Phosphorus stress of microphytoplankton community in the western subtropical North Pacific

Phosphorus stress in microphytoplankton (diatoms, dinoflagellates, Trichodesmium spp.) and small-sized unicellular cyanobacteria was assayed using a single-cell assay [enzyme-labelled fluorescence (ELF)]. The fractions of ELF-labelled cells varied within and between taxonomic groups along the 141.5E transect between 20.47N and 31.6N. High percentages of ELF-labelled dinoflagellates (6689) and Trichodesmium spp. (approximate to 100) were consistently observed at the surface over the study area. In contrast, the fractions of ELF-labelled diatoms increased in the surface layer from north to south (1083), along with the decrease in phosphate concentrations (96 nM to 4 nM). Phosphate amendment experiments conducted in the phosphate-depleted southernmost station showed that phosphorus stress of diatoms and dinoflagellates was alleviated following phosphate addition. In contrast to the microphytoplankton communities, low percentages of ELF-labelled small-sized cyanobacteria were observed throughout the study area, suggesting that these cells were not P-limited. Our results revealed that dissolved organic phosphorus utilization could potentially be important for microphytoplankton in the western subtropical North Pacific.