Application of chemotaxonomy to the study of the phytoplankton community structure in the Mediterranean Sea and in the Atlantic and Southern Oceans = Aplicación de la quimiotaxonomía al estudio de la estructura de la comunidad fitoplanctónica en el Mar Mediterráneo y en los oceános Atlántico y Antártico

Despite their small size, phytoplankton represents up to 50% of the global primary production of the Earth and serves as a connector between oceanic and atmospheric processes. Therefore, phytoplankton constitutes a central pillar in the comprehension of the cycles of mass and energy in the oceans. Their different attributes like size, abundance, community composition, functions, pigment complex and taxonomy provide relevant information to understand the processes of global change. In the past decades, the use of HPLC chromatography (High-Performance Liquid Chromatography) has proven as a very effective methodology to study marine phytoplankton. This technique, combined with the use of the statistic program CHEMTAX, allows to identify efficiently the contribution of certain taxonomic classes. This thesis is centered around the use of HPLC-CHEMTAX to identify the phytoplanktonic communities of three regions: the Mediterranean Sea, the Atlantic Ocean and the Antarctic Ocean, each characterized by different trophic conditions (from oligotrophic to eutrophic). With the results obtained, we expect to contribute to the comprehension of the local biochemical responses in a context of global change, as well as to improve future models of ecosystems and algorithms for remote sensing. To validate the data obtained, the results originated from the HPLC-CHEMOTAX were compared to other techniques used for identification and quantification, such as microscopy, flow cytometry and bio-optics. First, we focused on a temporal series (14 years) where we analyzed the seasonal and inter-annual dynamics of the phytoplanktonic community. We have verified that the phytoplanktonic community follows a seasonal pattern, just as had been observed in previous studies: diatoms dominated the blooms of late winter/early spring, while Synechococcus showed maximum abundances during the months of April and August. The cryptophytes and dinoflagellates showed a positive response to sporadic fertilization events, associated mainly to stormy events. In the next study of this thesis, we analyzed the size fractions obtained from sequential filtrations by HPLC-CHEMTAX (total, n+m>3 µm and pico