Thermodynamic predictions of redox metabolisms within Mars analogue hot springs

The toolkit of methods used in the search for life on other planets is growing vaster as research pushes new ways to examine the habitability of other planetary bodies. One method which can highlight the bioenergetic potential of our solar system involves thermodynamic calculations to estimate the Gibbs free energy produced by redox reactions. This method allows for predictions of the dominant biological reactions within environments such as Noachian-age martian hot springs and could be a useful indicator of habitability based on simple geochemical measurements capable by future Mars missions.

We utilise aqueous, gas and mineral measurements of key redox species within modern hot spring systems to predict the thermodynamic feasibility of chemolithoautotrophic metabolisms. These predictions are then compared to metagenomic and metatranscriptomic sequencing of these analogous microbial communities, to test the accuracy of Gibbs free energy calculations in predicting dominant redox metabolisms within primitive ecosystems. In addition, we model the outflow of these springs within a Noachian atmosphere to examine the differences in free energy availability and therefore dominant metabolisms compared to modern Earth systems.

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. We find that the proportion of reduced and oxidised mineral iron in models impacts the feasibility of many redox reactions, including those which do not use iron species, suggesting that redox conditions are impacted by mineralogy. In addition, the free energy yield of redox reactions varies before and after equilibrating with mineral and atmospheric species, encompassing the natural chemical gradients within both modern hot springs and ancient systems on Mars.

Combining geochemical methods with genomic sequencing in this way allows for a true interdisciplinary assessment of free energy predictions and habitability of early Earth and Mars hot spring habitats.