Phosphorus dynamics in the North Pacific subtropical gyre

Phosphorus (P) dynamics were studied during several research cruises to Stn ALOHA and in the 'Climax region' of the North Pacific subtropical gyre (NPSG) in 1996-1997. The aim of this study was to: (1) investigate the coupled uptake and regeneration of inorganic phosphate (Pi) and the production of dissolved organic phosphorus (DOP), (2) quantify the size of the biologically available P (BAP) pool, and (3) estimate the relative bioavailability of select organic P compounds to the natural microbial assemblages. At all stations, the microbial community was dominated by prokaryotes (>99.5% of total cell numbers); Prochlorococcus spp. was the dominant pigmented group (>97% by numbers), comprising 20 to 30% of the total prokaryotic population. Phosphate uptake rates were 3.0 to 8.2 nM d(-1) (median = 3.5 nM d(-1)) and P pool turnover times ranged from 2 to 40 d (median = 9 d). The BAP pool generally exceeded the Pi pool, suggesting rapid turnover of at least a portion of the much larger DOP pool. The net production of DOP was approximately 10 to 40% of the net P uptake. Both the dissolved and the particulate organic matter pools were enriched in carbon (C) and nitrogen (N) relative to P, compared to the Redfield molar stoichiometry of 106C:16N:1P. The half-saturation constant, K-m, values for P-i uptake were higher than the ambient Pi pool concentrations, and uptake rates were positively correlated with exogenous P-i additions over the range tested (P-i = 25 to 250 nM). The bioavailabilities of exogenous adenine and guanine nucleotides were generally higher than other organic P compounds we tested. The net P-i regeneration rate from nucleotides was up to 50 times higher than the net P-i uptake rates, indicating a large potential for the regeneration of P-i from specific organic compounds. These P pool dynamics observed in the NPSG are consistent with a microbial community currently under P control.