A new study of microorganisms in extremely salty water suggests that life may survive conditions previously thought to be uninhabitable. This research broadens the possibility of discovering life throughout the solar system and shows how changes in salinity affect life in aquatic habitats on Earth.
The research is part of a larger collaborative project called Oceans Across Space and Time, led by Britney Schmidt, associate professor of astronomy in Cornell University's College of Arts and Sciences and associate professor of earth and atmospheric sciences in Cornell Engineering. The project, funded by NASA's Astrobiology Program, aims to understand how ocean worlds and life co-evolved, producing detectable signs of life in the past or present.
The new study, titled "Single-cell analysis in hypersaline brines predicts water activity limits for microbial anabolic activity," was recently published in the journal Science Advances. The study is based on analysis of the metabolic activity of thousands of single cells in saltwater from industrial ponds along the Southern California coast.
Research led by Stanford University expands our understanding of potentially habitable space throughout the solar system and the possible consequences of some of Earth's aquatic habitats becoming saltier due to drought and water diversion.
"Saline environments exist throughout the solar system, from Mars to Jupiter's moon Europa. Understanding how microbes interact with and survive this environment on Earth is critical to the search for life elsewhere," Schmidt said.
Scientists interested in detecting life beyond Earth have long studied salty environments because they know liquid water is necessary for life, and salt allows water to remain liquid over a wider range of temperatures. Salt also preserves signs of life, such as pickles in brine.
The multi-institute team collected samples from the South Bay Salt Works, home to some of the saltiest waters on Earth. They filled hundreds of bottles with brine from ponds of varying salinity at the salt factory and analyzed the brine.
Most microorganisms stop dividing at a water activity level below 0.9 (the amount of water available for biological reactions for microbial growth), while the absolute minimum water activity level reported to sustain cell division in a laboratory setting is slightly above 0.63. Researchers have predicted new limits for life, estimating that life could be active at levels as low as 0.54.
Previous studies looking for the activity limits of water of life have used pure cultures to look for the point at which cell division ceases, marking the end of life. But under these extreme conditions, the doubling of life is painfully slow. The study of cell division does not tell when life dies; in fact, cells may be metabolically active and remain vibrant even if they are not replicating.
Instead, the researchers considered the limits of cellular activity as a more flexible definition of life, arguing that both cell division and cell building are hallmarks of life.
Across hundreds of brine samples, some of which were as salty as molasses, they determined the activity levels of the water and how much carbon and nitrogen the cells in the brine absorbed. With this method, they were able to detect when a cell's biomass increased, even by half (1%). In contrast, traditional methods that focus on cell division can detect biological activity only after the cell's biomass has approximately doubled. Then, based on how the process slowed down as water activity decreased, the scientists predicted the dividing line for when the process would stop completely.
This study challenges previous ideas about the activity limits of living water. While most microorganisms stop dividing at water activity levels below 0.9, this study shows that life is active at levels as low as 0.54. By focusing on cellular activity, including cell building, researchers are able to detect signs of life in conditions undetectable by traditional methods.