Life’s first toehold on Earth was likely in a watery place, and it might have looked something like Last Chance Lake. New research reveals that the small body of water in Canada exhibits the highest known natural levels of phosphate, a compound critical to the formation of RNA in the absence of enzymes. Natural concentrations of phosphate are typically orders of magnitude too low to support that process, but Last Chance Lake—and others like it—circumvents that problem because of its unique biogeochemistry, the researchers suggested.
Last Chance Lake is a shallow, murky lake in southern British Columbia. Its waters don’t even reach a person’s knees, but black, sticky sediments on its bottom lie ready to coat anyone foolish enough to wade in.
Sebastian Haas, a biogeochemist at the University of Washington in Seattle, braved the muck to collect water and sediment samples in 2021 and 2022. By his third trip there, he had learned to navigate its quicksand-like mud by ditching his boots in favor of neoprene booties. “It made for more efficient sampling,” he said.
“That is so much more phosphate than you usually find in the environment.”
When Haas and his colleagues analyzed the samples back in the laboratory, they measured anomalously high levels of phosphate, the inorganic form of phosphorus. The team noted values thousands of times higher than levels typically measured in nature, Haas said. “That is so much more phosphate than you usually find in the environment.”
Last Chance Lake’s phosphate levels are, in fact, comparable to those used in laboratory experiments to create the precursors of RNA. The concentrations of most natural sources of phosphate, on the other hand, are far too low to facilitate making such molecules.
Last Chance Lake—and other lakes like it—therefore offers a work-around to this so-called phosphate problem of the origin of life, Haas and his colleagues suggest. “We are proposing a solution to this phosphate problem,” Haas said.
High, but Why?
To better understand the origin of Last Chance Lake’s off-the-charts phosphate levels, Haas and his colleagues dug into the biogeochemistry of both Last Chance Lake and another lake located just 100 meters away, Goodenough Lake.
The team started by measuring calcium, an element that commonly reacts with phosphate. In both Last Chance Lake and Goodenough Lake, the researchers measured lower levels of calcium in the water than in the lakes’ sediments.
That was a tip-off, Haas said, that calcium was precipitating out of the water to form other compounds, such as calcium carbonate, and settling to the bottom of the lake. Indeed, the team found that a significant fraction of the lakes’ sediments was composed of dolomites, which have a lot of calcium carbonate.
“Normally, calcium is around and gobbles it up.”
Having a high concentration of carbonate on hand is key for locking up calcium, Haas said. And that’s where geology comes in: Both Last Chance Lake and Goodenough Lake sit atop basaltic rock, which weathers to produce both phosphate and the carbonate that eventually bonds with calcium. “That bedrock has a really important role to play in the whole story,” Haas said.
Having less calcium in the waters of the lakes allowed phosphate concentrations to build up over time, said Matthew Pasek, a geoscientist at the University of South Florida in Tampa not involved in the research. “Normally, calcium is around and gobbles it up.” And shallow lakes such as Last Chance Lake and Goodenough Lake additionally undergo a lot of evaporation, which further concentrates phosphate and other compounds.
Hungry Microbes
But despite the similar chemistries and geological setting of Last Chance Lake and Goodenough Lake, the lakes differed in a fundamental way: Average phosphate levels in Goodenough Lake are less than 1% of the values measured in Last Chance Lake. Something must be consuming the phosphate in Goodenough Lake, the research team surmised.
The culprit, the team discovered, was life: Haas and his collaborators observed microbial mats roughly 10 centimeters (4 inches) thick on the bottom of Goodenough Lake that persisted year-round. Similar algae-dominated mats persisted only around the edges of Last Chance Lake during the wet season, however. All that life in Goodenough Lake makes quick work of any phosphate present, Haas said. “It’s taken up by biology and locked up in organic matter.”
Microbial mats aren’t as widespread in Last Chance Lake because its water is up to 13 times saltier than seawater, according to Haas and his colleagues. That inhibits life, allowing the lake to maintain its high levels of phosphate, the team concluded. These results were published in Communications Earth and Environment.
These results are intriguing, Pasek said, but there’s still more to investigate. For instance, follow-up work is necessary to better understand how salty conditions—the very ones that enable high concentrations of phosphate to exist in Last Chance Lake in the first place—might well have also impeded Earth’s earliest life-forms, he said. “It’s hard to say whether or not chemistry would have been tolerant of those high concentrations of salt.”
—Katherine Kornei (@KatherineKornei), Contributing Writer