Inorganic nitrogen acquisition by the tropical seagrass Halophila stipulacea

The tropical seagrass H alophila stipulacea is dominant in most regions of the I ndo‐ P acific and the R ed S ea and was introduced into the M editerranean S ea after the opening of the S uez canal. The species is considered invasive in the M editerranean S ea and has been progressively colonizing new areas westward. Growth and photosynthetic responses of H . stipulacea have been described but no information is yet available on the nitrogen nutrition of the species. Here we simultaneously investigated the uptake kinetics of ammonium and nitrate and the internal translocation of incorporated nitrogen in H . stipulacea using 15 N ‐labelled substrates across a range of N i levels (5, 25, 50 and 100 μ m ). The ammonium uptake rates exceeded the nitrate uptake rates 100‐fold, revealing a limited capacity of H . stipulacea to use nitrate as an alternative nitrogen source. The uptake rates of ammonium by leaves and roots were comparable up to 100 μ m 15 NH 4 Cl . At this concentration, the leaf uptake rate was 1.4‐fold higher (6.22 ± 0.70 μmol·g −1 DW h −1 ) than the root uptake rate (4.54 ± 0.28 μmol·g −1 DW h −1 ). The uptake of ammonium followed Michaelis–Menten kinetics, whereas nitrate uptake rates were relatively constant at all nutrient concentrations. The maximum ammonium uptake rate ( V max ) and the half‐saturation constant ( K m ) of leaves (9.79 μmol·g −1 DW h −1 and 57.95 μ m , respectively) were slightly higher than that of roots (6.09 μmol·g −1 DW h −1 and 30.85 μ m , respectively), whereas the affinity coefficients (α = V max /K m ) for ammonium of leaves (0.17) and roots (0.20) were comparable, a characteristic that is unique among seagrass species. No substantial translocation (<2.5%) of 15 N incorporated as ammonium was detected between plant parts, whereas the translocation of 15 N incorporated as nitrate was higher (40–100%). We conclude that the N i acquisition strategy of H . stipulacea , characterized by a similar uptake capacity and efficiency of leaves and roots, favors the geographical expansion potential of the species into areas with variable water‐sediment N levels throughout the M editerranean.