| Summary: | Programa de Doctorat en Ciències del Mar / Tesi realitzada a l'Institut de Ciències del Mar (ICM-CSIC) [eng] Marine phytoplankton are responsible for approximately half of the photosynthetic production of organic matter (OM) and oxygen in Earth. The composition and reactivity of phytoplankton- derived OM influences two of the main C-sequestration mechanisms of the ocean: the biological carbon pump and the microbial carbon pump. Phytoplankton-derived OM can be classified as particulate (POM) or dissolved (DOM) and these size-fractions are subject to diverse production, consumption and transport processes involving biotic and abiotic interactions. Understanding how these processes influence OM composition and reactivity is essential to accurately describe the role of phytoplankton ecology in the marine Carbon cycle and ultimately in the regulation of Earth climate. This thesis aims, precisely, to better understand the controls over these processes. To do so, we combined fluorescence spectroscopy and elemental analysis of POM and DOM with multiple biotic and abiotic parameters during the development and decay of phytoplankton proliferations in micro- and mesocosm experiments and under natural conditions. The microcosm degradation experiment revealed that POM derived from diatom-dominated proliferations is degraded at a much slower rate than that of POM produced by a mixed phytoplankton community. In addition, accumulation of DOM of apparent recalcitrant nature was observed during the processing of diatom-derived POM. The analysis of four phytoplankton proliferations in Antarctic waters revealed that protein-like fluorescent OM was contributed by dissolved and particulate materials. The abundance and composition of phytoplankton and their interactions with viruses and grazers were identified as the main controls over the quantity and fractionation of protein-like fluorescent OM. By contrast, humic-like substances were mostly in the dissolved fraction, and their composition was related to photochemical degradation ...
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