Testing the metabolic feasibility of Noachian hot springs on Mars

Abstract

Mars exploration missions aim to search for signs of past microbial life during the planet’s most habitable era, the Noachian period. Volcanism during this period led to surface hydrothermal environments, which are hypothesised to be a conducive environment for prebiotic chemistry and are often inhabited by microorganisms employing primitive metabolisms. Modern terrestrial hot springs and their microbial inhabitants serve as a living representation of possible geobiological cycling on Noachian Mars. Biological fixation of nitrogen is a key part in unravelling the habitability of early martian environments. The nitrogen cycle on Mars remains enigmatic, and early martian communities would need to convert N₂ into bioavailable forms. Likewise, fixation of inorganic carbon forms the basis of the biological carbon cycle and is predicted to be a keystone metabolism for a Noachian biosphere.

This thesis will establish biological carbon and nitrogen cycling within Mars analogue hot springs and identify how these metabolic pathways manifest in the sedimentary record. Thermodynamic habitability estimates of analogue environments provide geochemical context to detailed genomic characterisation of key chemolithoautotrophic metabolisms. Through novel coupling of metagenomic analysis with stable isotope geochemistry, I test the hypothesis that δ¹³C and δ¹⁵N fractionations record the carbon and nitrogen fixation pathways described above.

Results reveal thermodynamically feasible carbon, iron and sulfur metabolisms and a ubiquitous reliance on biological fixation of inorganic N₂ and carbon within the hot spring communities. Deep geothermal processes indirectly influence biogeochemistry by controlling solubility of bio-essential metals used in metalloenzyme structures. This is reflected in biomass and sedimentary δ¹⁵N signatures, which are lighter than those seen for conventional N₂ fixation in acidic, sulfidic springs due to scarcity of molybdenum which is required for the MoFe nitrogenase enzyme. δ¹⁵N values instead agree with those produced by rare alternative nitrogenases which utilise vanadium and iron detected in these springs.

Overall, this thesis broadens our knowledge of keystone metabolisms within Mars analogue hot spring microbial communities and highlights the potential of nitrogen stable isotopes to record dominant biological nitrogen processes within Mars hot spring deposits.

Date of Award	2 Dec 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Eva Stueeken (Supervisor) & Claire Cousins (Supervisor)