Members of the research team walk across the surface of Last Chance Lake in September 2022. At the end of the summer, the water has almost all evaporated, leaving a salty crust on the surface. But water persists below in pockets and hollows, and soft sediments sit beneath, creating a somewhat treacherous crème brûlée structure to walk on.Zack Cohen/University of Washington

Phosphate, as a component of RNA, DNA, and cell membranes, is a key element thought to have spurred the origin of life. But the phosphate concentrations required to form these biomolecules in the lab are some 10,000 times higher than levels normally found naturally in water – the proposed medium for life’s origins. Globally, the most phosphate-rich lake yet known is in central British Columbia, explains University of Washington postdoctoral biogeochemist Sebastian Haas. So he plus fellow UW scientists Kimberly Sinclair and David Catling, ventured there to study the biogeochemistry of this enigmatically named Last Chance Lake.

“We need a lot of phosphate for the origin of life, we think,” says Haas. However high phosphate environments are unusual. This is a conundrum referred to as the “phosphate problem” – that life needs a lot of it, but it’s typically rare. So, focusing on phosphorus and nitrogen cycling, his team compared this wetland to one with more typical phosphate levels — neighbouring Goodenough Lake.

Last Chance is very shallow, explains Haas. Water almost entirely evaporates to form a salt crust in dry summers and the lake is only about half a metre deep at its wettest around November. So samples of water, sediment, and salt crust samples were collected in November 2021, June 2022, and September 2022 to explore lake chemistry in variable water volumes.

Characterizing ion concentrations, the team found an abundance of dissolved inorganic carbon, sodium, chloride, sulfate, potassium, phosphate, and magnesium, with lower concentrations of calcium, iron, and dissolved inorganic nitrogen. Dissolved oxygen, pH, and turbidity were also measured with sensors. Stable isotope analysis was used to examine background ratios of Nitrogen-15 relative to Nitrogen-14 and the prevalence of N2-fixation and ammonia volatilization. The mineralogy of salts evaporated during dry periods was examined using X-ray diffraction. Lake water was also bottled for a fertilization experiment examining microbial growth and nitrogen fixation in the presence of 15N-enriched N2.

In most lakes, dissolved phosphate rapidly combines with calcium to form calcium phosphate, the insoluble material in tooth enamel. This removes phosphate from water. But in Last Chance Lake, they found that calcium combines with abundant carbonate and magnesium to form the mineral dolomite. Calcium conversion to dolomite limits its availability as a bonding partner for dissolved phosphate, making phosphate concentrations rise, thus explaining the high phosphate in this unusual lake.

“This study is valuable in that it helps provide mechanistic insights into the factors that permit these alkaline lakes to achieve such extreme phosphorus concentrations,” says geochemist Ben Tutalo at the University of Calgary. “The methods it employs are valid, but this study is really just the tip of the iceberg when it comes to understanding the behaviour of these lakes and their relevance to the origin of life on Earth,” he adds.

Prebiotic organic geochemist Laura Rodriguez of the Lunar and Planetary Institute, in Houston, Texas commended the study for field testing hypotheses developed by those simulating early Earth environments in the laboratory. “A lot of the hypotheses that we put out there make sense, but the natural world is always more complex than what’s happening in our beakers.” There are limits to what can be simulated, she explains.

When we talk about analogs for conditions during the Hadean, the eon preceding life, “it’s never going to be a perfect analog,” says Rodriguez. Nevertheless, when looking for life on other worlds, as her team is doing on Mars, “it’s useful to keep these findings in mind,” she says.

“We know that the Hadean Earth was relatively enriched in iron,” she adds. “So one of my [additional] questions would be, where is the iron? Is it precipitated out?”

“I would like a follow-up with the iron story,” says Rodriguez. Her wish may be granted. In addition to exploring the phosphorus cycle more deeply, Haas plans to probe iron’s role in Last Chance Lake.

This lake, “is not a perfect analog. The water is oxygenated. There is life. But we plan to do an experiment where we turn the lake anoxic and look at the role of iron there,” says Haas.

“In the popular imagination, people tend to think about hydrothermal vents as a competing hypothesis for the origin of life setting,” says Haas. “But we think that early land surfaces and high phosphate lakes like this one were promising, plausible sites for the origin of life.”

BC’s Last Chance Lake may yet provide more hints about how Earth got its first chance at life.