Abstract
This work considers the hypothetical viability of microbial nitrate-dependent Fe2+ oxidation (NDFO) for supporting simple life in the context of the early Mars environment. This draws on knowledge built up over several decades of remote and in situ observation, as well as recent discoveries that have shaped current understanding of early Mars. Our current understanding is that certain early martian environments fulfill several of the key requirements for microbes with NDFO metabolism. First, abundant Fe2+ has been identified on Mars and provides evidence of an accessible electron donor; evidence of anoxia suggests that abiotic Fe2+ oxidation by molecular oxygen would not have interfered and competed with microbial iron metabolism in these environments. Second, nitrate, which can be used by some iron oxidizing microorganisms as an electron acceptor, has also been confirmed in modern aeolian and ancient sediment deposits on Mars. In addition to redox substrates, reservoirs of both organic and inorganic carbon are available for biosynthesis, and geochemical evidence suggests that lacustrine systems during the hydrologically active Noachian period (4.1–3.7 Ga) match the circumneutral pH requirements of nitrate-dependent iron-oxidizing microorganisms. As well as potentially acting as a primary producer in early martian lakes and fluvial systems, the light-independent nature of NDFO suggests that such microbes could have persisted in sub-surface aquifers long after the desiccation of the surface, provided that adequate carbon and nitrates sources were prevalent. Traces of NDFO microorganisms may be preserved in the rock record by biomineralization and cellular encrustation in zones of high Fe2+ concentrations. These processes could produce morphological biosignatures, preserve distinctive Fe-isotope variation patterns, and enhance preservation of biological organic compounds. Such biosignatures could be detectable by future missions to Mars with appropriate instrumentation.
Highlights
Mars, the red planet, has inspired the search for extraterrestrial life since the early days of the telescope, and continues to do so with perceptions of its habitability—or even inhabitation— changing with advances in exploration capabilities and knowledge of martian environments from images and data (Filiberto and Schwenzer, 2017)
Nitrate-dependent Fe2+ oxidation (NDFO) microorganisms oxidase Fe2+ compounds while reducing nitrates under anaerobic, circumneutral conditions. These environments are proposed to have existed on Mars, providing the electron donors and acceptors required for nitrate-dependent Fe2+ oxidation (NDFO) metabolism
This implies that NDFO is a feasible and logical avenue for investigating hypothetical early martian life
Summary
The red planet, has inspired the search for extraterrestrial life since the early days of the telescope, and continues to do so with perceptions of its habitability—or even inhabitation— changing with advances in exploration capabilities and knowledge of martian environments from images and data (Filiberto and Schwenzer, 2017). With estimated life-times of 150–200k years even for modest craters (100–180 km diameter) the size of Gale, and with cycles of continuous mineral dissolution and precipitation maintaining the availability of redox substrates during that time, impact-generated hydrothermal systems could have provided localized hospitable zones (Abramov and Kring, 2005; Schwenzer and Kring, 2009) These two examples of martian environments (lacustrine and impact-generated hydrothermal systems) demonstrate the diversity of potentially habitable environments (as we understand them today) on ancient Mars. Laboratory-based Mars simulation experiments, using analog regolith or brine, and theoretical modeling have suggested that chemolithotrophic life could persist in the sub-surface martian environment across a wide range of pH, salinity, desiccation, and temperature (Parnell et al, 2004; Amils et al, 2007; Jepsen et al, 2007; Gronstal et al, 2009; Chastain and Kral, 2010; Smith, 2011; Popa et al, 2012; Hoehler and Jørgensen, 2013; Montoya et al, 2013; Summers, 2013; Bauermeister et al, 2014; Oren et al, 2014; King, 2015; Fox-Powell et al, 2016; Schuerger and Nicholson, 2016)
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