| Summary: | [eng] N2-fixers bacteria or diazotrophs reduce atmospheric nitrogen (N2) into ammonia (NH3) and play a significant role in the ocean's N-cycles by providing new inorganic nitrogen to planktonic and benthic primary producers. The epiphytic and endophytic N2-fixing population found in association with the endemic Mediterranean seagrass Posidonia oceanica can potentially provide the entire N-demand to this seagrass. The Mediterranean Sea is one of the most oligotrophic seas in the world and at the same time subject to global climate change factors (e.g., ocean acidification and warming) and emerging pollutants (e.g., plastic pollution). Considering the importance of N2-fixers in the health and functioning of seagrasses, nothing is known about the effects of current environmental challenges in these microorganisms. From a physiological and molecular point of view, this thesis investigates the role of nutrient availability, climate change factors (i.e., temperature, pH and CO2) and emerging pollutants (i.e., plastics) through pioneering multifactorial experiments. Different species of N2-fixing phototrophic and heterotrophic bacteria found in association with P. oceanica were used as model test species. The thesis is divided into two main sections, revealing the role of (I) nutrient availability [phosphorus (P), iron (Fe) and nitrogen (N)] and (II) anthropogenic factors (CO2 and its concomitant effect on ocean acidification and warming, and microplastics and their organic additives) in regulating the functioning of N2-fixers associated with P. oceanica. The results show that the responses of the N2-fixing bacteria to environmental factors and emerging pollutants tested are species-specific, suggesting that the N2-fixing community structure associated with Posidonia oceanica will change in response to these factors. [spa] Las bacterias fijadoras de N2 o diazótrofos reducen el nitrógeno atmosférico (N2) en amoníaco (NH3) y desempeñan un papel esencial en los ciclos del N del océano, al proporcionar nuevo nitrógeno ...
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