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.
