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

Abstract

Seagrasses can use both ammonium (NH4+) and nitrate (NO3-) as inorganic nitrogen (N) 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 d under 2 levels of light (low and high) with 2 inorganic N concentrations supplied at the same dose, NO3- (25 µM) and NH4+ (25 µM), and a control, following a 2-factorial design. Results showed a differential response in Z. noltei depending on the inorganic N source and 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 had a positive effect on Z. noltei survival compared with the control treatment 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 N used. Light levels play a crucial role in this response by potentially shifting the effects from toxic (under low light) to beneficial (under high light) when NO3- is the main N source. Our findings highlight that N form in eutrophication events should be considered when evaluating the potential impacts of nutrient enrichment and light reduction on seagrass communities.