The role of microbes and biofilms in leaf fossilization

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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.