Alkaline phosphatase activity and regulation in the North Pacific Subtropical Gyre

Duhamel, Solange; Dyhrman, Sonya; Karl, David M.

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, espectively, 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.

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Limnology and Oceanography

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