Aperçus taxonomiques et génomiques des communautés microbiennes face à un environnement fluctuant dans la Polynie des Eaux du Nord (Canada-Groenland)

Authors: Freyria, Nastasia J.
Advisor: Lovejoy, Connie.Boudreau, Denis
Abstract: As the Arctic Ocean freshens due to multiyear ice and Greenland Ice Sheet melt, the microbial communities are being exposed to greater salinity fluctuations over the growth season and across wide geographic areas. Although remote sensing and limited in situ data have shown that the highly productive North Water has undergone marked changes in the seasonal patterns of surface chlorophyll. Interpreting this data in terms assemblages of species is difficult since the community phenology and spatiotemporal variability is unknown. Understanding the dynamics of phytoplankton and other microbial species assemblages is critical to understand ecosystem responses to global change. Moreover, greater salinity fluctuations or salt stress and osmotic shock are among the main environmental factors limiting the growth and productivity of plants and microorganisms. To address this knowledge gap, firstly, high throughput amplicon sequencing was applied to investigate summer-fall Arctic microbial communities from two sides of Northern Baffin Bay over 12 years (2005 to 2018), that are subjected to very different stratification and major current regimes. Secondly, we investigated the transcriptional response of an ice-associated microalga exposed to progressively decreasing salinities to gain a deeper understanding of the genetic capacity of this alga to tolerate salinity fluctuations, as well as to identify key stress genes. And lastly, we combined comparative metagenomic approach with 18S rRNA amplicon sequencing to investigate the functional capacities, diversity and dynamics of the microorganisms in upper water column of the Northern Baffin Bay during two consecutive summers (2017-2018). We found that seasonality was a major factor determining communities with summer species complex Chaetoceros socialis-gelidus and Micromonaspolaris dominating phytoplankton and Alpha and Gammaproteobacteria dominating bacterial community. In autumn uncultured undescribed open water dinoflagellates were favored. Our results suggest that Arctic autumn production due to longer open water conditions may not mirror a diatom dominated spring bloom as projected in some scenarios of longer ice-free seasons. Furthermore, a gradual response and specific adaptation to cold saline Arctic conditions were seen with differential expression of several antifreeze proteins, an ice-binding protein and acyl-esterase involved in cold adaptation. Our results suggest that ice-binding protein may also have a crucial role for the adaptation of a rapid change of salinity, by providing survival advantage to the cells. Moreover, through functional annotation, we examined potential genes involved in photosystem protection, osmotic regulation and antifreeze proteins, to be crucial for acclimatization to cold environment. The use of comparative genomics would reveal evolutions of gene families conferring adaptation of microorganisms to transitions between salt and fresh water. The combination of these several methodologies provides invaluable insights into the composition, the metabolic repertoire and putative functional profile of a microbial assemblage. This thesis work will have implications not only for the Arctic where surface communities are subject to large fluctuations in salinity and light but will also contribute to fundamental knowledge in the fields of biofuel production and harmful algal bloom prediction.
Document Type: Thèse de doctorat
Issue Date: 2021
Open Access Date: 10 January 2022
Permalink: http://hdl.handle.net/20.500.11794/71432
Grantor: Université Laval
Collection:Thèses et mémoires

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