Earth’s subsurface is often isolated from phototrophic energy sources and characterized by chemotrophic modes of life. These environments are often oligotrophic and limited in electron donors or electron acceptors, and include continental crust, subseafloor oceanic crust, and marine sediment as well as subglacial lakes and the subsurface of polar desert soils. These low energy subsurface environments are therefore uniquely positioned for examining minimum energetic requirements and adaptations for chemotrophic life. Current targets for astrobiology investigations of extant life are planetary bodies with largely inhospitable surfaces, such as Mars, Europa, and Enceladus. Subsurface environments on Earth thus serve as analogs to explore possibilities of subsurface life on extraterrestrial bodies. The purpose of this review is to provide an overview of subsurface environments as potential analogs, and the features of microbial communities existing in these low energy environments, with particular emphasis on how they inform the study of energetic limits required for life. The thermodynamic energetic calculations presented here suggest that free energy yields of reactions and energy density of some metabolic redox reactions on Mars, Europa, Enceladus, and Titan could be comparable to analog environments in Earth’s low energy subsurface habitats.
Though there is increasing understanding of the ecology, diversity and activity of microbial life in deep sediments, little is known about the microbial life inhabiting deep oceanic crust. It is not yet known what proportion, if any, microorganisms in the rock biosphere are actively growing. The work proposed in this fellowship application will be to identify and quantify active microbial biomass from Atlantis Massif oceanic crust samples collected during IODP 357. Fluorescence based microscopy techniques (BONCAT) will be used as a means to sort active cells using flow cytometry for downstream 16S rRNA gene, metagenomic and single celled sequencing. This combination of approaches for presents great potential to reveal if communities are actively growing in/on the rock, and will give insight into the taxonomy and function of active microorganisms in the Atlantis Massif deep oceanic crust.