Diel and vertical patterns of alkaline phosphatase activity in the oceanic dinoflagellate Pyrocystis noctiluca1

Abstract

Alkaline phosphatase activity (APA) was measured at stations in the Caribbean and Sargasso Seas on batches of 100–150 Pyrocystis noctiluca isolated from double‐closing net tows at various depths at 4–6‐h intervals. A diel pattern of activity was suggested, with the highest activity during the day and the lowest several hours before sunrise. The APA was highest in the mixed layer and lowest at the base of the euphotic zone. The dissolved N:P ratio declined 2–3‐fold when hydrolyzable organic phosphorus was included as part of the phosphorus pool, implying that organic phosphorus could contribute significantly to the phosphorus available to phytoplankton having alkaline phosphatase. In laboratory experiments the activity of the enzyme was depressed at an orthophosphate concentration as low as 0.05 µM, and 3.0 µM completely repressed alkaline phosphatase within 120 min. APA was proportional to the light intensity during preconditioning rather than during incubation. A diel pattern of activity was observed in laboratory cultures of P. noctiluca due to changes in light cycle alone. In the field, daily variations in light, mediated by associated changes in cell metabolism, appear to be the effectors of the observed diel rhythm of APA in P. noctiluca.