Alkaline phosphatase activity and regulation in the North Pacific Subtropical Gyre

Alkaline phosphatase (AP) activity (APA) was measured at several stations in the North Pacific Subtropical Gyre in July 2008, and in a series of nutrient addition experiments: nitrate plus ammonium (+N) or phosphate (+P), to study APA regulation and to evaluate the capacity of picoplankton organisms (i.e., in the 0.2-2-μm size range) to access the AP-hydrolyzable fraction of dissolved organic phosphorus (DOP). The data indicated a primary limitation of the biomass by nitrogen. Both total (measured with a soluble DOP analog) and cell-specific (measured with the enzyme-labeled fluorescence [ELF] phosphate cell labeling method) APA were enhanced in the +N samples and reduced in the +P samples, suggesting that DOP is an important resource for picoplankton nutrition. Cell-free APA represented > 65% of the APA in all samples, but its contribution to total APA significantly decreased in the +N treatment as microbial biomass increased. In the +N treatment, < 5% and up to 96% of the cells in the heterotrophic bacteria-enriched and picophytoplankton-enriched fractions, respectively, were ELF-alcohol-labeled after 5 d. Following N enrichment, the microbial assemblage shifted from cell-free phosphatase dominated under N limitation and P stress (i.e., physiological response) to picophytoplankton-based phosphatase dominated under P limitation (i.e., production or growth rate limitation). If, as predicted, the ocean evolves towards P limitation, DOP availability would become of major importance to sustain productivity.