Destruction, decay, and preservation: Early fossilization of leaf compressions

在植物开放的多种石化途径中，木材矿化和叶片碳化是最常见的途径，导致硅化木材和叶片压缩的形成。通过炭化过程，在池塘、湖泊和河流等淡水相中经常产生叶片压缩。在淡水保护lagerstätten中，化石动物的出色保存归功于覆盖在腐烂生物体上的细菌生物膜的保护特性。然而，微生物和生物膜的形成对叶片保存的影响尚不清楚。树叶要变成化石，它必须首先通过风或水流进入水体。传统观点认为，叶子必须迅速进入缺氧环境才能保存下来。然而，我们知道细菌或真菌微生物的定植和降解可以在叶片进入缺氧环境之前很早就发生。微生物在叶片上和叶片内的定植速率可能取决于角质层形态等内在因素。我们还观察到，叶子上生物膜的发育可以导致矿物结壳的自然发育，这将保护叶子表面免受运输和无脊椎食草动物的磨损和损害，减缓细菌进一步活动的腐烂，并保持叶子的形态特征。然而，究竟是哪种微生物，甚至是细菌还是真菌，导致了腐烂或石化还不得而知。在这里，我们开始了一项关于化石早期阶段的跨学科研究，其中我们调查了微生物和生物膜的时间和发展，这些微生物和生物膜参与了树叶的破坏、腐烂和保存。我们研究的核心是DNA提取和微生物组测序，以识别和表征参与生物降解或保存过程的微生物。在自然淡水环境和实验水族馆中，将对四个植物组进行为期数周的实验，这些植物组在新生代植物记录中出现的频率不同，包括睡莲叶、木贼茎、松针和枫叶。以Liquidambar为例，将对绿叶以及秋天的黄叶和红叶进行试验，以检查天然叶色素对病原体防御的差异。在整个实验过程中，将对叶片上的微生物生长、微生物区系的演变、水离子化学的波动和叶片表面的矿物积累进行分类测定。叶片的内在差异，如角质层形态也将被研究。在C4项目中，对叶子石化下一阶段的跨学科研究——在实验条件下形成矿物面纱或壳层——将继续在相同的植物群上进行。

Among the various fossilization pathways open to plants, wood mineralization and leaf carbonization are the routes most commonly taken, which lead to the formation of silicified wood and leaf compressions. Through the process of carbonization, leaf compressions are frequently produced in freshwater facies such as ponds, lakes, and rivers. In freshwater conservation lagerstätten, the excellent preservation of fossil animals is attributed to the protective properties of bacterial biofilms covering the decaying organism. Yet little is known about the influence of microorganisms and biofilm formation on the preservation in leaves. For a leaf to become fossilized, it must first enter a body of water, for example, by wind or the flow of water. Conventional wisdom says that the leaf must then rapidly enter an anoxic environment to become preserved. However, we know that leaf colonization and degradation by bacterial or fungal microbes can occur early on, well before the leaf enters an oxygen-less environment. The rate of microbial colonization on and in the leaf may depend on intrinsic factors such as cuticle morphology. It has also been observed that the development of a biofilm on leaves can lead to the natural development of a mineral encrustation, which would protect a leaf surface from abrasion and damage from transport and invertebrate herbivory, slow down decay from further bacterial activity, as well preserve morphological features of the leaf. However, it is unknown which microorganisms, or even whether they are bacterial or fungal, are responsible for decay or fossilization. Here we begin an interdisciplinary study on the early stages of fossilization in which we investigate the timing and development of the microbes and biofilms involved in the destruction, decay, and preservation of leaves. Central to our research is DNA extraction and microbiome sequencing to identify and characterize the microbes involved in the biodegradation or preservation processes. Experiments of several weeks' duration will be conducted on four plant groups that are found in varying frequencies in the Cenozoic plant record—Nymphaea water lily leaves, Equisetum stems, Pinus needles, and Liquidambar leaves—in both natural freshwater settings and in experimental aquaria. In the case of Liquidambar, trials with green leaves, as well as with autumnal yellow and red leaves, will be run to check for differences in pathogen defense due to natural leaf pigments. Taxonomic determination of the microbial growth on the leaves, successional changes in the microbial flora, fluctuations in water ion chemistry, and mineral accumulation on the leaf surfaces, will be tracked throughout the experiments. Intrinsic differences in leaves such as cuticle morphology will also be studied. Interdisciplinary research on the next stage of leaf fossilization—the development of a mineral veil or encrustation under experimental conditions—will continue on the same plant groups in Project C4.