Unravelling the origin of ultralaminae in sedimentary organic matter; the contribution of bacteria and photosynthetic organisms

Conditions for OM preservation depend on both the chemical nature of constituents and environmental factors. Among known preservation pathways, the selective preservation is based on refractory molecules such as algaenans, which are algal components. So far, ultralaminae have been considered as remains of microalgal cell walls on the basis of microscopic and organic geochemical similarities between fossil kerogen and recent algae. This high-resolution microscopic study focuses on organic matter (OM) from different fossil environments, i.e., from shallow to deep water. This OM is compared with recent organic analogs, i.e., microbial mats and biofilm, using transmission electron microscopy. Four possible origins of ultralaminae can be interpreted: algal and bacterial cell walls, thylakoids, and filamentous organisms. This highlights the importance of ultralaminae as a major indicator for paleoenvironmental interpretation. The multiple origin of ultralaminae implies new considerations about OM preservation and depositional conditions. In this way, the role of bacteria in OM accumulations can be reevaluated and the presence of ultralaminae such as thylakoids has a potential to become a new indicator of photosynthesis in fossil sediments.