PRIMARY PRODUCERS IN THE ~1GYRS-OLD MBUJI-MAYI SUPERGROUP (DR CONGO)

Primary producers, mostly phototrophic organisms, are the base of the trophic networks, controlling the productivity at all ecosystemic levels. Since the Mesozoic, these primary producers are mostly chlorophyll c algae Proterozoic marine productivity and primary producers are, however, very poorly constrained. The biomarker record suggests that cyanobacteria and other photosynthetic bacteria were the main primary producers for most of the early Proterozoic, before the rise of phototrophic eukaryotes (green and red algae) during the Neoproterozoic. However, the bulk biomarker record and the fossil record are not necessarily linked and the identity of many fossils remain enigmatic.

The combination of morphological, ultrastructural and spectroscopic analyses to synchrotron-based X-ray fluorescence and X-ray absorption techniques has shown its potential to identify, directly within the microfossils, the presence of Ni-porphyrinic moieties that are remnants of chlorophyll. This allowed to the unambiguous identification of a phototrophic multicellular eukaryote, A. tetragonala, in the shales of the 1 Gyrs-old Mbuji-Mayi Supergroup (DRC).

To further reconstruct the Mbuji-Mayi ecosystem, we applied this approach to five additional taxa of organic-walled microfossils co-occurring with A. tetragonala. These taxa were chosen based on their previously proposed biological affinity (two possible cyanobacteria, one possible xanthophyte alga, one possible osmotroph and one possible alga). These taxa are ubiquitous in the coeval fossil record and display frequent intracellular inclusions (ICIs), corresponding to condensed cytoplasm, an essential feature to detect porphyrinic moieities.

All taxa display metal enrichment in their inclusions but only the cyanobacteria and the algae display a specific Ni enrichment that can be linked with Ni-porphyrinic moieties. This suggests that these four taxa are indeed phototrophic organisms, supporting our approach of phototrophy detection in the fossil record. For the first time, we are able to identify several primary producers present in the ocean at the Mesoproterozoic–Neoproterozoic transition.