Nitrogen Assimilation and Partitioning in the Mediterranean Seagrass Posidonia oceanica

Abstract

Abstract. The supply of nitrogen (N) often limits the productivity of marine macrophytes. In vitro and in vivo assays of glutamine synthetase (GS) activity were employed to investigate patterns of N assimilation by the Mediterranean seagrass Posidonia oceanica (L.) DELILE. Biomass‐specific GS activity wa measured in root tissue, in leaves within a shoot, in shoots collected at two sites during two season and over a depth range of 5–33 m. Root tissue was less important than shoot tissue in assimilating inorganic N in P. oceanica, due both to the small roots' biomass (ca. 3% of total plant biomass and greatly lower (10‐ to 50‐fold) GS activities. While the GS activity and N assimilatory potentia (biomass × GS activity; μmol N‐h‐‐1) were greatest in leaf 2, leaves 1 and 3–5 assimilated N a significant rates. Shoots from a site characterized by elevated N availability in the winter water column and no significant sediment N reservoir exhibited GS activities that were 9‐times higher than shoot from a more oligotrophic site. Shoot GS activities in July increased linearly from 5 to 33 m and wer correlated with light availability as defined by Hsat, (daily period during which photosynthetic reaction are light‐saturated). This may represent metabolic compensation by P. oceanica to maintain N influx. Factors contributing to the ecological success of P. oceanica include the ability to assimilate N under conditions of severe light limitation (< 35 μmol photons. m‐2. s‐1), and metabolic plasticity to ensur the de novo generation of N‐containing organic compounds.