Large-scale biogeography of marine pelagic bacteria and archaea
- Autores: Guillem Salazar Guiral
- Directores de la Tesis: Josep M. Gasol (dir. tes.), Silvia G. Acinas (codir. tes.), Vicente Gracía García (tut. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Carles Pedrós-Alió (presid.), María Sebastián López (secret.), Gerhard J. Herndl (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencias del Mar por la Universidad de Barcelona y la Universidad Politécnica de Catalunya
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- Resumen
The dark ocean contains about 70% of the ocean’s microbial cells and 60% of its heterotrophic activity, which is mainly fueled by the flux of organic particles produced in the surface ocean and exported to the bathypelagic ocean (1,000 – 4,000 m depth). The bathypelagic ocean represents a nonhomogeneous environment and contains a variety of particles that are considered as the main supply of organic carbon to this environment. The microorganisms inhabiting this realm play a pivotal regulatory role in the biogeochemical cycles at a planetary scale. Accordingly, the study of these microorganisms is an essential step to decipher the ecological functioning of the deep ocean.
Chapters 1 to 3 in this Thesis are dedicated to the description of the prokaryotic community composition in the bathypelagic ocean at a global scale through the sequencing of ribosomal DNA and RNA fragments using data collected during the Malaspina 2010 expedition. Chapter 1 identifies the dominant prokaryotes in the deep ocean and reveals a high proportion (~50%) of previously undescribed prokaryotes. The water masses and the structure of the deep ocean’s floor, organized into basins, are identified as the main drivers of their biogeography. Chapter 2 addresses the differences between free-living and particle-attached bathypelagic prokaryotic communities. This is shown to be a phylogenetically conserved trait, indicating that the bathypelagic particles and the water surrounding them constitute two distinct niches and that transitions from one to the other have been rare at an evolutionary timescale. Finally, in Chapter 3 we identify a linear relationship between the 16S RNA/DNA ratio and particle attachment preference, suggesting a global relationship between the prokaryote’s preference for a particle-attached lifestyle and their growth rate.
While the deep ocean is a highly unexplored environment, a more complete knowledge exists for the epipelagic ocean (0 – 200 m depth). Steep gradients of light intensity and quality, temperature and nutrient availability characterize the oceans and impact on the distribution of species. However, different processes, such as the sinking of particles and the vertical movement of water masses, have been described as mechanisms capable of connecting the surface and deep layers of the ocean. These same processes could transport entire prokaryotic communities, a process theoretically proposed but never tested. In Chapter 4 we develop a tool (mtagger) for the extraction of short 16S ribosomal reads from metagenomes to describe the taxonomical composition of microbial communities. We propose and evaluate technical improvements compared to previous versions as a benchmark for its use in the last chapter. Chapter 5 is dedicated to the development of a modeling tool (disperflux) for the analysis of prokaryotic communities’ connectivity using data collected during the Tara Oceans expedition. We observe and describe a fast-decay relationship between community similarity and depth, which is consistently fitted by a power-law across the whole dataset, with the exception of 5 stations that are compatible with events of whole community export from the photic ocean to the mesopelagic.
In summary, this Thesis significantly contributes to the knowledge on the ecological functioning of marine prokaryotes by describing the structure of prokaryotic communities along the bathypelagic realm and the vertical gradient of the ocean and by the development of original methodological tools that may be applied to a variety of environments.
