above those required to support H2-utilizing microorganisms. More recently, geochemical, microbiological, and molecular analyses of alkaline saline groundwater at 2.8 kilometers depth in Archaean metabasalt in South Africa revealed a microbial community dominated by a single phylotype of sulfate-reducing bacteria (SRB) belonging to Firmicutes.34 These SRB were reported to be sustained by geologically produced sulfate and H2 at concentrations sufficient to maintain activities for millions of years. The discovery of hyperthermophilic autotrophic methanogens in conjunction with H2 at millimolar concentrations beneath an active deep-sea hydrothermal field in the Central Indian Ocean Ridge further extended the SLiME concept to hyperthermophilic marine systems.
For reasons presented above, the subsurface is the most likely environment to harbor extant life and preserved tracers of life (biomarkers) on Mars today. Deeply buried sediments such as those beneath the seafloor on Earth are another important subsurface analog system that has yielded a wealth of new information on lithologic controls on microbial activity, especially at very low organic carbon concentrations, and on the ability of microbes to persist under extremely low nutrient conditions, leading to the requirement for refined definitions of life and death for very-slow-growing microorganisms. Parkes and colleagues have established that there is a diverse and active microbial community in deeply buried marine sediments35 and that these organisms can persist for extended periods in spite of a relative lack of circulating fluids. Although relatively little is known about the phylogeny of these organisms, molecular analyses of deeply (~ 200 m) buried sediments from the Peru Margin indicate that the communities were dominated by bacteria in the gamma-Proteobacteria, Chloroflexi (green nonsulfur bacteria), and Archaea in the Miscellaneous Crenarchaeotic Group and South African Gold Mine Euryarchaeotic Group, and that the community composition changed with depth.36
Direct in situ evidence for hydrothermal activity on Mars is forthcoming; however, chemical studies of SNC meteorites suggest some history of aqueous or hydrothermal alteration in Mars’s past, and current juxtaposition of water-rich and volcanic systems combined with geomorphological evidence points strongly to the existence of hydrothermal systems on Mars today or in the past. Martian hydrothermal systems, be they cold, warm, or hot, could have created environments conducive to the development and support of life. Critical studies in hydrothermal Earth analog systems (especially the low-temperature systems found in the Arctic) include studies of the initial imprinting, preservation, and subsequent alteration of potential biosignatures; determination of fossilizable components of hydrothermal deposits that may harbor biosignatures; and characterization of biogenic patterns recorded in a suite of related chemical/isotopic measurements. Hydrothermal systems on Earth have been the sites of discovery of numerous novel extremophilic organisms as well as the most ancient organisms yet found on Earth. For example, hydrothermal systems harbor many deeply branching microorganisms such as members of the Aquificales37 as well as many anaerobic thermophilic archaea of the genera Pyrococcus, Archaeoglobus, and Methanococcus.38 Additionally, hydrothermal systems may also preserve prebiotic compounds and serve as sites of prebiotic chemical synthesis.
Findings from Mars exploration over the past decade have yielded data to suggest the occurrence of evaporative sedimentary environments and both alkaline and acidic aqueous environments during Mars’s history. Accordingly, evaporative systems hosting microbial mat communities and aqueous alkaline and acid systems—groundwater and lakes (e.g., Mono Lake and Rio Tinto), and terrestrial hot springs and marine hydrothermal systems (e.g., seafloor serpentinization at Lost City, mid-ocean ridge hydrothermal deposits and seamounts)—are currently being explored for the presence of microorganisms and biosignatures preserved within the fossilizable (mineral) portions of these systems. A molecular-based study of Mono Lake microbial communities39 revealed that most of the 212 sequences retrieved from the samples fell into five major lineages of the domain Bacteria: alpha- and gamma-Proteobacteria