Stand on the railroad causeway dividing the Great Salt Lake into two different bodies of water, and the tipping point for life in the lake becomes clear.

This week, the blue-green waters of the south arm resonate with the calls of thousands of eared grebes. By contrast, the pinkish water of the north arm, about 3.5 feet lower and over two times saltier than the south, doesn’t attract any birds. But on Tuesday, it echoed with the sounds of an excited scientist tapping on a polygon pattern of rocks that have emerged.

“It’s a rare opportunity. You never see these things,” John Spear, a professor at the Colorado School of Mines, said of the rocks. “The time it took for one of these things to form, it’s quite the process.”

For many researchers throughout the nation, those rocks are what make the Great Salt Lake special. They’re called microbialites and they’re formed by living microorganisms, layer by layer, decade by decade. Microbialites also form the foundation for bigger life thriving in the lake, from brine shrimp to brine flies to birds. And they provide a glimpse of how all life on the planet got its start.

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John Spear, a professor at the Colorado School of Mines, walks along exposed microbialites in the north arm of the Great Salt Lake on May 10, 2016.

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Microbialite rocks are visible in the north arm of the Great Salt Lake on May 10, 2016. The rocks are formed by microrganisms over decades, although the north arm is too salty for them to continue growing.

“Structures like these, in the fossil record, are some of the earliest evidence of life on Earth, dating back 3.5 billion years ago,” said Melody Lindsay, a doctoral student in microbiology at Montana State University. “So being able to study them in a modern environment, and how they might have functioned on early Earth, that’s exciting.”

On Earth today, only a few sites with living microbialites remain. Many, like Hamelin Pool in Australia’s Shark Bay, are protected as World Heritage sites. The Great Salt Lake has around 300 square miles of microbialites. Experts say that’s the largest living crop in the world.

“It’s much bigger, much more extensive, and here we are, walking around, having fun, doing science,” Lindsay said.

She’s worked on analyzing the DNA of the microbialite life in the lake’s south arm and compared it with the microrganisms living in the north arm’s microbialites, partly to figure out what’s building the rocks and what moved in after.

The likely architects of the lake’s microbialites are cyanobacteria. Cyanobacteria string together and weave into mats. Those mats trap carbonate in the lake and cement it into rock over time.

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Melody Lindsay, left, and Dan Coleman take measurements in the north arm of the Great Salt Lake on May 10, 2016. The researchers study organisms that survive in extreme environments at Montana State University.

In the south arm, where the cyanobacteria still thrive, the microbialite rocks continue to grow. Some are the size of a Volkswagen Beetle. Some look like round mounds, some look like mushrooms. Some offer clues of the lake’s long history.

“If you take the causeway all the way to the other side, there used to be a town called Lakeside,” said Bonnie Baxter, director of the Great Salt Lake Institute at Westminster College. “There are giant carbonate structures that are out of the water, vestiges, like a big coral reef. I don’t know at what lake level that accumulated.”

Baxter said she thinks a diversity of microbialite structures live in this lake. While more scientists have taken an interest in the lake’s unique life, they’re still just scratching the surface. But one thing’s clear — the entire ecosystem depends on them.

A thick layer of biofilm covers the rock, feeding brine fly larvae. The microbialite mats also feed brine shrimp when their primary source of food, phytoplankton, gets gobbled up by mid-summer.

Those flies and shrimp, in turn, feed the millions of birds migrating through the lake each year.

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Thousands of eared grebes gather in the south arm of the Great Salt Lake on May 10, 2016. The southern part of the lake is currently around 12 percent salt, and supports millions birds, billions of brine flies and a community of diverse microorganisms.

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Exposed rocky microbialites are visible in the north arm of the Great Salt Lake as seen from the railroad causeway on May 10, 2016. The northern part of the lake is 3.5 feet lower than the south and too salty to support birds, brine shrimp or brine flies.

The microbialites might play other roles, too — like powering the lake with photosynthesis or changing the lake chemistry. 

But in the north arm, the rocks have stopped growing and the causeway’s the culprit. It has effectively sealed off the northern part of the lake from freshwater sources, creating a hyper-saline environment. 

“Right now, in the north arm, all microbes that created microbialites are dead,” said Mike Vanden Berg, a geologist with the Utah Geological Survey. “Maybe when they put a breach in the causeway, and the salinity levels equal out again, maybe some of those microbes will recolonize these old structures.”

Even in the extreme salinity of the north arm, some extremophiles thrive. Peel the salty crust off an exposed microbialite, and there’s a thick, brown, smelly film. Signs of life. 

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Signs of life are visible under the salty crust of an exposed microbialite rock in the north arm of the Great Salt Lake on May 10, 2016.

“Tenants, lots of tenants,” Spear said. “We think they’re not the things that formed them, but the things that are living there now ... we call it builders versus inhabitants, or builders versus tenants.”

Those tenants are likely remnants of when the Great Salt Lake was one lake, before the causeway cut the lake in two.

And while that’s exciting for microbiologists, it’s also a signifies the vulnerability of life elsewhere in the lake. The north arm might still have some microscopic life, but to the birds, it’s essentially dead.

“Brine flies can’t live on the (microbialites) because they’re too salty,” said Maureen Frank, a doctoral student at the University of Utah, at a recent forum hosted by FRIENDS of the Great Salt Lake. “So from a shorebird’s point of view, this habitat up here is no longer available.”

The birds are losing habitat elsewhere on the lake, too. The southern Great Salt Lake is within a couple feet of an all-time low. (The north arm already reached its record low late last year.) As the water drops, salinity goes up and more microbialites become exposed. It threatens the entire food chain depending on those tiny organisms and the big rocks they build.

Given their vitality, Baxter said it’s “high time” for the lake’s microbialite communities to get attention from the world’s scientific community.

“It’s never been more obvious to me … how many different lenses we need to understand these things,” she said. “You can’t be a geologist and study them without thinking about biology. You can’t be a biologist and without thinking about the geology, and there’s a hell of a lot of chemistry going on.”

And with millions of birds depending on the lake’s geologic crusts, “You don’t get much more interdisciplinary than that,” she said.

Contact Reporter Leia Larsen at 801-625-4289 or llarsen@standard.net. Follow her on Facebook.com/leiaoutside or on Twitter @LeiaLarsen

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