Intracellular bound chlorophyll residues identify 1 Gyr-old fossils as eukaryotic algae

Abstract

The acquisition of photosynthesis is a fundamental step in the evolution of eukaryotes. However, few phototrophic organisms are unambiguously recognized in the Precambrian record. The in situ detection of metabolic byproducts in individual microfossils is the key for the direct identification of their metabolisms. Here, we report a new integrative methodology using synchrotron-based X-ray fluorescence and absorption. We evidence bound nickel-geoporphyrins moieties in low-grade metamorphic rocks, preserved in situ within cells of a ~1 Gyr-old multicellular eukaryote, Arctacellularia tetragonala. We identify these moieties as chlorophyll derivatives, indicating that A. tetragonala was a phototrophic eukaryote, one of the first unambiguous algae. This new approach, applicable to overmature rocks, creates a strong new proxy to understand the evolution of phototrophy and diversification of early ecosystems.

Fig. 1. Microphotographs of A. tetragonala specimens.

a–g Branched specimens with node cell, equal-diameter branches, lanceolate folds (lf), and intracellular inclusions (ICI). h, i Uniseriate filaments of the microfossils j, Sketch of A. tetragonala, displaying the main features of the microfossil. Black stars correspond to specimens that have been analysed by SR-μXRF.

Fig. 2. Nickel-specific enrichment in intracellular inclusions.

a, g Microphotographs of the studied specimen. b–f Fe and Ni SR-μXRF maps obtained at SLS (b, c pixel: 1 μm, 200 ms/px), at SS (d, e pixel: 400 nm, 100 ms/px) and, the associate composite image (f, R: Fe, B: Ni). These maps show a specific enrichment of Ni uncorrelated of Fe in the intracellular inclusions (ICI) while the walls appear homogeneously enriched in Fe and Ni. h–j Fe and Ni SR-μXRF maps obtained at SLS (h, i pixel: 1 μm, 1 s/px) and the associated composite image (j, R: Fe, B: Ni). These maps show that some ICIs can also be enriched in Fe, but that Fe is present as small hotspots in the ICI while Ni is mainly homogeneously distributed. Color scales correspond to normalized counts. (s.) is for sulfides and (fl.) is for fluorides attached to the surface of the fossils.

Fig. 3. Presence of tetrapyrrole moieties highlighted by XANES analyses on intracellular inclusions.

a Microphotograph of the studied specimens. b, c Fe and Ni SR-μXRF maps obtained at SLS (pixel: 1.5 μm, 200 ms/px) showing the enrichment in Ni and Fe of the intracellular inclusions (ICI). Color scales correspond to normalized counts. d XANES spectra at the Ni K-edge of 4 ICIs performed in zones with low Fe content (gray circles) and their linear combination fitting (red lines) and XANES spectra of two Ni-porphyrin (NiTPP: Ni(II)-tetraphenylporphine, NiOEP: Ni(II)-octaethylporphine), asphaltene, and NiO standards. The shoulder and the spectral line shape are typical of Ni in coordination (IV) in bound Ni-porphyrinic species. Differences between the fitted spectra and the data come from the molecular heterogeneities between the standards used for the fitting and the incorporated tetrapyrroles moieties in the kerogen.