Response of marine microbial communities to natural fertilization events

Abstract

Surface microbial communities are essential to the functioning of marine ecosystems. Phytoplankton and bacteria drive key ecological and biogeochemical processes in the upper ocean, including carbon fixation, nutrient cycling and organic matter transformation. These processes are strongly modulated by nutrient availability, which varies across oceanic regions and over different time scales. In the subtropical Northeast Atlantic, natural fertilization events–such as Saharan dust deposition and shallow submarine hydrothermal emissions–release pulses of nutrients and trace elements to surface waters. Although these inputs influence surface chemistry and productivity, their effects on microbial ecology remain underexplored. This thesis combines experimental bioassays with long-term field observations to study how such geochemical perturbations influence phytoplankton and bacterial communities, focusing on changes in cell abundance, metabolic activity and community structure. Microbial responses were assessed under diverse environmental conditions, including surface productivity gradients and seasonal variability, to understand how these responses are also shaped by broader oceanographic processes and climate variability at local and regional scales. The effects of Saharan dust deposition on surface microbial communities were examined in the Mauritanian-Senegalese coastal transition zone, an area subject to frequent dust intrusions and strong surface productivity gradients. Dust-enrichment experiments revealed increased microbial metabolic activity, particularly bacterial production relative to primary production, especially under low-nutrient, oligotrophic conditions. Dust additions increased the concentrations of bioavailable nutrients in seawater, such as nitrate, phosphate and, to a lesser extent, silicate, which selectively favored the growth of specific microbial groups based on their distinct metabolic requirements and ecological traits. Future increases in atmospheric dust deposition, combined with a projected weakening in the intensity of the Mauritanian-Senegalese upwelling, could thus favor heterotrophic processes and potentially reduce the efficiency of this region for carbon sequestration. Meanwhile, the influence of shallow-sea hydrothermal activity on microbial communities was examined at the Tagoro submarine volcano (Canary Islands, Spain), where persistent diffuse emissions of nutrient-rich, low-temperature and low-pH fluids often reach sunlit surface waters. Using over a decade of hydrographic data, we assessed the spatial extent and temporal evolution of these hydrothermal inputs, characterized by silicate and phosphate among other compounds, and their effects on microbial dynamics. Experimental simulations of natural dilutions of hydrothermal fluids showed that these nutrient-rich inputs promoted primary production, particularly favoring small eukaryotic phytoplankton, as well as photo- and chemoautotrophic bacteria. Seasonal variations influenced the magnitude of microbial responses, highlighting the importance of background environmental variability when assessing microbial responses to natural geochemical perturbations. Furthermore, field observations revealed distinct microbial community structures across hydrothermally influenced epipelagic and benthic habitats, dominated by specialized microbial taxa associated with nitrogen, iron, sulfur and methane cycling. These findings reveal broader biogeochemical processes than previously recognized in the area, along with notable novel microbial diversity with potential biotechnological applications, underscoring the need for further research into these often overlooked, geochemically dynamic shallow hydrothermal systems. Altogether, this thesis demonstrates that distinct natural geochemical fertilization events can significantly alter surface marine microbial communities, with implications for nutrient cycling, carbon export, productivity and biodiversity in the subtropical Northeast Atlantic, particularly under future climate scenarios of increasing environmental variability.other compounds, and their effects on microbial dynamics. Experimental simulations of natural dilutions of hydrothermal fluids showed that these nutrient-rich inputs promoted primary production, particularly favoring small eukaryotic phytoplankton, as well as photo- and chemoautotrophic bacteria. Seasonal variations influenced the magnitude of microbial responses, highlighting the importance of background environmental variability when assessing microbial responses to natural geochemical perturbations. Furthermore, field observations revealed distinct microbial community structures across hydrothermally influenced epipelagic and benthic habitats, dominated by specialized microbial taxa associated with nitrogen, iron, sulfur and methane cycling. These findings reveal broader biogeochemical processes than previously recognized in the area, along with notable novel microbial diversity with potential biotechnological applications, underscoring the need for further research into these often overlooked, geochemically dynamic shallow hydrothermal systems. Altogether, this thesis demonstrates that distinct natural geochemical fertilization events can significantly alter surface marine microbial communities, with implications for nutrient cycling, carbon export, productivity and biodiversity in the subtropical Northeast Atlantic, particularly under future climate scenarios of increasing environmental variability.