Soutenance de thèse de Mathilde BON

Mathilde BON - Laboratoire PhLAM - UMR 8523 - Équipe PMI

Titre : Molecular characterization of microfossils by mass spectrometry at the cellular scale

Résumé : 

We will explore times gone by, with a cross-disciplinary approach combining organic geochemistry (LOG), analytical chemistry (Phlam), and palynology (Gand).

The objective is to analyze organic microfossils on an individual scale using mass spectrometry. Rocks that show no obvious trace of life to the naked eye may in reality be a treasure trove of information regarding the evolution of life.Here we focus on their organic composition in order to better identify these traces of life.  My research focuses on organic molecules derived‒among others‒from fatty acids, proteins, and sugars contained in microscopic organic fossils that have withstood burial for hundreds of millions of years.  The rocks we study date from 460 to 420 million years ago (Upper Ordovician to Silurian).

To access these organic-walled microfossils, also known as palynomorphs, we first have to remove all the mineral content from the rocks using acids so that only the fossil organic matter remains. 

We then have to separate the kerogen (insoluble organic matter) from the bitumen (soluble organic matter).  The kerogen contains our palynomorphs, such as microalgae, specimens of relatively unknown biology (acritarchs), probable egg remains of Metazoans (chitinozoans), and others.  We analyzed such individualized microfossils with mass spectrometry assisted by laser beams focalized down to the size of cells.  This reveals chemical formula of their molecules and distinguish their relative abundances.  This way, we have acquired new fundamental knowledge on the structure of the insoluble fraction of fossil organic matter. Ultimately, this should help better understand fossilization processes.  Distinct compositions were observed between different microfossils, in particular based on their oxygen atom content. 

This improves our perspective for molecular identification of fossil microorganisms.  This could then help us understand their nature, track their ancestors in even older rocks, and may help the search for traces of life beyond Earth.