Deciphering Fossilization Pathways: Sediment Composition Impacts Biofilm-Forming and Biomineralizing Microbial Communities in Early Stages of Leaf Taphonomy

The formation of leaf compressions is dependent on the type of sediment in which the leaves are buried and on burial depth because greater burial depth leads to a more anoxic environment conducive to fossilization. Recent research has hypothesized that the presence of a microbial biofilm on leaf surfaces in the early stages of decay also enhances preservation. In decaying leaves, the biofilm community is likely influenced by the same factors: sediment type and burial depth. Here we investigate experimentally the microbial community composition of microbial biofilms formed on floating and buried leaves of living Ginkgo in four sediment types—montmorillonite clay, kaolin clay, quartz sand, and pond mud. Leaves were placed in aquariums with pond water under identical light conditions and room temperatures for three months. The leaves, sediments, and pond water were then evaluated with 16S and ITS sequencing to identify the bacterial and fungal communities. We found that the biofilms on the floating and buried leaves differed in their basic microbial community composition. The leaves buried in the kaolin clay showed the most distinctive microbial communities, while the montmorillonite clay buried leaves contained several genera noted for biomineralization. In general, the buried leaves had microbial communities that were more diverse than those on the floating leaves and richer in anaerobic microbes and biomineralizers. These results suggest that biofilms form best in very fine-grained sediments with low organic content, such as kaolin and montmorillonite clays, and under burial conditions fostering anaerobic environments and the incorporation of minerals that enhance biomineralization on leaf surfaces. Our results provide new insights into the role of microbial biofilms and microbe–sediment interactions in the early stages of leaf fossilization.