Study in Italian Cave Shows Life Comes At You Fast

Inside limestone caves, sparkling crystals of gypsum (CaSO4· 2H2O) form when hydrogen sulfide gas dissolved in deep aquifers degasses and turns into sulfuric acid that reacts with the cave walls. This white mineral can form slowly from purely chemical reactions. Or, it can form many times faster with the help of bacteria that rapidly transform the hydrogen sulfide into sulfuric acid. These microbial activities leave behind clues called “biosignatures” inside the gypsum, a property that may one day be useful to find signs of life on Mars and other remote locations.

In a project funded by NASA, Deep Energy and Deep Life Community member Jennifer Macalady (Pennsylvania State University, USA), and colleagues, analyzed gypsum in the Frassassi cave system in Italy to better understand these biosignatures. The researchers measured ratios of sulfur isotopes, which are atoms that have differing numbers of neutrons in the nucleus. They discovered distinct gradient patterns in the sulfur isotope ratios occurring across the walls of the cave, caused by the movement of the hydrogen sulfide gas and the actions of bacteria. The researchers published their findings in a new paper in Astrobiology [1].

Macalady worked with Penn State colleague and isotope geochemist Matthew Fantle and former Ph.D. student Muammar Mansor, to study the sulfur isotopes in the cave. Initially, they had collected gypsum samples to gather evidence for a different hypothesis but were intrigued when their results came back and the gypsum samples had different isotopic concentrations, just centimeters apart. “This project was a surprising development, as often happens in research,” said Macalady.

The research team discovered that the patterns of isotope ratios only made sense when the researchers modeled the path of the airflow over the cave walls and compared them to the isotope data. As bacteria rapidly consumed hydrogen sulfide gas from the air, they altered the isotopic composition of the gas as it flowed downstream. Microbial activity created sulfur isotopic gradients that correlated with the direction of airflow, and became recorded in the gypsum. Gypsum that forms slowly, in the absence of bacteria, would not show this distinct pattern.

Scientists have described this isotopic phenomenon before, where isotope ratios change as a reaction progresses along a flow path, but this is the first time that researchers have applied the phenomenon to detect life. “The isotope signature is generalizable to any microbial process where a fluid flows past a solid,” said Macalady. “That’s the exciting part.” For example, bacteria consuming carbon dioxide dissolved in a trickle of water could create a similar pattern.

This signature could be used to find signs of life on other planets, like Mars. While no one has discovered limestone caves on the red planet, it does host lava tubes that could be a good place for microbes to hide out.

One drawback of this new analytical approach is that researchers must first have a good understanding of the speed and direction of the fluid and must also be able to measure isotopic composition along a flow path. These tasks are straightforward on Earth, but may be difficult to accomplish when taking samples remotely, such as with a Mars rover.

Macalady thinks that future research should explore the limits of these biosignatures, including how the rates of microbial activity and fluid flow affect the resulting isotopic patterns.

“Because life is fast, we need to think about the signatures that might emerge from life, as compared to those that arise from rates of abiotic reactions,” said Macalady. By considering the speed of life, scientists may uncover exciting new research directions.

Matthew Fantle samples gypsum from the Frasassi cave system in Italy.  Credit: J. Macalady
Matthew Fantle samples gypsum from the Frasassi cave system in Italy.
Credit: J. Macalady


Top image: Gypsum crystals form along the walls of the Frasassi cave system. Credit: Macalady research group

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