Marie Catherine Sforna and colleagues have discovered chlorophyll remnants in a microfossil from the Congo Basin. What makes this extraordinary is that the remnants are in situ in a billion-year-old fossil of Arctacellularia tetragonala, a multicellular algae. The discovery was made possible by a new technique, combining morphological, chemical, and ultrastructural analyses with synchrotron-based X-ray Fluorescence (SR-XRF) and X-ray Absorption Spectroscopy (SR-XAS).
Chlorophyll is challenging to track in fossils, and it can easily break during burial or diagenesis – the physical and chemical process that is the first stage of fossil formation. They can be transformed into geoporphyrins, more stable chemical structures that retain features allowing palaeontologists to identify the source molecules. The presence of the right geoporphyrins in fossil material has allowed palaeontologists to recognise the existence of chlorophyll associated with fossils from a billion years ago. However, techniques haven’t allowed scientists to identify chlorophyll derivatives with specific individual fossils.
In addition, you need to have the right rock for your fossil. If your rock overheated during its maturation, the biomarkers you’re looking for will have broken down. The new technique solves both these problems, finding the necessary molecules even in overmature rocks. The key to unlocking the fossils was nickel enrichment in the condensed cytoplasm of cells. These nickel-geoporphyrins still contain the structure necessary to identify them as chlorophyll derivatives.
A press release from the Early Life Traces & Evolution Laboratory (Astrobiology / Faculty of Science) at the University of Liège states: “This new methodology, applicable to billion-year-old supermature rocks, provides a new approach to understanding the evolution of eukaryotic phototrophy during the Precambrian and the diversification of primary producers in early ecosystems.”
In this case, the word ‘early’ means very early. Sforna and colleagues think their technique could possibly reach more than twice as far back into the past. In their paper, the authors write: “This approach offers the possibility to track porphyrins, and thus phototrophy, much further back in time, perhaps even into the Archean. Indeed, the ability of XANES to detect tetrapyrrole structures bound to insoluble organic matter reduces the risk of contamination and allows the assignment of bound porphyrin derivatives to individual microfossils.”
Sforna, M.C., Loron, C.C., Demoulin, C.F., François, C., Cornet, Y., Lara, Y.J., Grolimund, D., Ferreira Sanchez, D., Medjoubi, K., Somogyi, A., Addad, A., Fadel, A., Compère, P., Baudet, D., Brocks, J.J. and Javaux, E.J. (2022) “Intracellular bound chlorophyll residues identify 1 Gyr-old fossils as eukaryotic algae,” Nature Communications. https://doi.org/10.1038/s41467-021-27810-7