Unravelling the molecular taphonomy of organic-walled dinoflagellate cysts via infrared spectroscopy

Dinoflagellates are an extant group of unicellular, eukaryotic microalgae occurring in a wide range of aquatic environments. They can form resistant, organic-walled resting stages (dinocysts); the earliest body fossils appear in the Middle Triassic. Dinocyst walls are composed of a heavily crosslinked, nitrogenous, cellulose-like carbohydrate, ‘dinosporin’, which is compositionally distinct from other resistant biomacromolecules such as sporopollenin (pollen and spores) and algaenan (green algae). Macromolecular analyses of modern dinosporin via attenuated total reflection microFourier transform infrared (ATR micro-FTIR) spectroscopy revealed a variable compound: sometimes strongly influenced by the presence of brown colour-inducing (possibly eumelanin) pigments, or with additional aromatic (‘sporopollenin-like’) or aliphatic (‘algaenan-like’) moieties. Here we used ATR micro-FTIR spectroscopy to collect a large dataset from a wide range of Meso- and Cenozoic dinocysts to investigate their molecular taphonomy. These data reveal an expected increase in the aliphaticity of dinosporin over time, likely related to early-diagenetic in situ polymerization of lipids. Furthermore, modern dinosporin variability was detected in fossil dinocysts up to the late Paleocene, highlighting the palaeoecological and taxonomical value of these chemospecific signatures. Finally, it is hypothesized that some dinosporin types might be better suited for low-salinity (aliphatic type) or high-UV (pigmented and aromatic types) aquatic environments.