STUDY ON PHOSPHATE-UPTAKE OF THE MARINE CYANOPHYTE SYNECHOCOCCUS SP NIBB-1071 IN RELATION TO OLIGOTROPHIC ENVIRONMENTS IN THE OPEN-OCEAN

Inorganic phosphate (Pi) uptake by the marine cyanophyte Synechococcus sp. NIBB 1071 was studied using cells grown in an artificial seawater medium. The phosphate uptake was markedly enhanced in cells grown in the medium of low phosphate concentrations (phosphate-limited cells) t han in cells grown in the phosphate-rich medium (phosphate-replete cel ls). The diagnosis of kinetics of instantaneous phosphate-uptake showe d that V-max of the former was more than two orders of magnitude great er than that of the latter, and the k(m) of the former was about 1/20 of that of the latter. The enhancement of the phosphate uptake was com pleted after a 40-h incubation of phosphate-replete cells in the phosp hate-free medium. The activation was suppressed by chloramphenicol, an inhibitor of protein synthesis. The uptake developed in phosphate-lim ited cells was energy dependent and susceptive to osmotic shock, which suggests the involvement of a periplasmic phosphate-binding protein, analogous to that found in heterotrophic gram-negative eubacterial cel ls. The relationship between phosphate quota and growth rate, together with the kinetical data for phosphate uptake, predicted that ambient phosphate as low as 0.5 nM could support cell growth at a rate of one division per day. Results indicate that cells can grow rapidly even at phosphate concentrations as low as nanomolar levels. A possible regul atory mechanism of phosphate uptake in marine Synechococcus spp. is di scussed in relation to a wide distribution of this picophytoplankton i n the ocean environment.