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Differential ecophysiological responses to inorganic nitrogen sources (ammonium versus nitrate) and light levels in the seagrass Zostera noltei

Rocío Jiménez-Ramos*, Beatriz Villazán, Luis G. Egea, Rosa Cantero, José Lucas Pérez-Lloréns, Juan José Vergara, Fernando G. Brun

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ABSTRACT: Seagrasses can use both ammonium (NH4+) and nitrate (NO3-) as inorganic nitrogen sources. However, NO3- uptake and assimilation are energetically more expensive and tightly regulated than NH4+ uptake. The objective of this study was to test the complex interactive effects between different forms of N enrichment (NH4+ and NO3-) and light levels on the morphological and physiological traits in the intertidal seagrass Zostera noltei. Plants were cultured over 40 days under two levels of light (low and high) and two inorganic nitrogen concentrations supplied at the same dose: NO3- (25 µM), NH4+ (25 µM) and control, following a 2-factorial design. The results showed a differential response in Z. noltei depending on the inorganic nitrogen source and the light dose. NH4+ enrichment negatively affected almost all morphometric and dynamic variables analyzed, both in isolation and combined with low light conditions. In contrast, NO3- enrichment, compared with the control treatment, had a positive effect on Z. noltei survival in terms of net growth rate and rhizomatic growth, mainly under high light conditions. Therefore, our study demonstrated that the effects promoted by nutrient enrichment largely depend on the source of nitrogen used. Light levels play a crucial role in this response by potentially shifting the effect from toxic (under low light) to beneficial (under high light) when NO3- is the main nitrogen source. Our findings highlight that nitrogen form in eutrophication events should be considered when evaluating the potential impacts of nutrient enrichment and light reduction on seagrass communities.