Light and dark phosphate (PO43-) uptake rates were investigated in the North Pacific Subtropical Gyre (NPSG) and along a coastal to open ocean transect in the South Pacific Ocean. PO43- uptake rates were consistently higher when incubated in the light, but the ratio of uptake in the light and dark (L: D) decreased with depth. In the NPSG, the L:D ratio of euphotic-layer integrated PO43- uptake was 1.58 +/- 0.08 (+/- SE, n = 15 profiles), and the average L:D ratio was 1.60 +/- 0.45 (+/- SE, n = 42) between 5 and 45 m and 1.25 +/- 0.70 (+/- SE, n = 28) between 150 and 175 m. The L: D ratio was higher for the pigmented plankton-enriched size fractions (0.6 to 2 mu m and > 2 mu m), but results were difficult to interpret at the oligotrophic stations where non-pigmented and pigmented plankton cell sizes overlapped. Group-specific measurements obtained using flow cytometric cell sorting demonstrated that Prochlorococcus PO43- uptake rates were higher when the samples were incubated under ambient light. Adenosine-5'-triphosphate (ATP) uptake by Prochlorococcus was also higher in the light for the uptake of both the terminal PO43- group ([gamma-P-33]ATP) and adenine moiety ([2,8-H-3]ATP). This could be the result of secondary uptake of PO43- and/or adenine after ATP cleavage by non-pigmented picoplankton. There was no significant difference in P-assimilation by non-pigmented picoplankton between light and dark incubated samples. Light dependence of phytoplankton PO43- uptake could thus influence the functioning of the microbial loop and the flows of matter and energy in marine environments by creating temporal patterns of resource utilization.