Nutrient limitation of picophytoplankton photosynthesis and growth in the tropical North Atlantic

Identification of the proximal nutrient limiting primary production is a necessary first step toward evaluating the physiological state of phytoplankton communities and the biogeochemical constraints on the current oceanic carbon cycle. We conducted 48-h nutrient addition bioassay experiments to evaluate nitrogen, phosphorus, and iron limitation of primary productivity, net chlorophyll synthesis, and net increase in cell numbers of the dominant picophytoplankton from the tropical North Atlantic. Our results indicate that N was the proximal limiting factor for primary production during the autumn of 2002, followed by P and then Fe. Net chlorophyll synthesis was significantly stimulated by addition of N alone and further stimulated by addition of P. Analysis of picophytoplankton populations by analytical flow cytometry revealed a more complex response. Cellular red fluorescence, an index of cell chlorophyll content, increased in Prochlorococcus, Synechococcus, and picoeukaryotes in response to addition of NH4NO3 but was not affected by single or combined additions of P and Fe. In contrast, cell abundance in these picophytoplankton populations increased only after combined N and P (63% of comparisons) or N and Fe (21% of comparisons) additions. Thus, our experiments revealed that chlorophyll synthesis and primary production were limited by the availability of nitrogen alone, while net increase in cell abundance was colimited by N and P or N and Fe in the majority of these picophytoplankton populations.