Abstract :
[en] Cyanobacteria are well-known for being involved in events that profoundly modified
the early biosphere and Earth. Cyanobacteria are the only prokaryotes doing the
oxygenic photosynthesis. Thanks to this ability, they played a role in the oxygenation
of the atmosphere and oceans during the Great Oxidation Event that took place
around 2.4 Ga, which generated the development of new oxygenated ecological
niches. Moreover, they are the ancestors of the chloroplast, the organelle where
oxygenic photosynthesis takes place in eukaryotes. The chloroplast was formed
during primary endosymbiosis from an engulfed and undigested cyanobacterium by a
primitive unicellular eukaryote. Through the development of oxygenic niches and the
formation of the chloroplast, cyanobacteria had a key role in the diversification of
eukaryotes. Consequently, their study is of great importance to understand the
evolution of the early Life and Earth. Despite this importance and the abundance of
microfossils interpreted as cyanobacteria, their unambiguous fossil record remains
scarce. Only three microfossils were identified for certainty as cyanobacteria using a
set of criteria unique to this group of microorganisms.
This thesis focuses on the detailed study of microfossils interpreted as probable
cyanobacterial microfossils, which may in fact represent also other prokaryotes or
eukaryotes. The objective is to evaluate, using new criteria, and confirm or infirm with
certainty their identification and improve our understanding of their early
diversification. The morphology, morphometry, ultrastructure, chemical composition
and elements distribution of selected Proterozoic fossil taxa (Polysphaeroides
filiformis, Arctacellularia tetragonala, Chlorogloeaopsis spp., Navifusa majensis) were
analyzed using a combination of techniques. These data enabled to interpret reliably
two microfossil taxa as cyanobacteria, Polysphaeroides filiformis (Mbuji-Mayi
Supergroup, Democratic Republic of the Congo) and Navifusa majensis (Tawallah
Group, Australia; Shaler Supergroup, Canada). The multicellular branching P.
filiformis was interpreted as a cyanobacterium belonging to the Stigonematacean
family. Moreover, the ultrastructure of N. majensis highlighted the oldest thylakoids
preserved in fossil cells, bringing a direct evidence for oxygenic photosynthesis. Also,
the detection of nickel in intracellular inclusions within fossil cells highlighted the
preservation of degraded chlorophyll in the form of Ni-geoporphyrins. Therefore, this
thesis proposes two new calibration points for cyanobacterial molecular clocks: 1. a
first point (minimum age ~1 Ga) for heterocytous nitrogen-fixing cyanobacteria; 2. a
second point (minimum age ~1.75 Ga) for the divergence of thylakoid-bearing
cyanobacteria from the thylakoid-less cyanobacteria (Gloeobacter). Finally, this work
also shows that the study of modern lineages is important for the comparison with
microfossils as modern microorganisms may produce compounds exclusively found
in a particular group, e.g. cyanobacteria with the production of scytonemin and
gloeocapsin, UV-screening cyanobacterial pigments, which could be looked for in
microfossils.
