Moissl-Eichinger says.
Understanding how all these disparate pieces fit together may change our definition of what it means to be alive. According to McKay, extraterrestrial environments for life may be like those found on Earth, or they could differ vastly. At present, given our sample size of only one confirmed life-bearing world, both possibilities appear equally plausible.
“If [extraterrestrial life] is too [similar] to our life on Earth, people will argue that it is something that we brought with us. But if it is too different, will we be able to see it?” says Moissel-Eichinger, “Now that is the question that drives us.”
Quelle: SCIENTIFIC AMERICAN