Dataset: APA In-situ Experiments

Sampling and analytical procedures:

 Seawater samples were collected in triplicate from Niskin bottles (12 L), at three stations (St. 1, St. 2 and St. 3) along a transect from coastal Georgia to offshore waters. Latitude (lat) and longitude (lon) of sampling sites are provided in .mat files. Stations 1 and 2 were on the continental shelf (15–30 m depth), while Station 3 was on the shelf break (220 m depth) adjacent to the Gulf Stream. At each station, samples were collected at 2-3 depths between the surface (4–5 m) and 50 m for alkaline phosphatase activity measurements (APA).

APA was determined fluorometrically using 4-methylumbelliferyl phosphate (MUF-P), as representative of phosphomonoesterase activity. APA was measured in triplicate samples from both bulk (total APA) and filtered (<0.2 µm, dissolved APA) seawater for each sampling depth. Hydrolysis rates were determined by separately incubating 200 µL of seawater with 8 different concentrations (0, 0.1, 0.2, 0.5, 1, 5, 10, 20 µM; final concentrations) of MUF-P in 96-well black microtiter plates. To ensure linearity of the hydrolysis rate, the increase in fluorescence was measured (excitation/emission wavelength: 359/449 nm) at multiple time points over an incubation period of 24 h.  Maximum hydrolysis rate (Vmax) and Michaelis-Menten constant (Km) were determined, using a nonlinear regression based on the rectangular hyperbolic function following: V=Vmax x S/ Km+ S, where S and V are the concentrations of the substrate and the hydrolysis rates, respectively.   

Instruments:  Sampling was performed using Niskin bottles (12 L) mounted on a rosette.  Reading of fluorescence was performed on a plate reader (SpectraMax® M2, Molecular Devices).

Location:  Northwestern Atlantic surface waters. Depth: surface-50 m.