Polysphaeroides filiformis, a proterozoic cyanobacterial microfossil and implications for cyanobacteria evolution
Catherine Demoulin, Marie-Catherine Sforna, Yannick Lara, Yohan Cornet, Andréa Somogyi, Kadda Medjoubi, Daniel Grolimund, Darío Ferreira Sánchez, Remi Tucoulou Tachoueres, Ahmed Addad, Alexandre Fadel, Philippe Compère, Emmanuelle Javaux
Abstract
Deciphering the fossil record of cyanobacteria is crucial to understand their role in the chemical and biological evolution of the early Earth. They profoundly modified the redox conditions of early ecosystems more than 2.4 Ga ago, the age of the Great Oxidation Event (GOE), and provided the ancestor of the chloroplast by endosymbiosis, leading the diversification of photosynthetic eukaryotes. Here, we analyze the morphology, ultrastructure, chemical composition, and metals distribution of Polysphaeroides filiformis from the 1040–1006 Ma Mbuji-Mayi Supergroup (DR Congo). We evidence trilaminar and bilayered ultrastructures for the sheath and the cell wall, respectively, and the preservation of Ni-tetrapyrrole moieties derived from chlorophyll in intracellular inclusions. This approach allows an unambiguous interpretation of P. filiformis as a branched and multiseriate photosynthetic cyanobacterium belonging to the family of Stigonemataceae. It also provides a possible minimum age for the emergence of multiseriate true branching nitrogen-fixing and probably heterocytous cyanobacteria.