Mécanismes physiologiques et moléculaires impliqués dans la survie de Fragilariopsis cylindrus (diatomée polaire) à une période d’obscurité prolongée

Authors: Sciandra, Théo
Advisor: Babin, MarcelBowler, Chris
Abstract: The polar regions are characterized by extreme environmental conditions whose variations challenge the acclimation capabilities of marine sessile and planktonic organisms. One of the major challenges faced by autotrophic organisms is to survive in darkness during the long winter months. In the spring, the surviving microalgae (mostly unicellular eukaryotes) form large blooms that support the rest of the food web for summer production. Diatoms, particularly well adapted to turbulent nutrient-rich oceanic zones, dominate primary production at the poles. They are often the first to initiate spring blooms, illustrating their extraordinary ability to survive the polar night, but also to resume growth after a very long period of inactivity. Although studied many times in the past, most of the processes involved in survival remain poorly understood. The Green Life in the Dark project, in which this thesis was carried out, aims at elucidating the physiological and genetic mechanisms involved in the survival of diatoms during and just after the polar night. To do so, cultures of Fragilariopsis cylindrus (polar pennate diatom) were subjected in the laboratory to four periods of darkness lasting from one to five months, each followed by a period of re-illumination. F. cylindrus often dominates bloom production in the Arctic and Antarctic. It can also grow attached under the ice and in the water column, making it a relevant representative of polar diatoms. Its genome has also been published. We first set out to test the value of using flow cytometry in the study of survival. This technique allowed us to follow the variations of different physiological parameters of the cultures at the single-cell level, a first in this field of research. The results illustrated the importance of considering the potential inter-individual variations occurring within a population of cells during a long acclimation to darkness. After several weeks in the dark, two subpopulations derived from the initial population were observed and their physiological characteristics studied. Three causes of mortality were identified. The large reservoirs of carbon-rich molecules (sugars, lipids and proteins) are used to store photosynthates. The use of these reserves allows organisms to compensate for a deficit in exogenous energy supply, by providing the metabolites necessary for the functioning of the cell. Associated with a slowing down of the metabolism, this strategy is used by some diatoms to survive the polar night. However, the underlying mechanisms remain poorly understood. The second objective of this thesis was therefore, through a dual approach, to follow the managment of the metabolism of F. cylindrus in the dark. First, by studying the regulation of the transcription of metabolic pathways associated with large carbon compartments, and then by following the variations of the size and nature of the latter. Analyses confirmed the degradation of different carbon stocks accumulated before darkness, as well as the expected metabolic slowdown. The transcriptomic data allowed to underline the importance of the degradation pathways of some branched and aromatic amino acids. The analysis of the different families of molecules showed that reserve lipids and carbohydrates were degraded in the short and medium term, while proteins, the functional biomass, were recycled in a second phase, probably to compensate for the decrease in energy from other compartments.
Document Type: Thèse de doctorat
Issue Date: 2022
Open Access Date: 9 May 2022
Permalink: http://hdl.handle.net/20.500.11794/73362
Grantor: Université Laval
Collection:Thèses et mémoires

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