Biosignatures of fossil and modern cyanobacteria
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".