Mars’ surface is not currently conducive to human life. It presents extreme challenges, including a tenuous atmosphere, freezing temperatures, and heightened radiation levels. While Earth’s extremophiles can tackle some obstacles, they can’t handle them all simultaneously. If Martian life exists, how do these microbes manage to survive in such an environment?

The answer might lie within caves. Many researchers believe that ancient lava tubes on Mars formed billions of years ago when the planet was warmer and had liquid water. Caves serve as shelters against radiation and severe temperatures found on the Martian surface. They also host the nutrients and minerals necessary for sustaining life. Although scientists cannot yet explore Martian caves directly, they are examining analogous sites on Earth to establish parameters for searching for life on Mars.

A research team, led by C.B. Fishman from Georgetown University, investigated the microorganisms inhabiting the lava tubes of Mauna Loa, Hawaii, to learn about their survival mechanisms. Thanks to careful conservation efforts by Native Hawaiians, these lava tubes remain undisturbed by human activity. Researchers believe that both the rock structures in Mauna Loa Cave and the minerals formed from sulfur-rich gases bear similarities to Martian cave formations.

The team analyzed five samples from well-lit areas near the cave entrance, two from dimly lit zones with natural openings known as skylights, and five from the cave’s darkest recesses. Samples were chosen based on rock characteristics, including secondary minerals like calcite and gypsum, and primary iron-bearing minerals such as olivine and hematite.

Findings revealed significant variation in mineralogy within the cave, even over small distances. The bright samples were predominantly gypsum, while the dark samples lacked these key minerals. Instead, one dark sample was rich in iron-bearing minerals, while another contained mainly calcite, gypsum, and thenardite.

To identify the microorganisms within the samples, the team employed the 16S rRNA gene to recognize known microbes and understand their relationships. They also reconstructed complete genomes from cave samples using a method called metagenomic analysis. This technique is akin to following instructions to assemble various models from mixed DNA fragments. Such insights help researchers grasp how both known and unknown microorganisms thrive in their respective environments.

The team discovered that approximately 15% of the microbial genomes were unique to specific locations, with about 57% appearing in less than a quarter of the samples. Furthermore, microbial communities in dark regions exhibited less diversity and were more specialized compared to those in well-lit areas. While dark sites were not as varied as bright ones, each supported its own distinct microbial community.

To explain this difference, the researchers proposed that dark microbes have limited survival strategies since photosynthesis is impossible without light. Instead, these microbes extract chemical energy from rocks and decaying organic matter, much like how humans derive energy from breaking down food.

The findings from metagenomic data indicated that even though sulfur minerals were abundant, very few microorganisms specialized in sulfur consumption were present. This aligns with expectations in oxygen-rich environments, as oxygen tends to react with sulfur, making it unavailable to microorganisms. The researchers suggested that sulfur-metabolizing microbes may be more commonly found in low-oxygen environments, such as Mars.

Additionally, the study revealed that a majority of the microorganisms found in these caves were previously undescribed by science, contributing to what is referred to as microbial dark matter. The existence of such unknown microorganisms hints at novel survival strategies.

The research team concluded that lava tube caves could be a crucial source of new microorganisms, aiding astrobiologists in their quest to understand potential life forms on Mars. They recommended that future investigations into Martian caves should focus on detecting small-scale microbes in various mineral contexts. Over time, the interplay between cave conditions and Martian microorganisms may be unveiled as Mars becomes less habitable.

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