Price Alex, Pearson Victoria K, Schwenzer Susanne P, Miot Jennyfer, Olsson-Francis Karen
Faculty of Science, Technology, Engineering and Mathematics, The Open University, Milton Keynes, United Kingdom.
CNRS, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Muséum National d'Histoire Naturelle, Université Pierre et Marie Curie - Sorbonne Universités, UMR 7590, Paris, France.
Front Microbiol. 2018 Mar 20;9:513. doi: 10.3389/fmicb.2018.00513. eCollection 2018.
This work considers the hypothetical viability of microbial nitrate-dependent Fe 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 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 Fe has been identified on Mars and provides evidence of an accessible electron donor; evidence of anoxia suggests that abiotic Fe 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 Fe 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.
这项研究探讨了在早期火星环境背景下，微生物硝酸盐依赖型铁氧化作用（NDFO）支持简单生命存在的假设可行性。这借鉴了几十年来通过遥感和实地观测积累的知识，以及最近的发现，这些发现塑造了我们目前对早期火星的理解。我们目前的认识是，某些早期火星环境满足了具有NDFO代谢的微生物的几个关键要求。首先，在火星上已发现大量的铁，这为可获取的电子供体提供了证据；缺氧的证据表明，在这些环境中，分子氧对铁的非生物氧化不会干扰和与微生物铁代谢竞争。其次，一些铁氧化微生物可作为电子受体利用的硝酸盐，也已在火星上的现代风积物和古代沉积物中得到证实。除了氧化还原底物外，有机和无机碳库都可用于生物合成，地球化学证据表明，在水文活跃的诺亚纪时期（41亿至37亿年前）的湖泊系统符合硝酸盐依赖型铁氧化微生物的中性pH要求。除了可能在早期火星湖泊和河流系统中作为初级生产者外，NDFO的非光依赖性质表明，只要有足够的碳和硝酸盐来源，这些微生物在地表干燥后很长时间内可能会在地下含水层中持续存在。NDFO微生物的痕迹可能通过在高铁浓度区域的生物矿化和细胞结壳保存在岩石记录中。这些过程可以产生形态学生物特征，保留独特的铁同位素变化模式，并增强生物有机化合物的保存。通过未来前往火星的任务，使用适当的仪器可能会检测到这种生物特征。
