Plasticity in the grazing ecophysiology of Florenciella(Dichtyochophyceae), a mixotrophic nanoflagellate that consumes Prochlorococcusand other bacteria

Mixotrophic nanoflagellates can account for more than half of the bacterivory in the sunlit ocean, yet very little is known about their ecophysiology. Here, we characterize the grazing ecology of an open‐ocean mixotroph in the genus Florenciella(class Dictyochophyceae). Members of this class were indirectly implicated as major consumers of Prochlorococcusand Synechococcusin the oligotrophic North Pacific Subtropical Gyre, but their phagotrophic capabilities have never been investigated. Our studies showed that Florenciellareadily consumed Prochlorococcus, Synechococcus, and heterotrophic bacteria, and that the ingested prey relieved nutrient limitations on growth. Florenciellagrew faster (3 d−1) in nitrogen‐deplete medium given sufficient live Synechococcus, than in nitrogen‐replete K medium (2 d−1), but it did not grow in continuous darkness. Grazing rates were substantially higher under nutrient limitation and showed a hint of diel variability, with rates tending to be highest near the end of the light period. An apparent trade‐off between the maximum clearance rate (5 nL Florenciella−1h−1) and the maximum ingestion rate (up to ∼ 10 prey cells Florenciella−1h−1) across experiments suggests that grazing behavior may also vary in response to prey concentration. If the observed grazing rates are representative of other open‐ocean mixotrophs, their collective activity could account for a significant fraction of the daily cyanobacterial mortality. This study provides essential parameters for understanding the grazing ecology of a common marine mixotroph and the first characterization of mixotrophic nanoflagellate functional responses when feeding on unicellular cyanobacteria, the dominant marine primary producers in the oligotrophic ocean.