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

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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.