The role of microbes and biofilms in leaf fossilization

化石是经过一系列复杂的生物、化学和地质过程形成的。植物的部分，如叶子，通常是碳化的，或者在木材的情况下，是硅化的。在对动物尸体的研究中，已经表明细菌产生的生物膜在软组织的石化中起作用。通过生物膜，软组织结构可以被精确地保存下来。带负电荷的生物膜表面有利于生物膜中矿物质的沉淀。在第一个资助期，项目B3研究节肢动物的分类，重点研究小龙虾。碳酸钙在小龙虾体内的沉淀只需要两天时间。微生物组分析表明，小龙虾的定植是由一系列不断减少氧气需求的细菌种群组成的。占优势的细菌属是蛋白酶、几丁质酶和脂肪酶的积极生产者。在较高的温度下，即使在缺氧的情况下，它们也会导致小龙虾的快速腐烂；在较低的温度下，该过程的速率大大降低。与动物的生物膜相比，微生物生物膜在化石叶片压缩发生中的作用鲜为人知。只有少数研究表明，叶子生物膜可以形成，也可以吸收矿物质。矿化生物膜可能具有抵抗食草动物和机械损伤的保护功能。在2685研究组的框架内，C3项目将研究自然环境和水族馆中这种生物膜的形成。在项目C4中，生物膜形成背后的复杂过程将被阐明，包括哪些条件和哪些细菌或真菌可以导致矿物沉淀或叶组织降解。为此，将从生物膜中分离出细菌和真菌的个别菌株。这些微生物将在无菌的实验室条件下与叶片一起或单独培养，并分析它们对叶片保存或腐烂的影响。一旦生物膜形成，就会测试哪些离子浓度或哪些代谢过程（磷酸盐的释放、尿素的分解、铁的氧化）是细菌在叶子上沉淀矿物质的先决条件。我们第一个资助期的结果也表明，微生物的衰变活动必须被阻止或减慢，这样化石化才能继续进行。这可能发生在沉积物沉积之后，大概是由于细菌在缺氧条件下产生的酸释放重金属。因此，重金属对生物膜的影响将在实验室进行研究，以建立叶片石化早期阶段的机制模型。

Fossils are formed through a complex series of biological, chemical, and geological processes. Plant parts such as leaves are commonly carbonized, or in the case of wood, silicified. In studies on animal carcasses, it has been shown that biofilms produced by bacteria can play a role in the fossilization of soft tissues. Through biofilms, soft-tissue structures can be preserved in exact detail. The negatively charged surface of a biofilm facilitates the precipitation of minerals in the biofilm. In the first funding period, Project B3 investigated the taphonomy of arthropods with a focus on crayfish. Precipitation of calcium carbonate in the crayfish can occur after only two days. Microbiome analysis documented that colonization of the crayfish consisted of a succession of bacterial populations with continuously decreasing oxygen demand. Dominant were bacterial genera that are aggressive producers of proteases, chitinases, and lipases. At higher temperatures, they led to the rapid decay of the crayfish, even in the absence of oxygen; at lower temperatures, the rate of the process was strongly reduced. In contrast to biofilms in animals, the role of microbial biofilms in the genesis of fossil leaf compressions is poorly known. It has been shown in only a few studies that leaf biofilms can form and also incorporate minerals. A mineralized biofilm may have a protective function against herbivores and mechanical damage. Within the framework of Research Unit 2685, the formation of such biofilms in natural settings and in aquaria will be studied in Project C3. In Project C4, the complex processes behind biofilm formation will be elucidated, including which conditions and which bacteria or fungi can lead to mineral precipitation or to the degradation of leaf tissues. To this end, individual strains of bacterial and fungal species will be isolated from the biofilms. Together or alone, these microbes will be incubated with leaves under sterile laboratory conditions, and their influence on the leaf preservation or decay will be analyzed. As soon as a biofilm has been formed, it will be tested which ion concentrations or which metabolic processes (release of phosphate, breakdown of urea, oxidation of iron) are carried out by the bacteria as a prerequisite for the precipitation of minerals on the leaves. Our results from the first funding period have also shown that the decay activity of microorganisms must be stopped or slowed down so that fossilization can continue. This may happen after deposition in sediment, presumably through the release of heavy metals from the acid produced by bacteria in anoxic conditions. For this reason, the effect of heavy metals on biofilms will be investigated in the laboratory to develop a mechanistic model of an early phase of leaf fossilization.