par Lapeyra Martin, Jon 
Président du jury Flot, Jean-François
Promoteur Gypens, Nathalie
Publication Non publié, 2022-05-27
Président du jury Flot, Jean-François
Promoteur Gypens, Nathalie
Publication Non publié, 2022-05-27
Thèse de doctorat
| Résumé : | Since the rediscovery of plastid retention by ciliates and ingestion of prey by some phytoplankton in the80s, many studies have documented the widespread occurrence of the combination of phago- andphototrophy within marine protists. Despite the widespread occurrence of mixotrophy in marine plankton(mixoplankton) and acknowledgement of its importance, it has been difficult to re-conceptualize protistplankton in this light and research is often still focused on protists as either phytoplankton orzooplankton. Accordingly, the assessment of the mixoplankton diversity, and in particular, of its differentfunctional types (MFTs) is in its infancy. Thus, the overarching objective of this thesis was to characterizethe spatio-temporal diversity trends of mixoplankton in the North Sea, particularly focusing on the twomajor MFTs: constitutive (CM) and non-constitutive (NCM) mixoplankton. The main approach employedto address the objective was DNA metabarcoding, which has become the gold standard technique toassess protist diversity in nature. Multiple oceanographic campaigns were performed to obtain qualityfield samples and assess plankton diversity. The spatio-temporal scope covered (i) more than a yeartime-series in the Belgian Coastal Zone (BCZ); (ii) summertime period with a broad latitudinal scope inthe North Sea; and (iii) a late spring post-phytoplankton bloom snapshot in Southern North Sea. Wegenerated a metagenomic dataset and the distinct spatio-temporal diversity dynamics of MFTs, as wellas the ones of strict autotrophs and heterotrophs were evidenced. Their diversity was correlated toenvironmental variables to better understand their ecology. In the next step, the controversialquantitative value of metabarcoding datasets was addressed by proposing a correction factor (CF) toone of the most important biological biases impeding an accurate community composition: the largelyvarying 18S rRNA gene copy number per cell. The CF was applied to a metabarcoding dataset andresults compared to their corresponding microscopy (morphological) measurements, showing that thequantitative value of certain plankton groups can be improved. Finally, aiming to make molecular dataemployable for biogeochemical ecosystem models representing mixotrophy, it was developed apigment-based approach (chlorophyll-a) to attribute biomass values (proxy) to metabarcodes. Using analready existing model representing the ecosystem of the BCZ, the necessary forcing data wasassembled and a simulation representing the year 2018 was conducted. Model results were comparedto metabarcoding field observations (either applying the CF or not) to explore the potential usefulnessof molecular datasets in the validation stage of biogeochemical models. Overall, this thesis addressesthe information gap regarding MFT diversity trends across broad geographical and temporal scales inthe North Sea. The results provided precious baseline and complementary knowledge on thequantitative significance of eukaryotic plankton metabarcoding. Finally, the contemporary challengesmarine plankton and mixoplankton research are facing to combine molecular and modelling approacheswere highlighted. |
