Life and death ofCrocosphaerasp. in the Pacific Ocean: Fine scale predator-prey dynamics

In the North Pacific Subtropical Gyre, the daily pulse of photosynthetic carbon (C) fixation is closely balanced by losses. This concert of growth and loss is driven by a diverse assemblage of plankton, including the diazotrophCrocosphaerasp. While primary production is relatively well characterized in this ecosystem, the extent of C transfer to secondary producers is poorly constrained. Here, we use automated imaging flow cytometry and population modeling to study the coupling of C production byCrocosphaeraand subsequent grazing by nanoplanktonic protists.Crocosphaeracells represent on average 30% of the nanoplankton detected by the Imaging FlowCytoBot in the surface layer of mesoscale eddies during summertime. The size spectra show a maximum in the frequency ofCrocosphaeradoublet cells just prior to mitotic division at midday, with an average estimated growth rate of 0.8 +/- 0.5 d(-1). We also identified potential predators by fitting a Lotka-Volterra model to plankton time series observations. Significant predators include the dinoflagellatesProtoperidiniumandDinophysisas well as the ciliateStrombidium, which were all imaged withCrocosphaerain food vacuoles. The estimated C demand of the main grazers fluctuated between 25% and 250% ofCrocosphaeranew production in an anticyclonic eddy where we observed the onset of aCrocosphaera-driven bloom. HeterotrophicProtoperidiniumdrove most of the estimated C demand, with grazing rates nearly equivalent toCrocosphaeragrowth rates (0.6 +/- 0.4 d(-1)on average), but saturating at high prey concentrations. Our novel results demonstrate tight coupling between specific protistan predators and a diazotrophic prey.