Characterization of alkaline phosphatase activity in the north and south pacific subtropical gyres: Implications for phosphorus cycling

We characterized alkaline phosphatase (AP) activity (APA) and report AP kinetic parameters in the North and South Pacific Subtropical Gyres (NPSG and SPSG, respectively) to evaluate the relative importance of APA in dissolved organic P (DOP) remineralization. APA potential hydrolysis rates were low but measurable (0.10 ±0.06 nmol L^-1 h^-1, n =12, and 0.13 6 0.08 nmol L^-1 h^-1, n = 6) in surface samples from the NPSG and central SPSG, respectively. When compared to dissolved inorganic P (DIP) uptake rates, the potential utilization of DOP by APA may be important for microbial communities in these oligotrophic habitats. Assuming an upper (80%) and a lower (10%) fraction of the DOP to be AP-hydrolyzable, APA could represent 25% 6 18% (n = 11) and 4% 6 3% (n = 11) of the DIP uptake, respectively, in surface waters of the NPSG and 147% 6 86% (n = 6) and 20% 6 12% (n = 6), respectively, in surface waters of the central SPSG. The ratio of APA to DIP uptake was significantly larger in the SPSG compared to the NPSG despite having higher DIP and lower DOP. The halfsaturation constant (K_m) was in the range of DOP concentrations measured in the two gyres, meaning that microorganisms can adjust their DOP utilization according to DOP availability. Dissolved APA was a large fraction of total APA in the NPSG, contributing between 12% and 100% (41 ± 23%, n 5 23) of total APA at Sta. ALOHA. Utilization of the AP-hydrolyzable fraction of the DOP probably contributes to production in the extensive Pacific oligotrophic gyres.