A bacterium in an acidic, hot volcanic lake may hold clues to life on ancient Mars

The Laguna Caliente lake inside the volcano Poás in Costa Rica, which acts as a Mars analog. Image credit: Kobold66/Wikimedia Commons

A group of microbes found in a highly acidic, hot, mineral-rich volcanic lake in Central America may give us clues about life on ancient Mars.

Current orbiters including NASA’s Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express, as well as the Spirit, Opportunity and Curiosity rovers, have found geological evidence suggesting that there was hydrothermal activity on Mars approximately 3.7 billion years ago. Volcanic gases reacted with localized sources of water, which created hot, highly acidic environments and supported the formation of minerals rich in iron, silica and sulfur.

Surprisingly, similar extreme environments existing on Earth, termed “Mars analogs,” are often host to flourishing populations of microbes called “extremophiles.” Scientists speculate that ancient Martian hydrothermal sites could have supported similar organisms, and they are using Mars analogs to better understand what sort of life could have existed on early Mars.

A group of scientists from the University of Colorado at Boulder, the Rensselaer Polytechnic Institutein New York, the National University of Costa Rica and Universidad de Costa Rica set out to identify and characterize microbes from one such analog: Laguna Caliente, a crater lake in the volcano Poás in Costa Rica. Their findings were published online in the journal Astrobiology

The study’s investigators have described the lake as “one of the most extreme habitats of our planet.” Its water temperature has ranged from room temperature to the boiling point of water over the course of weeks to months, and the reaction of volcanic gases with the water has lowered the pH to a range from –1 to 1.5, or stronger than battery acid. Changes in volcanic activity and precipitation have caused the lake volume to vary over decades, from a cubic kilometer to having almost totally evaporated. All of these conditions support a rich variety of chemical reactions. In addition, volcanic minerals rich in iron, sulfur or silica have been found on Poás. Notably, similar minerals have been found on Mars in Gusev Crater (explored by Spirit), Valles Marineris, Meridiani Planum, and parts of Gale Crater (being explored by Curiosity). 

A rock outcrop in Gusev Crater on Mars, where NASA’s Spirit rover discovered minerals similar to those found on the Poás volcano. Image credit: NASA/JPL/Cornell.

DNA testing

To characterize organisms that can survive in such environments, the scientists isolated and sequenced the DNA from lake water samples. The scientists examined the sequence of the 16s RNA gene, because while it is found in all organisms, part of its sequence varies across species. These sequence variations are used to identify related species.

In most of the water samples, they found a DNA sequence that belongs to a single species of the genus Acidiphilium, a group of bacteria that thrive in acidic hydrothermal regions on Earth. In addition, Acidiphilium species can survive at 45 degrees Celsius (113 degrees Fahrenheit), which is the average temperature of the Laguna Caliente samples. However, the pH of the lake was an acidic 0.29, whereas known Acidiphilium species survive at pH levels between 1.5 to 5.5. The scientists therefore proposed that they had found an especially acid-tolerant species of Acidiphilium that has not previously been discovered.

This is one of the few ecosystems on Earth where a single species is found to exist as a “monoculture,” according to Brian Hynek, who is a geologist and professor at the University of Colorado’s Department of Geological Sciences and Laboratory for Atmospheric and Space Physics, and lead author of the study.

“It is really rare to find a single species in an ecosystem,” Hynek tells Astrobiology Magazine. “Usually we find no life if the environment is too harsh, or abundant types of species in less extreme field sites. This lake is at the fringes of what Earth life can tolerate due to the dynamic conditions and unforgiving characteristics.”

 At the same time, the lake water samples were compared in more detail with known Acidiphilium species. The scientists found that the sample DNA sequences were similar to those of species that metabolize iron or elemental sulfur and thiosulfate.

 Acidic Martian bugs

Given the abundance of different types of sulfur and iron minerals on the Martian surface, and the evidence that hydrothermal systems with acid-sulfate interactions existed on ancient Mars, the scientists propose that organisms similar to Acidiphilium might be similar to what could have existed on early Mars. This study will help inform future missions to Mars searching for signs of past microbial life.

Dr Jack Farmer, a geologist and astrobiologist at the Arizona State University School of Earth and Space Exploration, points out how important it is for research such as this to study life in extreme water environments.

“I think many in the planetary community would agree with the idea that habitability is more than just “follow the water”,” he tells Astrobiology Magazine. “We also need to understand details of the quality of thatwater, e.g., its pH, temperature, chemistry and microbiology, and how these factors may play out against each other in shaping habitability and life’s perceived limits on Earth.”

However, Farmer cautions that “We still don’t know the details of what controls the distribution of life on Earth, and we need to be careful about how we use potential terrestrial analogs. That said, extremophiles research, [as in Hynek’s study], is an exceedingly valuable step along this path to refine what we mean by habitability.”

The research was supported by NASA Early Career Award # NNX12AF20G, NASA MDAP (Mars Data Analysis Program) Award # NNX14AG90G, and the University of Colorado Center for the Study of Origins.