Resumen de Respuestas ecofisiológicas al enriquecimiento de amonio en angiospermas marinas
Anthropogenic nutrient loading in coastal areas is one of the major causes of seagrass decline worldwide. One of the main negative effects is indirectly caused by the proliferation of fast-growing species promoting light attenuation, sediment anoxia and sulphide intrusion risks. However, the parallel direct toxic effect caused by high ammonium concentrations on seagrasses has received little attention, in spite of some pioneer works. Adverse effects of ammonium enrichment have been traditionally explained by internal accumulation of ammonium. To prevent toxic effects, plants must assimilate this nutrient into amino acids and other non-toxic organic compounds, generating strong internal demand for ATP and organic carbon skeletons, which must be provided by photosynthesis or from C-reserves. Thus, any factor affecting ammonium uptake and/or assimilation could intensify or ameliorate ammonium toxicity in seagrasses. This PhD Thesis studies the interactive effect of ammonium enrichment and different environmental factors in the genus ¿Zostera¿. In the first two chapters, the interaction between ammonium and phosphate uptake rates and the interactive effect of light, hydrodynamics and ammonium enrichment was studied in ¿Z. noltei¿. In these assays ammonium was taken up following a diffusive trend, and while a shortcoming of phosphate uptake was found in leaves of ¿Z. noltei¿ when ammonium was present, ammonium uptake was unaffected by phosphate presence. As well, a non-linear response to ammonium enrichment with flow velocity was recorded, with the strongest negative effect at intermediate flow. The effects of light and ammonium on morphology, physiology, nitrogen metabolism and carbon reserves were studied in ¿Z. marina¿ (chapter 3). Light reduction had negative and synergistic effects with ammonium enrichment, which were related with a drastic drop in carbon reserves and a remarkable increase in amino acid concentrations, indicating a tight coupling between carbon an nitrogen metabolisms. The response to the interaction between ammonium loading and hyposaline stress was analyzed on different physiological and morphological properties of ¿Z. marina¿ (chapter 4). The negative and interactive effect between high ammonium concentrations and low salinity was correlated with an increase in intracellular ammonium content and a decrease in photosynthetic rates and non-structural carbohydrates, causing a drop in growth and survival of plants. Thus, hyposaline stress enforced the toxic effect of ammonium by increasing the competition between ammonium assimilation and osmotic regulation for energy and C-skeletons. In summary, this Thesis highlights that toxic effects promoted by ammonium on seagrasses can be intensified or alleviated depending in the presence of other natural stressors. In nature, organisms are rarely stressed by only one factor, and the interaction among multiple stressors may drive synergistic, antagonist or even non- linear responses. The understanding of nature of such interactions is important for developing better predictions and managing policies for seagrass ecosystems, since ammonium load is expected to increase in the near future in coastal areas, jointly with the modification of a broad range of environmental factors as a consequence of the global change.
