Microbial Life in Mauna Loa Lava Tubes as a Martian Analog

Subsurface environments on Mars offer protection from the planet's harsh surface conditions. Shielded from extreme radiation and temperature fluctuations, life may have developed and survived in basaltic lava tubes on Mars. The stable conditions in these lava tubes may also preserve relic signs of past life. Given this biotic potential, lava tubes are key targets for astrobiology investigations.

Basaltic lava tubes on Mauna Loa, Hawaii can be used as a model to evaluate the potential for life in Martian lava tubes. The active volcanism and intermittent water films present on Mauna Loa are analogous to early Mars when Mars was warm enough to sustain liquid water and more conducive to life. Mauna Loa's basaltic lava tubes were formed recently (~200 years ago), mirroring the conditions in which pioneer microbes could have emerged on the Red Planet. Minerals in the lava tubes could provide substrates for chemosynthetic microbes, and comparable deposits are thought to have formed in Martian lava tubes.

To better understand the presence and preservation of biosignatures, learn more about the habitability of these subsurface environments, and assess the metabolic pathways utilized by their microbial life, we characterized the microbial life in secondary mineral deposits in a Mauna Loa lava tube. In August 2019, we collected samples and, in the months that followed, conducted 16S rRNA gene and metagenomic sequencing. After analyzing DNA sequences with multiple bioinformatic pipelines, we found Actinobacteria and Proteobacteria to be most prevalent. A diverse array of aerobic and anaerobic microbes, however, was identified that play roles in carbon, nitrogen, and sulfur cycling. We also identified heat- and radiation-tolerant extremophilic orders and pathways for sulfate reduction and methanogenesis, which are particularly relevant given current conditions and potentially limited nutrient availability on Mars. These findings reveal the types of microbes that can survive in this analog environment and, when integrated with parallel mineralogical analyses, can inform specific microbe-mineral interactions that will help improve our search for life and biosignatures on Mars.