Dissolved organic nitrogen uptake by seagrasses

We examined the ability of seagrasses to take up dissolved organic nitrogen (DON) with leaves (in situ) and roots (laboratory) in an oligotrophic tropical offshore meadow in Indonesia using 15N-labeled nitrogen (N) substrates. We compared the uptake of urea and amino acids with that of ammonium (NH z4 ) and nitrate (NO { 3 ) and determined uptake kinetics of amino acids for the seagrasses Thalassia hemprichii, Halodule uninervis, and Cymodocea rotundata in comparison with the macroalgae Sargassum sp. and Padina sp. Uptake rates of small DON substrates for macroalgae were higher than those for seagrass leaves for all N substrates, but the seagrass roots also had a considerable uptake capacity. Seagrass leaves preferred urea, NH z4 , and NO { 3 over amino acids, and there were differences between species. Seagrass roots, however, took up amino acids at rates comparable to NH z4 , whereas uptake rates of urea and NO { 3 were much lower. The ability to take up DON enables seagrasses and macroalgae to shortcut N cycling and gives them access to additional N resources. In oligotrophic environments, uptake of amino acids by roots may provide seagrasses with a competitive advantage over macroalgae. Seagrasses, like all other plants, need nitrogen (N) to maintain their high productivity. In contrast to many terrestrial plants, resorption of N from senescing leaves in seagrasses is low and loss due to leaf detachment is high (Stapel and Hemminga 1997; Romero et al. 2006). Because seagrasses flourish in oligotrophic areas, it seems paradoxical that they can collect sufficient N to survive. It is well established that seagrasses take up N by both leaves and roots in amounts depending on the relative availability in the sediment and in the water column (Stapel et al. 1996; Lee and Dunton 1999). The majority of the research on N uptake by seagrass leaves and roots has focused on uptake of ammonium (NH z4 ) and nitrate (NO { 3 ; Stapel et al. 1996; Romero et al. 2006). As concentrations of these inorganic N sources are generally low in seagrass habitats (Lee and Dunton 1999; McGlathery et al. 2001), this raises the question if other important N sources exist. Recent insights indicate that the dissolved organic nitrogen (DON) pool contains a ‘‘doughnut’’ fraction composed of relatively labile compounds such as amino acids and urea, with a relatively high turnover (Bronk et al. 2007). Dissolved amino acids and urea are released during decomposition of organic material. Besides, urea is also produced by animal excretion (Bronk 2002) and is often present in run-off from agricultural fields to coastal waters (Glibert et al. 2006). Particularly in oligotrophic environments, where most N is available in the dissolved organic form (Bronk 2002; Bronk et al. 2007), small DON molecules could serve as potentially important N substrates for seagrasses. Whereas it is clear that certain groups of organisms such as phytoplankton may obtain a substantial part of their N nutrition from this DON fraction (Bronk et al. 2007), it remains largely unknown whether seagrasses can do the same. Recent studies demonstrated that N present in phytodetritus from plankton trapped within the sediment can be rapidly taken up by seagrass plants (Evrard et al. 2005; Barrón et al 2006). These in situ labeling experiments, however, did not reveal whether N was taken up in the dissolved inorganic form (DIN) after complete mineralization or as DON. Various terrestrial plants (Chapin et al. 1993; Persson and Nasholm 2001) and microand 1 Corresponding author ([email protected]). Acknowledgments We thank Piet Nienhuis and two anonymous reviewers for useful comments on the manuscript, Alfian Noor for administrative arrangements in Indonesia, and Saido for assistance with field work. This work was funded by NWO-WOTRO grant W86-168. Limnol. Oceanogr., 53(2), 2008, 542–548 E 2008, by the American Society of Limnology and Oceanography, Inc.