Defense of thesis Mathilde Bon

defense of thesis  Mathlde Bon - laboratory LOG/PhLAM

Abstract :

The origin of the fossil organic matter in very ancient rocks often remains obscure, in part due to the simple morphology of its constituent fossil microorganisms. Obtaining molecular signatures at the scale of the individualized fossil microorganisms (or doubtful fossils, for the oldest candidates) would allow for a better description of the evolution of microbial biodiversity over the past 3.5 billion years. Spatially-resolved methods such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) made it possible to identify bio(macro)molecules such as melanin or haemoglobin derivatives in macrofossils. This project developed further molecular analysis of fossils down to the cellular scale in unicellular microorganisms extracted by acid maceration from rocks (palynomorphs).
We analyzed an exceptional collection of microfossils aged from ~460 to ~421 million years old, including various palynomorphs such as Gloeocapsomorpha prisca (unresolved: cyanobacterium or algae), identified prasinophyte algae (Tasmanites, Cymatiosphaera nimia), other probable phytoplanktonic groups (Leiosphaeridia, other acritarchs), as well as chitinozoans (Metazoan eggs). To decipher the composition of their organic matter, we principally used laser-assisted mass spectrometry including single and two-step laser-desorption laser-ionization high-resolution mass spectrometry (LDI- and L2-HR-MS), and laser desorption-ionization Fourier-transform ion cyclotron resonance mass spectrometry (LDI-FT-ICR-MS). Thanks to its exceptional mass resolution, the latter technique allowed us to perform a petroleomics-type characterization of the kerogen (the solvent-insoluble organic matter) of microfossils, whereby chemical formula can be assigned to almost all peaks of the resulting spectrum. ToF-SIMS and micro-infrared spectroscopy were also used. Bulk kerogen was characterized by Rock-Eval analysis, and bulk bitumen (solved-extracted organic matter) with the laser-assisted techniques.
For all microfossils, the laser-assisted mass spectrometry revealed polyaromatic-rich fragments that are interpreted to reflect the asphaltene-like building blocks of the macromolecular organic matter forming their kerogen. Abundant, and variable contributions of nitrogenated fragments could reflect the encapsulation and/or grafting of protein derivatives (or other molecules) onto the original biopolymers of the fossils. G. prisca yields polyaromatic fragments, some associated with oxygen and/or nitrogen atoms, which had not been documented before, as well as short alkyl chains consistent with previous observations. This composition differs significantly, in particular in terms of abundances of oxygenated fragments, from that of other palynomorphs analyzed in two other source rocks. In one source rock, the composition of Tasmanites and C. nimia was unravelled. Additional analyses and statistical investigation such as principal component analysis are required to fully exploit these heterogeneities as a potential chemotaxonomic tool. In turn, the coupling of molecular composition and morphology could provide better constraints on the paleobiological affinity of these organisms and could ultimately enable the search for their lineage and oldest forms. The techniques applied here to micropaleontology can be transposed to exobiology, as, for instance, a laser desorption-ionization mass spectrometer has been developed for Mars exploration.

Keywords : paleontology, mass spectrometry, organic geochemitry