New research led by Stanford University scientists predicts that life could survive in extremely salty environments, beyond the limits of what was previously thought possible. The study, published Dec. 22 in the journal Science Advances, was based on analysis of the metabolic activity of thousands of single cells in saltwater from industrial ponds along the coast of Southern California.
The findings expand our understanding of potentially habitable space throughout the solar system, as well as the possible consequences of some of Earth's aquatic habitats becoming saltier due to drought and water diversion.
"We can't look everywhere, so we have to really think carefully about where and how to look for life on other planets," said study senior author Anne Dekas, assistant professor of Earth system science at Stanford University's Dole School of Sustainability. "Learning as much as possible about where and how life exists in extreme environments on Earth allows us to prioritize life-detection missions elsewhere and increase our chances of success."
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 can also preserve signs of life, such as pickles in brine. "We think places with salt are good places to find signs of past or present life," said study lead author Emily Paris, a doctoral student in Earth system science and a member of the Decas lab. "Salt could be what makes another planet habitable, although high concentrations of salt on Earth are also inhibitors of life."
The new research is part of a larger collaborative project 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: Understand how the ocean world and life co-evolved, producing detectable signs of life in the past or present. Understanding the habitable conditions in ocean worlds and developing better ways to detect signals of biological activity are steps toward predicting the potential for life elsewhere in the solar system.
Effects of salinity changes on the Earth
We should consider the impact of changes in salinity on Earth's ecosystems. For example, falling water levels in Utah's Great Salt Lake are causing an increase in salinity, which could affect all life in the food chain.
"In addition to being from a life detection perspective, understanding the effects of salinity is also important for the conservation and sustainability of the planet," Paris said. "Studies have shown how increasing salinity changes the composition of microbial communities and the rate of microbial metabolism. These factors can affect nutrient cycling and the life of crustaceans and insects, which are important food sources for migratory birds and other aquatic animals."
Discovering life in the saltiest waters on Earth
When marine research teams visit salt ponds like the South Bay Salt Works—where the samples for this study were collected—or those along the San Francisco Bay, they find a kaleidoscope of neon green, rust red, pink and orange glowing from some of the most active microorganisms on Earth. The patchwork of colors reflects the ability of aquatic microorganisms to adapt to survive in different salinities, what scientists call "water activity" -- the amount of water available for biological reactions in which the microorganisms grow.
"We're interested in knowing at what point the water activity becomes too low, when the salinity becomes too high, and when microbial life can no longer survive," Parris said. Seawater has a water activity level of about 0.98, while pure water has a water activity level of 1. Most microorganisms stop dividing when their water activity falls below 0.9, and it has been reported that the absolute minimum water activity required to sustain cell division in a laboratory setting is slightly above 0.63.
In new study, researchers predict new limits to life. They estimate that life could be active at water activity levels as low as 0.54.
Stanford scientists worked with colleagues from across the country to collect samples from the South Bay Saltworks, home to some of the saltiest water on Earth. They collected brine from ponds of varying salinity at the salt plant, filled hundreds of bottles, and shipped them back to Stanford for analysis.
discover life faster
Previous studies looking for the water activity limit 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. If researchers relied on cell division to test when life ends, they would face years of lab experiments, which is not realistic for graduate students like Paris. Even studies of cell division cannot tell when life dies; in fact, cells may be metabolically active and remain vibrant even if they are not replicating.
So Paris and Decas studied microorganisms in open-air salt ponds to determine another limit of life—the limits of cellular activity.
The research team made three key improvements on previous studies. First, they didn't use pure cultures, which are scientists' best guess at which particular species or strain would be the hardiest, but an actual ecosystem. In salt pans, the environment naturally selects for complex communities of organisms best adapted to these specific conditions.
Second, the researchers adopted a more flexible definition of life. They saw not only cell division but also cell building as a hallmark of life. "It's a bit like watching humans eat or grow. It's a sign of active life and a necessary prelude to replication, but it's much faster to watch," Decasse said.
Across hundreds of brine samples, some of which were as salty as molasses, they determined the activity levels of the water and how well the carbon and nitrogen in the brine were incorporated into the cells. With this method, they were able to detect how much the cells' biomass increased, with the smallest increase being just half of 1%. In contrast, traditional methods that focus on cell division can detect biological activity only after the cell's biomass has roughly doubled. Then, based on how the process slows down as water activity decreases, the scientists predict the process will stop altogether.
Third, while other scientists measured the massive incorporation of carbon and nitrogen into brine, the Stanford team conducted a cell-by-cell analysis using a rare Stanford instrument, NanoSIMS, one of only a handful in the United States. This sensitive technique allowed them to observe the activity of individual cells among other "marinated" cells whose presence would mask the activity signal in bulk analyses, allowing for low detection limits.
"Single-cell viability analysis of environmental samples is still very rare," Decasse said. "Single-cell viability assays are key to what we do here, and as single-cell viability assays become more widely available, I think we will see advances in microbial ecology that have broad relevance, from understanding global climate to human health. Our understanding of the microbial world at the single-cell level is only beginning."
Compiled from /ScitechDaily