ORIGINAL ARTICLE Inorganic nitrogen uptake and related enzymatic activity in the seagrass Zostera noltii Ana Alexandre, Joa ˜ o Silva & Rui Santos Marine Plant Ecology Research Group, CCMAR, Centre of Marine Sciences, Universidade do Algarve, Gambelas, Faro, Portugal Problem Seagrasses are marine angiosperms inhabiting shallow coastal areas, sheltered bays and coastal lagoons. Nutrient recycling is one of their most relevant ecological functions (Hemminga & Duarte 2000). Inorganic nitrogen is avail- able both as nitrate and ammonium, either in the water column or in sediment porewater. In seagrass habitats, the nitrate concentration in the water column typically ranges from 0 to 8 lm, whereas ammonium ranges from 0 to 3.2 lm. In sediment porewater, ammonium concen- tration is usually much higher, ranging from 1 to 180 lm, whereas nitrate concentration is almost negligible (cf. Touchette & Burkholder 2000). Young, actively growing seagrass roots take up most of the porewater nitrogen as ammonium, whereas leaves take up both ammonium and nitrate from the water column (Pedersen & Borum 1992; Hemminga et al. 1994; Stapel et al. 1996; Pedersen et al. 1997; Terrados & Williams 1997). Typically, seagrasses have higher affinity and higher uptake rates for ammo- nium than for nitrate (Lee & Dunton 1999; Touchette & Burkholder 2000; Dudley et al. 2001) mostly because the ammonium assimilation process is energetically less expensive (Bloom et al. 1992). Leaves are more efficient than roots in absorbing low levels of ammonium (Lee & Dunton 1999). To avoid toxic effects inside the cells, ammonium must be rapidly converted into glutamate, a process driven by the enzyme glutamine synthetase (Touchette & Burkholder 2000). Following its absorption into the tissues, nitrate is reduced to nitrite and then to ammonium. The first step of this process is the reduction of nitrate to nitrite, a reaction mediated by the enzyme nitrate reductase. Recently, the capacity for organic nitro- gen uptake (urea and amino acids) was demonstrated in two seagrass species (Vonk et al. 2008). The organic nitrogen uptake rates were always lower than those of inorganic nitrogen, except the uptake of amino acids by the roots. The objective of this study was to identify the preferen- tial inorganic nitrogen source of the seagrass Zostera noltii (= Nanozostera noltii). Rates of ammonium and nitrate uptake were determined at different concentrations and when supplied separately or simultaneously. Leaf and root nitrate reductase and glutamine synthetase activity was Keywords Glutamine synthetase; nitrate reductase; nitrogen; seagrass; uptake. Correspondence Ana Alexandre, Marine Plant Ecology Research Group, CCMAR, Centre of Marine Sciences, Universidade do Algarve, Gambelas, 8005-139 Faro, Portugal. E-mail: aalexandre@ualg.pt Accepted: 29 March 2010 doi:10.1111/j.1439-0485.2010.00378.x Abstract The preferential inorganic nitrogen source for the seagrass Zostera noltii was investigated in plants from Ria Formosa, South Portugal. Rates of ammonium and nitrate uptake were determined at different concentrations of these nutri- ents (5, 25 and 50 lm), supplied simultaneously (NH 4 NO 3 ) or separately (KNO 3 and NH 4 Cl). The activity of the enzymes nitrate reductase (NR) and glutamine synthetase (GS) was also assessed. The results showed that ammo- nium is the preferential inorganic nitrogen source for Z. noltii, but, in the absence of ammonium, the species also has a high nitrate uptake capacity. The simultaneous availability of both inorganic nitrogen forms enhanced the uptake rate of ammonium and decreased the uptake rate of nitrate compared to when only one of the nitrogen forms was supplied. The activity of both enzymes was much higher in the leaves than in the roots, highlighting the importance of the leaves as primary reducing sites in the nitrogen assimilation process. Marine Ecology. ISSN 0173-9565 Marine Ecology 31 (2010) 539–545 ª 2010 Blackwell Verlag GmbH 539