Shedding synchrotron light on the fossil record of early plant evolution

陆地植物的进化改变了地球的表面、大气和海洋——通过生物合成和加速风化作用来固定碳，以及通过建立一个适合动物居住的陆地环境。我们对这是如何产生的理解，不是基于对进化关系知之甚少的原始活植物（苔藓、苔类植物和角苔类植物），而是来自最早植物的微观化石遗骸。两种石化机制为我们提供了独特的见解：(i)晚泥盆世Rhynie Chert热喷口群落的快照，其中邻近的植物被包裹并保存在热流体中的矿物质中；（ii）在志留纪和泥盆纪的许多更古老的沉积物中，野火使植物炭化。炭化过程是瞬间石化——它使植物材料化学惰性和物理脆性，但抵抗细菌腐烂，并将解剖结构保存到细胞水平——使以这种方式保存的化石成为植物进化史上无与伦比的档案。原始陆生植物的远古炭化遗迹可以从细胞层面上了解它们的解剖结构，但要解开这些信息非常困难。这种宝贵的材料传统上是通过破坏性的解剖来研究的，通过在偶然方向上打开的裂缝来瞥见内部解剖结构。整个解剖结构必须通过破坏许多标本来拼凑在一起，内部结构之间的联系是推断的；许多结构根本就没有被揭示出来。另一个明显的选择是使用非侵入性计算机断层扫描- CT扫描。我们在2007年介绍了一种非常高分辨率的方法，使用同步加速器（一种粒子加速器）研究高级种子和开花植物的炭化残留物。然而，对最古老的陆地植物遗骸的分析结果是可怕的——黄铁矿（愚蠢的黄金）的晶体在化石遗骸的细胞空间内生长，黄铁矿（高吸收和散射）和木炭（极低吸收）之间的x射线吸收差异巨大，使得不可能分辨出任何解剖结构。自2007年以来，同步加速器束线测量技术有了改进，而且收集的数据处理也有了改进。特别是，原始测量数据转换为x射线切片的阶段，已经通过过滤算法得到了改进，使我们能够看到化石解剖结构的清晰细节。我们的项目将需要优化数据处理阶段的测量参数和算法，通过与我们长期的项目合作伙伴，瑞士光源同步加速器的Marco Stampanoni教授合作，他建立了仪器并为数据处理编程算法。我们将利用这些在CT扫描早期植物遗骸的新材料，我们将从什罗普郡的关键化石矿床中恢复。我们在利用同步加速器数据对化石进行计算机建模方面拥有丰富的经验，我们将与这些生物生物学的世界专家Dianne Edwards FRS教授密切合作，分析化石的计算机模型，对它们进行虚拟解剖，以更好地了解这些早期植物的解剖学和生理学。这项工作将使我们能够了解早期陆地植物进化的进化步骤顺序，反过来，这项工作将有助于我们理解陆地植物是如何改变地球的，也可能在解决有关其现存近亲的进化关系的争议方面发挥关键作用。

The evolution of land plants transformed the surface, atmosphere and ocean of our planet - by fixing carbon through biosynthesis and increased rates of weathering, as well as through establishing a terrestrial environment habitable by animals. Our understanding of how this was brought about rests less on primitive living plants (mosses, liverworts and hornworts) whose evolutionary relationships are poorly understood but, rather, from the microscopic fossil remains of the earliest plants. Two fossilization mechanisms have provided us with unique insights: (i) the snapshot of a hot vent community in the Late Devonian Rhynie Chert where the neighboring plants were enveloped and preserved in minerals from the hot fluids, and (ii) the charcoalification of plants by wildfires in a number of much older deposits through the Silurian and Devonian. The process of charcoalification is instant fossilization - it makes plant material chemically inert and physically brittle but resistant to bacterial decay and preserves anatomical structure to the cellular level - making fossils preserved in this way an unparalleled archive in of plant evolutionary history. Ancient charcolified remains of primitive land plants hold insights into their anatomy to the cellular level, but unlocking this information is very difficult. This invaluable material has traditionally been studied by destructively dissection, yielding glimpses of the internal anatomy along cracks that open up in chance orientations. The overall anatomy has to be pieced together by destroying many specimens and the linkage between internal structures are inferential; many structures are never revealed at all. The obvious alternative is to use non-invasive computed tomography - CT scanning. We introduced a very high resolution method in 2007 of studying charcolified remains of advanced seed and flowering plants using a synchrotron - a kind of particle accelerator. However, the results of analyses of the most ancient land plant remains were terrible - crystals of pyrite (fools gold) have grown within the cell spaces of the fossil remains and the huge difference x-ray absorption between the pyrite (high absorption and scattering) and the charcoal (extremely low absorption) making it impossible to make out any anatomical structure. Since 2007 there have been improvements in the synchrotron beamline measurement technology, but also in the processing of the data that it collects. In particular, the stage in which the raw measurements are converted to x-ray slices, has been improved with filtering algorithms that allow us to see the anatomy of the fossil in pin-sharp detail. Our project will entail optimising the measurement parameters and of algorithms involved in the data processing stages, through collaboration with our long-standing Project Partner Prof Marco Stampanoni at the Swiss Light Source Synchrotron - who build the instruments and program the algorithms for data processing. We will exploit these advances in CT scanning new material of early plant remains that we will recover from the critical fossil deposit in Shropshire. We have extensive experience of computer modelling of fossils from synchrotron data and we will analyse computer models of the fossils, dissecting them virtually to better understand the anatomy and physiology of these early plants, in intimate collaboration with the world expert in the biology of these organisms, Prof Dianne Edwards FRS. This work will allow us to understand the sequence of evolutionary steps in early land plant evolution and, in turn, this work will contribute to our understanding of how land plants transformed the planet, and may also prove pivotal in resolving controversy concerning the evolutionary relationships of their living relatives.