Presence of dissolved nucleotides in the North Pacific Subtropical Gyre and their role in cycling of dissolved organic phosphorus

Dissolved and particulate nucleotide triphosphate (NTP) concentrations were measured in the upper 1000 m of the water column during 3 summer and 3 winter months at Stn ALOHA (22.75 degrees N, 158 degrees W) in the oligotrophic North Pacific Subtropical Gyre. In the euphotic zone (0 to 175 m) particulate adenosine-5'-triphosphate (P-ATP) and dissolved ATP (D-ATP) concentrations were positively correlated in summer (0 to 175 m; r(2) = 0.61, p < 0.001, n = 24), but not in winter (r(2) = 0.02, n = 24). D-ATP comprised > 65 % of the total ATP (T-ATP) pool in the summer, and similar to 50% in winter. Dissolved guanosine-5'-triphosphate (D-GTP) inventories were 5- to 6-fold greater in July and August than those observed in mid-June (1.1 +/- 0.1 vs. 8.0 +/- 0.6 and 6.5 +/- 0.3 mu mol D-GTP m(-2), respectively) and winter concentrations were on average lower than the mean summer concentrations. The particulate GTP (P-GTP) inventories were almost twice those measured in winter (mean 1.1 +/- 0.4 in summer vs. 0.6 +/- 0.1 mu mol P-GTP m(-2) in winter, n = 3). These results are consistent with higher microbial growth rates in summer. Uptake of D-ATP showed multi-phasic kinetic patterns. The half-saturation constants (K(t)) ranged from 1 to 26 nM at D-ATP concentrations of 0.2 to 30 nM, and V(max) ranged from 0.3 to 1.4 nM d(-1). At concentrations > 30 nM, K(t) exceeded 100 nM and V(max) was 10.4 nM d(-1). The calculated net production rates of D-ATP ranged from 40 to 150 pM d(-1), and the turnover time of the ambient D-ATP pool was estimated to be 1 to 2 d. The P flux through the D-ATP pool could potentially be 5 times faster than that of the bulk DOP pool, implying that P derived from nucleotides may be an important pathway in the P cycle of oligotrophic oceans.