Nitrate-reducing Fe(II)-oxidizing microorganisms in a geochemical and mineralogical Mars terrestrial analogue (Rio Tinto, Spain)

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The identification of Fe(II)- and nitrate-containing minerals at the Gale crater on Mars suggested that nitrate-reducing Fe(II)-oxidizing (NRFeOx) microorganisms could have acted as primary producers in early Martian fluvio-lacustrine systems and could have persisted in subsurface aquifers long after the desiccation of the planetary surface. Such an environment during the early Hesperian Eon was subjected to physio-chemical variations such as periodical desiccation, changes in salinity, pH, etc. In this environment NRFeOx microorganisms could participate in N and Fe cycles resulting in formation of gaseous nitrogen species, Fe(III) (oxyhydr)oxide and Fe(III) hydroxysulfate minerals. Analogous Fe(III) mineral-rich biospheres characterized by physico-chemical fluctuations are typical for terrestrial estuarine systems affected by acid mine drainage. For example, the tidal activity in the estuary of the Rio Tinto river (Spain) modulates fluctuations in pH, Eh, salinity, etc. while dry seasons lead to desiccation of floodplains surrounding the estuary. The primary production in this unique anoxic biosphere can be supported by chemosynthetic NRFeOx activity. This possibility could have a crucial value for hypothetical Martian life after desiccation of the surface. Therefore, the first goal of the present proposal is to isolate NRFeOx microorganisms from anoxic Fe(II)-rich sediments at the estuary and headwaters of the river and from subsurface core samples collected in the Rio Tinto region. The second goal is to identify and characterize the isolated strains of NRFeOx microorganisms. The third goal is to evaluate their metabolic activity and contribution to Fe(II) mineral oxidation, primary production and N2O formation. These goals will be achieved by a combination of geochemical, physiological and molecular approaches, including metagenomics and metatranscriptomics. The results from this project will help us to reveal i) the environmental niches and the role of NRFeOx microorganisms in sediments of the estuary and headwaters as well as in the deep subsurface of the Rio Tinto region, ii) the metabolic characteristics of NRFeOx microorganisms inhabiting the estuary and headwater sediments and subsurface rock, and iii) the hypothetical viability of NRFeOx microorganisms to support growth using Fe(II)-bearing minerals similar to those that were found on Mars.