Microbial Survival on Exoplanets: Habitability and the Role of Melanin

Abstract

Exoplanet science remains a challenging field of study due to the large distances separating Earth from other star systems, and the consequent limitations in observational resolution. Conventionally, an exoplanet's habitability is evaluated by checking whether its orbit lies in the zone where surface liquid water might exist. This approach provides a reductionistic view of habitability, excluding bodies situated outside this zone which may still foster conditions conducive to life, as has been hypothesized in the Solar System (Nimmo and Pappalardo 2016) and beyond (Seager 2013). In this thesis, I integrated insights from biology and astrophysics to develop streamlined methods to study exoplanet habitability. Firstly, models and observations were used to estimate the temperature and radiation environment (flare UV and X-ray doses) of exoplanets orbiting M-dwarfs, the most common stars in the Universe, using the Martian atmosphere as a model for shielding. Then, the results of the modelling were used to inform microbiology experiments related to astrobiology that assessed the survival of the melanin-rich fungus Aspergillus niger under exoplanet-like radiation conditions, including UV-C, X-rays, and cosmic rays. Additionally, recent research indicates that melanin may have had a crucial role on the origin of life on Earth. To investigate how primordial melanin-rich solutions may have been beneficial to microorganisms, I explored the role of solubilized melanin on the survival and germination of A. niger, both with irradiated and non-irradiated samples. The results obtained show that A. niger spores can endure superflare events on potentially habitable M-dwarf planets when shielded by a Mars-like atmosphere or by a thin layer of soil or water. Melanindeficient spores suspended in a melanin-rich solution showed higher survival rates and germination efficiency when compared to melanin-free solutions, even for non-irradiated samples. These findings highlight the importance of melanin for radiation protection and cell health, and reinforce the possibility that melanin-rich primordial solutions may have been beneficial to life’s development on Earth, and perhaps elsewhere. Overall, the models developed in this thesis pave the way for future research by establishing a framework for microbiological research in habitability studies. Additionally, the results acquired show that A. niger can survive the harsh radiation conditions on exoplanets, especially when in combination with multifunctional molecules like melanins.