A Study in Blue: Secondary Copper-Rich Minerals and Their Associated Bacterial Diversity in Icelandic Lava Tubes

Lava tubes on Mars hold exciting potential for the preservation of biosignatures, which may survive on geological timescales in these isolated, stable environments. To support the development of future astrobiological mission concepts, we turn to terrestrial lava tubes, host to a variety of microbial communities and secondary minerals. Following a multidisciplinary sampling protocol, we retrieved biological, molecular, and mineralogical data from several lava tubes in Iceland. We report on blue-colored copper-rich secondary minerals and their associated bacterial communities using a multi-method approach, and an amalgam of 16S rRNA gene sequencing, Raman spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy data sets. We found numerous bacterial genera known for their high metal resistance and ability to survive in low-nutrient environments. Both are characteristics to be expected for any potential life in Martian lava tubes, and should be considered when checking for contaminants in Mars mission preparations. Associated with the microbial mats, we identified several types of copper-rich secondary minerals, indicating localized copper enrichments in the groundwater, possibly stemming from overlying ash deposits and nearby hyaloclastite formations. Molecular analysis revealed carotenoid signals preserved within the copper speleothems. If found in Martian lava tubes, blue copper-rich mineral precipitates would be deserving of astrobiological investigation, as they have potential to preserve biosignatures and harbor life.Plain Language Summary Subterranean lava tubes on Mars are exciting locations to study in the potential discovery of signs of life outside of Earth, as the surface of Mars does not have conditions conducive to the preservation of life as we know it. In order to better study these Martian environments we look first to comparable lava tubes on Earth. Within Icelandic lava tubes we found blue-colored copper minerals, host to microbial life. The microbes that thrive in these caves are able to withstand extreme conditions, and leave behind detectable molecular traces indicative of life, a type of biosignature. Using a variety of tools and techniques, we describe the nature of the blue minerals and their provenance, the role of the microbial populations within them, and the value of the molecular traces as biosignatures. We discuss the potential for such minerals and microbes in Martian lava tubes, and how we might successfully sample them in future missions to Mars