A Stanford study of microbes in hypersaline water shows that life can survive in conditions previously thought to be uninhabitable. The research expands the possibilities for detecting life in our solar system and shows how changes in salinity can affect life in aquatic habitats on Earth.

A new study led by Stanford University scientists suggests that life can survive in hypersaline environments, beyond the limits previously thought possible.

Study published on December 22 Science Developments is based on analysis of metabolic activity in thousands of individual cells in the brine of industrial ponds off the coast of Southern California, where salt is collected by evaporating water from seawater. The results expand our understanding of the potential habitable space in our solar system and the possible consequences of some terrestrial aquatic environments becoming saltier as a result of drought and water outflow.

Search for extraterrestrial life

“We can’t look everywhere, so we need to be very careful about where and how we try to find life on other planets,” said senior study author Ann Decas, an assistant professor in the Department of Earth System Sciences at Stanford’s Doerr School. Sustainability. . “Having as much information as possible about where and how life survives under extreme conditions on Earth allows us to prioritize targets for life discovery missions elsewhere, increasing our chances of success.”

Scientists interested in discovering life outside Earth have long studied salty environments, knowing that liquid water is essential for life and that salt allows water to remain liquid over a wider range of temperatures. Salt can also preserve signs of life, such as pickled pickles. “We think salty places are good candidates for looking for signs of past or present life,” said study lead author Emily Paris, a graduate student in Earth systems science who is part of the Dekas Laboratory. “Salt may be what makes another planet habitable, but it is also an inhibitor of life on Earth in high concentrations.”

“Having as much information as possible about where and how life survives under extreme conditions on Earth allows us to prioritize targets for life discovery missions elsewhere, increasing our chances of success.”

— Ann Decas, Associate Professor of Earth System Sciences

The new research is part of a larger collaboration called Oceans Across Space and Time, led by Cornell University professor Britney Schmidt and funded by NASA’s Astrobiology Program, which brings together microbiologists, geochemists, and planetary scientists. Their goal: To understand how ocean worlds and life combine to produce visible signs of life, past or present. Understanding the conditions that make ocean worlds habitable and developing better ways to detect signals of biological activity are steps toward predicting where life might be found in the Solar System.

Impact of changing salinity on Earth

Paris says we also need to consider how changing salinity affects Earth’s ecosystems. For example, falling water levels in the Great Salt Lake in Utah have caused increased salinity, which can affect life at every stage of the food chain.

Discovery of life in the saltiest waters on Earth

Travelers flying over salt ponds like those at the South Bay Salt Works, where samples were collected for this study, or across the San Francisco Bay can see a kaleidoscope of some of the world’s most powerful microbes glowing in neon green, rust red, pink and orange. . The variety of colors reflects the array of aquatic microbes adapted to survive at different levels of salinity, or what scientists call “water activity”—the amount of water available for biological reactions that allow microbes to grow.

“We are interested in knowing at what point water activity becomes too low, when salinity becomes too high, and where microbial life can no longer survive,” Paris said. The water activity level of seawater is approximately 0.98 compared to 1 for pure water. Most microbes stop dividing at a water activity level of 0.9, and the absolute lowest water activity level reported to support cell division in the laboratory is just above 0.63.

In a new study, researchers predicted that life will have a new frontier. They estimate that life may have been active at 0.54.

Stanford scientists joined colleagues from across the country to collect samples from the South Bay salt pans, home to some of the saltiest waters in the world. They filled hundreds of bottles with brine from varying salinity ponds at the Saltworks, then took them back to Stanford for analysis.

Find life faster

Previous studies investigating the limit of life’s water activity have used pure cultures to find the point at which cell division stops; This marked the end of life. But in these extreme conditions, life doubles very slowly. If researchers rely on cell division as a test for when life will end, they will face years of laboratory experiments that are impractical for graduate students like Paris. Even if done, studies of cell division do not indicate when life dies; Indeed, cells can be metabolically active and still very much alive even when not proliferating.

So Paris and Decas studied microbes in outdoor salt ponds to determine another limit of life: the limit of cellular activity.

The research team made three important improvements over previous studies. First, instead of using pure cultures, which are scientists’ standard best guess as to which species or type of microbe will be most resistant, they went to an actual ecosystem. Salt works are a naturally selected environment for a complex community of organisms best adapted to these special conditions.

Second, researchers used a more flexible definition of life. They viewed not only cell division but also cell formation as a sign of life. “It’s a bit like watching a person eat or grow. This is a sign of active life and a necessary precursor to proliferation, but it is much quicker to observe,” Decas said.

In hundreds of samples of brine, some so salty and thick as syrup, they determined the level of water activity and the amount of carbon and nitrogen incorporated into cells in the brine. With this approach, they were able to detect that a cell increased its biomass by only half of 1%. In contrast, traditional methods targeting cell division can only detect biological activity after roughly doubling the biomass of cells. Scientists then predicted that the closure would stop altogether, based on how this process slowed as water activity decreased.

Third, while other scientists measured the carbon and nitrogen content of brines at the bulk level, the Stanford team performed cellular analysis at Stanford using a rare device called nanoSIMS, one of only a few in the country. This sensitive technique allowed them to observe activity in individual cells amidst other “pickled” cells, as their presence could obscure the activity signal in bulk analysis, and achieve low detection limits.