Methane-producing microbe grows in lab simulation of Enceladus ocean floor

Saturn's icy moon Enceladus may be more supportive of life than previously thought. A study from Ludwig-Maximilians-Universitat Munchen (LMU) shows that microorganisms could survive in its hidden ocean.

From a human perspective, Enceladus is far from a comfortable place to live: it is icy cold, has hardly any oxygen and, below its icy crust, holds a global subsurface ocean whose water is as corrosive as pipe cleaner. Even so, the moon is one of the few places in the solar system where the conditions for life to emerge could be present. A new study supervised by William Orsi, Professor of Geomicrobiology at LMU, presents experiments showing in detail that certain microorganisms could survive there, and how they do so.

Orsi's team recreated in the laboratory the conditions thought to exist on the seafloor of Enceladus. They found that hydrogen-consuming, methane-producing archaea appear able to adapt to an environment previously considered deadly for anaerobic archaea.

The evidence depended on collaboration between disciplines including geomicrobiology, biochemistry, geochemistry and planetary science. The study involved scientists from LMU, the Woods Hole Oceanographic Institution, the University of Regensburg and Freie Universitat Berlin, and was published in the journal Science Advances.

"Enceladus is considered to be one of the most promising places to search for extraterrestrial life," said Dr. Vanessa Helmbrecht, lead author of the study. "Our experiments show that its unique geochemistry could create conditions that are even more favorable for microbial life than we had previously thought."

Beneath Enceladus' thick ice shell lie an ocean of liquid water and a solid core. Data from NASA's Cassini mission showed that the water-rich plumes that regularly shoot up from the ice contain molecular hydrogen, methane and dissolved minerals. That is strong evidence of hydrothermal activity on the seabed and of water-rock geochemical reactions where the rocky core meets the ocean.

The researchers used a special anoxic chamber to create similar deep-sea conditions with a very low level of oxygen, a kind of "Enceladus simulant." The oxygen concentration was roughly 10,000 times lower than the level in Earth's atmosphere. Under these conditions, the researchers used carbonate salts to simulate a hypersaline liquid that replicated both the moon's alkaline soda ocean and its rocky ocean floor.

They then introduced Methanothermococcus okinawensis, a methane-producing archaeon that normally lives near deep-sea hydrothermal vents on Earth. The metabolic pathway this organism uses to conserve energy requires only H2 and CO2 gases, and is considered one of the most ancestral metabolisms still retained by life on Earth.

While the organism failed to grow in a conventional laboratory medium at a pH of 10 or 11, in the Enceladus simulant it continued to grow, producing methane using hydrogen generated by water-rock reactions.

One of the biggest challenges for potential life on Enceladus is the extremely low concentration of carbon dioxide caused by the ocean's high pH. The team demonstrated that Methanothermococcus okinawensis could adjust to these conditions and use its metabolism to scavenge the tiny amounts of available CO2 and keep growing.

"Our findings suggest that the chemistry of Enceladus itself can help overcome this major barrier to life," said Orsi. "The interaction between the rock and water not only produces hydrogen as a source of energy, but also creates conditions that allow microbes to keep accessing carbon, even though CO2 is extremely scarce."

The findings widen the range of conditions under which scientists consider Enceladus potentially able to sustain life.

"Our study doesn't prove that life exists on Enceladus," said Orsi, "but it does show that key geochemical features of its environment can support one of life's most ancient metabolisms. This strengthens the scientific case for future missions to retrieve samples from the moon's ocean-derived plumes."

ESA has recently announced plans to do that with its next major flagship space mission, "L4", currently foreseen to launch in 2042.

CONTACT: Dominic Anders, Ludwig-Maximilians-Universitat Munchen, [email protected]