Alkaline Phosphatase Activity in Zostera noltii Hornem. and its Contribution to the Release of Phosphate in the Palmones River Estuary

Alkaline phosphatase activity (APA) was studied in Zostera noltii Hornem., a sea-grass collected in the Palmones river estuary (southern Spain). The higher activity was found in the leaves, with minor contributions in the stem and the underground parts of the plant. The enzymatic activity showed a two-phase kinetic versus substrate concentration between 5 μM and 25 mM. The influence of some environmental factors important in nature (temperature, pH, salinity, photon irradiance and external phosphate) on the enzymatic activity is discussed. Over an ecophysiological range of these factors, maximum APA was found at 30 °C (22•6 μmol pNP released g dry wt-1 h-1), pH 8•8 (35•6 μmol pNP g dry wt-1 h-1) and salinity 43•8 (27•8 μmol pNP g dry wt-1 h-1). With regard to light, APA and phosphate uptake in shoots were light-saturated and showed similar values for maximum velocity and half-saturation constant. In the range of phosphate concentration tested (0-20 μM), APA was independent of the external phosphate concentration. Finally, as Z. noltii incorporated only 16% of the phosphate hydrolysed from the model phosphomonoester used in the assay, the significance of Z. noltii population in the enzymatic release of phosphate to the estuary was estimated. A minimum of 8•4 nM Pi liberated per day and a maximum of 99•8 nM Pi day-1 was found.