Nitrate-Dependent Iron Oxidation: A Potential Mars Metabolism

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.

The identification of sulfide oxidation as a potential metabolism driving primary production on late Noachian Mars

“…For example, this occurs in marine sediments and hydrothermal vents, where there is limited light and a gradient in the availability of oxygen and nitrate 86,87 , and in artificial environments, such as wastewater treatment plants, which have higher concentrations of nitrate 88,89 . On Mars, the oxidation of sulfur species could be coupled to these electron acceptors in a similar manner 90 . For example, oxygen has been detected in the martian atmosphere by the Curiosity rover 25 , with thermodynamic models indicating that subsurface environments on Mars could possess sufficient O 2 to allow for aerobic metabolisms to be viable 67 .…”

Redox Processes Impacting the Flux of Iron(II) from Shelf Sediments to the OMZ along the Peruvian Shelf

“…Anaerobic ammonium oxidation coupled to Fe(III) reduction (Feammox) 16 has been found in soils but not yet the ocean. Nitrate oxidation of Fe(II) has also been considered as possibly supporting life on Mars 17 . The balance between microbial and abiotic oxidation rates for Fe(II) has implications for both our present day understanding of Fe cycling and that of the past 18 .…”

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

“…Fungi are a complex (eukaryotic) form of life that appeared relatively late in Earth history . Organisms sustained by lithoautotrophic metabolic strategies plausible for hypothetical life on Mars (e.g., methanogenesis; nitrate‐dependent iron oxidation) are abundant in Earth's deep biosphere, but it has not been demonstrated that any of the candidate fossils from the deep biosphere represent such organisms, although some rather unusual examples might conceivably do so . More work is needed, in particular, to search for and characterize definitively prokaryotic fossil remains from the basalt‐hosted deep biosphere.…”