Abstract EANA2025-156

The Selvagens Islands, an isolated volcanic archipelago in the North Atlantic Ocean, represent a unique natural laboratory for studying microbial life in pristine, oligotrophic environments shaped by prolonged geological isolation and minimal anthropogenic disturbance. This study investigates the composition and functional potential of microbial communities in sediment cores collected from both terrestrial and marine caves on Selvagem Grande (Madeira, Portugal), the largest island in the archipelago, formed during the Oligocene (25–29 Ma). Core samples were obtained from a sloped terrestrial site in Galinhas Bay and a submerged marine cave in Cagarras Bay. Microbial diversity was characterized through 16S rRNA gene sequencing and culturing methods.

Terrestrial sediments were dominated by archaeal taxa, particularly members of the phylum Methanobacteriota, with Halococcus emerging as the most prevalent genus. Bacterial groups such as Actinomycetota, Pseudomonadota, and Bacillota were detected in lower abundance. In contrast, marine cave sediments exhibited a high representation of other archaeal lineages, including Thermoproteota, Halobacterota, Nanoarchaeota, and Thermoplasmatota. Among bacteria, Fusobacteriota, especially the anaerobic genus Propionigenium, was notably abundant.

Functional inference analyses revealed metabolic traits associated with survival in energy-limited and hypersaline environments. The prevalence of halophilic and extremophilic archaea across both terrestrial and marine settings underscores their critical role in maintaining microbial activity and ecosystem stability under extreme conditions. These findings suggest convergent evolutionary strategies that enable microbial resilience in nutrient-poor, saline, and geochemically challenging habitats.

Selvagem Grande Island has been recognized in the Catalogue of Portuguese Space Analogues by the Portuguese Space Agency as a Mars analogue site due to its geological, environmental, and biological features, specifically its basaltic lava formations, carbonate dykes, and slow tectonic activity. The detection of extremophiles such as chemolithotrophic archaea in cave sediments and salt pools, adapted to harsh and isolated conditions, provides valuable insights into possible biosignatures and microbial survival strategies beyond Earth. As one of the most undisturbed ecosystems in the North Atlantic, the Selvagens Islands serve as a relevant analogue site for the search for life on Mars and other planetary bodies, contributing to ongoing astrobiological efforts in planetary exploration and the development of life-detection missions.

Acknowledgements: This work was funded by the Portuguese Foundation for Science and Technology (FCT) through the MICROCENO project (DOI: 10.54499/PTDC/CTA-AMB/0608/2020), and by the Regional Government of Andalusia through the MICROLAVA project (PROYEXCEL_00185).