EVIDENCE OF PHOTOSYNTHESIS SHOWED BY NI-PORPHYRIN PRESERVATION IN ~1 GYR-OLD EUKARYOTES (Invited Presentation)

Photosynthetic eukaryotes are key constituents of modern ecosystems. Their appearance is an important step in the evolution of Life, leading to the complexification of trophic networks and making possible the later colonization of land. The early fossil record of eukaryotic algae is not well constrained as their identification is mostly based on their morphological comparison to modern algae, despite possible convergence between clades. Important work has been done to decipher the Precambrian fossil rock record and to characterize new taxonomic tracers within the eukaryotic domain. However, none of these approaches permit to detect the presence of chlorophyll, an essential criteria for evidencing phototrophy.

Here we show, by combining synchrotron-based X-ray fluorescence and X-ray absorption techniques, the earliest in-situ evidence of preserved Ni-porphyrins (degraded chlorophyll) within an organic-walled microfossil, Arctacellularia tetragonala, from the ca. 1 Ga Mbuji-Mayi Supergroup (Congo Basin, Democratic Republic of Congo). This geochemical approach combined to morphological, ultrastructural and spectroscopic analyses support the interpretation of this multicellular branching microfossil as a eukaryotic photosynthetic algae, part of the Archaeaplastidae supergroup.

This discovery, supports the evolution of eukaryotic photosynthesis and the supergoup Archaeaplastidae by at least 1 Ga, consistent with the red algae fossil record. For the first time we were able to identify Proterozoic eukaryotes not only in terms of morphology, but based on their metabolic machinery.