A team of interdisciplinary astrobiologists from NASA and other agencies is homing in on recognizing the microbial biosignatures for life, making it easier someday to identify life on other planets.

A scientific paper analyzing the team's research results, titled "Modern Freshwater Microbialite Analogues for Ancient Dendritic Reef Structures," was published in the journal Nature on October 5. The paper focuses on the study of mounded microbialite deposits - layers of living and non-living organisms - found at Pavilion Lake in Canada.

Microbialites are organic sedimentary mineral deposits covered by a thin layer of microbes that become entombed in the mounds as they grow outward.

"These unique and rare microbialite formations are important to NASA's astrobiology effort because they are big, macroscopic evidence of microscopic life," said Dr. Chris McKay, a scientist with the NASA Astrobiology Institute and one of the paper's authors. "They are helping us understand one of the big astrobiology questions - how early life took hold and began to flourish on Earth. These fossils are like seeing a billion-year-old footprint in the sand and comparing it to a modern human foot," he said.

NASA scientists and others began studying the mounded deposits growing in Pavilion Lake, British Columbia, Canada, in 1998. The microbialites were formed layer by layer with the oldest on the bottom. This structure provides a record of growth and yields important clues about the organisms that once lived there.

The odd-looking mound formations, discovered by recreational divers in 1997, are unique, scientists say, and differ from the Earth's oldest known structures called stromatolites, 3.5-billion-year-old formations in Western Australia, which show no direct evidence of life.

The microscopic organisms at Pavilion Lake created large visible structures that scientists need to explore further, according to McKay. The mounded structures are relatively young in geological terms, he added, being only about 12,000 years old.

"At spring-fed Pavilion Lake, we studied the rock textures in the mounds that reflect the different roles played by the living microbes who created the microbialites versus the natural environmental mineral precipitation activity there," said Dr. Sherry L. Cady, assistant professor of geology at Portland State University and editor of the journal Astrobiology. "It's a unique natural laboratory," she said. Cady analyzed the microbialites using electron microscopy to determine if the mounds were of organic (biological) or inorganic origin. "Teasing out which components of the mounds were biological is key," Cady said.

According to McKay, the first to dive and view the mounds, they are especially interesting to astrobiologists because their shape changes as the lake gets deeper. "The stuff on top was soft like cauliflower, while the stuff on the bottom was hard like artichokes. I had never seen anything like this before in the world, especially in a freshwater lake," he said. 

What Next?

Research is still ongoing about the different types and textures of mounds that formed in different parts of the lake, he added.  The research is also important because scientists may find similar structures on Mars, McKay said.  "One goal of this work is to help us better understand and recognize carbon-based microbial biosignatures for life on Mars." This research will help target future Mars landing sites where life is most likely to be found and help researchers fine-tune tools for Mars sample return missions, he said.

"When we walk on Mars, it'll be hard to spot a microscopic-sized fossil.  But if we see stromatolities or microbialites, we can send a rover there, and it may turn out to be a marker for Mars life."  Pavilion Lake is located in a box canyon, similar to some of the canyon features on Mars, according to McKay. Scientists say future research at the site will include a winter dive under the ice and studying the formations for a full year, under seasonal conditions.

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