Laguna Caliente in Costa Rica
A researcher analyzes minerals near the banks of Laguna Caliente. Credit: Geoffroy Avard, OVSICORI-UNA

Nestled on a stratovolcano in Costa Rica, Laguna Caliente is one of the most inhospitable places on Earth. Its ultra-acidic waters, heated by magma, can approach boiling. Clumps of sulfur float on its steamy surface, which ranges in color from bluish green to yellow. But is it a dead zone? Apparently not, new research has found.

A team of scientists sampled the lake’s waters and showed that Laguna Caliente contains life but predominantly just one form of it: a single genus of the bacterium known as Acidiphilium (“acid lover”). This is a surprise because most ecosystems on Earth are home to diverse communities or utterly devoid of life, said Brian Hynek, a planetary scientist at the University of Colorado Boulder who led the work.

“Laguna Caliente is one of the most extreme habitats on our planet and may well represent the edge of the habitable range,” he and his team wrote in a paper accepted for publication in Astrobiology. This place is otherworldly, the scientists also note; it probably resembles ancient Martian terrain from that planet’s wetter, more volcanic days. Hence, bacteria like those prevalent in Laguna Caliente may have thrived on Mars in the past, they propose.

Foreign and Wild

“Even in a full gas mask, your eyes are tearing up.”

Hynek and his team traveled to central Costa Rica in 2013 to investigate the minerals around Poás volcano and its acidic crater lake. They found an unpredictable and dangerous landscape: Geyser-like eruptions from Laguna Caliente launched ash and mud hundreds of meters, and sulfuric acid and hydrochloric acid permeated the air. “Even in a full gas mask, your eyes are tearing up,” said Hynek. “It’s a foreign, wild environment” with a geochemistry similar to that of Mars. What life-forms, the researchers wondered, might exist in this harsh place?

Hynek and his colleagues collected water and sediment from Laguna Caliente using test tubes attached to a 2-meter-long aluminum painting rod. The pole protected the researchers’ hands from the water, which had a pH of 0.29, 50 times more acidic than stomach acid. The researchers froze the samples and brought them back to Colorado for analysis. In the lab, the scientists extracted and sequenced the DNA entrained in the samples. This so-called environmental DNA revealed the organisms that had passed through Laguna Caliente’s waters.

Just a Single Thing

The scientists found that 98% of the environmental DNA from Laguna Caliente could be traced to that one genus of Acidiphilium. “There’s just a single thing there,” said Hynek. More intriguing, none of the known species of the Acidiphilium genus tolerate pH levels as low as Laguna Caliente’s, the researchers noted. Hence, it’s likely this is an organism that hasn’t been described previously, said Hynek.

Is it also otherworldly? The Red Planet is rich in sulfur and iron, the nutrients on which Acidiphilium thrives, Hynek and his colleagues noted. What’s more, ancient Mars’s volcanic hot springs would have provided wet, acidic niches for similar bacteria, they remarked in their paper.

These kinds of studies are “highly relevant” to finding evidence of previous life on Mars.

These kinds of studies are “highly relevant” to finding evidence of previous life on Mars, said Manfred van Bergen, an Earth scientist at Utrecht University in the Netherlands not involved in the research. “The more we know about what could be expected, the more efficient this quest can be.”

Nature’s Logistics

The team hopes to return to Costa Rica this year to continue to sample the area’s microbiology and conduct aerial surveys of Poás volcano using drones. Any trip would have to focus on sediments rather than water—Laguna Caliente itself is gone after draining last year when Poás volcano became active again. But the biggest stumbling block to traveling comes down to overcoming nature’s logistics, said Hynek. Poás Volcano National Park is currently closed because of “increased and unpredictable volcanic activity,” the park’s website notes.

—Katherine Kornei (email:; @katherinekornei), Freelance Science Journalist


Kornei, K. (2018), Scientists discover an environment on the cusp of habitability, Eos, 99, Published on 25 May 2018.

Text © 2018. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.