| Summary: | Programa de Doctorado en Oceanografía y Cambio Global por la Universidad de Las Palmas de Gran Canaria Invisible to the naked eye, planktonic prokaryotes—comprising both bacteria and archaea—represent a large fraction of marine biomass, with abundances typically ranging from thousands to millions of cells per millilitre. From primary producers (photo- and chemoautotrophic) to heterotrophs, they display extremely diverse metabolisms and are of the utmost importance for the biogeochemical cycles in the ocean. During their involvement in elemental cycling, prokaryotes interact with the organic matter continuum (particulate and dissolved), utilising it as source of energy and carbon. In doing so, they transform the organic matter pool, diversifying it and remineralising compounds into their inorganic constituents. Moreover, by converting organic matter into biomass, prokaryotes return carbon into the marine trophic webs, a process known as the microbial loop. Hence, this pathway is crucial for organic matter cycling in the oceans. Upwelling regions represent some of the most productive marine systems, with large pools of organic matter subject to dynamic interactions with prokaryotic communities. Such environments thus have major importance for carbon cycling at the global scale, making very relevant the study of the prokaryotes that inhabit them. In this thesis we combine mesocosm experiments and synoptic field samplings to study prokaryotic communities in upwelling environments, with special attention to how they relate to organic matter cycling. The first half of the thesis is devoted to an upwelling simulation experiment carried out in the oligotrophic waters of the subtropical Eastern North Atlantic. By simulating different upwelling intensities and frequencies (singular pulse versus recurring upwelling), we studied how the dissolved organic matter pool and the prokaryotic communities respond to variable upwelling scenarios. First, we measured the dissolved organic matter concentrations and optical ...
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