UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK.
School of Geosciences, Grant Institute, University of Edinburgh, Edinburgh, UK.
Geobiology. 2021 Sep;19(5):473-488. doi: 10.1111/gbi.12445. Epub 2021 May 5.
The search for a fossil record of Earth's deep biosphere, partly motivated by potential analogies with subsurface habitats on Mars, has uncovered numerous assemblages of inorganic microfilaments and tubules inside ancient pores and fractures. Although these enigmatic objects are morphologically similar to mineralized microorganisms (and some contain organic carbon), they also resemble some abiotic structures. Palaeobiologists have responded to this ambiguity by evaluating problematic filaments against checklists of "biogenicity criteria". Here, we describe material that tests the limits of this approach. We sampled Jurassic calcite veins formed through subseafloor serpentinization, a water-rock reaction that can fuel the deep biosphere and is known to have occurred widely on Mars. At two localities ~4 km apart, veins contained curving, branched microfilaments composed of Mg-silicate and Fe-oxide minerals. Using a wide range of analytical techniques including synchrotron X-ray microtomography and scanning transmission electron microscopy, we show that these features meet many published criteria for biogenicity and are comparable to fossilized cryptoendolithic fungi or bacteria. However, we argue that abiotic processes driven by serpentinization could account for the same set of lifelike features, and report a chemical garden experiment that supports this view. These filaments are, therefore, most objectively described as dubiofossils, a designation we here defend from criticism and recommend over alternative approaches, but which nevertheless signifies an impasse. Similar impasses can be anticipated in the future exploration of subsurface palaeo-habitats on Earth and Mars. To avoid them, further studies are required in biomimetic geochemical self-organization, microbial taphonomy and micro-analytical techniques, with a focus on subsurface habitats.
寻找地球深部生物圈的化石记录，部分是受火星地下栖息地潜在类比的推动，在古代孔隙和裂缝内发现了许多无机微丝和管的组合体。虽然这些神秘的物体在形态上与矿化微生物相似（有些还含有有机碳），但它们也类似于一些非生物结构。古生物学家通过将有问题的丝状结构与“生物成因标准”清单进行评估来应对这种模糊性。在这里，我们描述了一些材料，这些材料检验了这种方法的极限。我们对通过海底蛇纹石化形成的侏罗纪方解石脉进行了采样，这种水-岩石反应可以为深部生物圈提供燃料，并且已知在火星上广泛存在。在相隔约 4 公里的两个地点，脉中含有由镁硅酸盐和氧化铁矿物组成的弯曲、分支的微丝。我们使用了一系列广泛的分析技术，包括同步加速器 X 射线微断层扫描和扫描透射电子显微镜，结果表明这些特征符合许多已发表的生物成因标准，并且可与化石 cryptoendolithic 真菌或细菌相媲美。然而，我们认为蛇纹石化驱动的非生物过程可以解释相同的一组栩栩如生的特征，并报告了一个支持这种观点的化学花园实验。因此，这些丝状结构最客观地被描述为 dubiofossils，我们在此为这个名称辩护，并建议替代方法，但这仍然意味着僵局。在未来对地球和火星地下古栖息地的探索中，可能会出现类似的僵局。为了避免这种情况，需要进一步研究生物模拟地球化学自组织、微生物埋藏学和微分析技术，重点是地下栖息地。
