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 oceanic crust, in part due to the challenges inherent to low biomass settings. This C-DEBI project investigated the use of fluorescence activated cell sorting (FACS) as a means of concentrating microbial cells from low biomass samples for downstream DNA sequencing. Community level amplicon and metagenomic sequencing, and single microbial cell DNA analysis was carried out on Atlantis Massif oceanic crust and sediment samples collected during IODP x357. Oceanic crust communities were composed of low diversity and largely uncharacterized phlya. Functional gene analyses revealed microbial communities that have the genomic potential for using carbon monoxide as a carbon and/or energy source, as well as genes that hint at metal tolerance, motility and biofilm formation in the subsurface. In addition, this project investigated differential staining techniques to probe activity at the single cell level in low biomass settings. Individual microbial cells that were identified as active through the use of Bioorthogonal Amino Acid tagging (BONCAT) and Redox sensor green, were combined with community level and single cell sorting methods to separate and obtain genomic information from active and total crustal subsurface microbial communities at North Pond (RV Atlantis, Oct 2017). This work demonstrated distinct active and total communities, and generated partial genomes from over 300 individual microbial cells inhabiting oceanic crust.