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

Abstract

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 Prochlorococcus and Synechococcus in the oligotrophic North Pacific Subtropical Gyre, but their phagotrophic capabilities have never been investigated. Our studies showed that Florenciella readily consumed Prochlorococcus, Synechococcus, and heterotrophic bacteria, and that the ingested prey relieved nutrient limitations on growth. Florenciella grew 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 −1 h −1) and the maximum ingestion rate (up to $ 10 prey cells Florenciella −1 h −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.

Annual phytoplankton dynamics in coastal waters from Fildes Bay, Western Antarctic Peninsula

“…Dictyochophyceae were most abundant in the spring under low light conditions. Some of the main ASVs were assigned to Pedinellales, which are known mixotrophs 58 , and also to the genus Florenciella, which has been very recently determined to be mixotrophic feeding on heterotrophic bacteria as well as cyanobacteria 59 . In contrast, Pyramimonadophyceae which harbor several mixotrophic species 57,60 were most abundant in the summer, suggesting that the occurring species were probably not mixotrophic.…”

Diel-Regulated Transcriptional Cascades of Microbial Eukaryotes in the North Pacific Subtropical Gyre

“…Ochrophytes were represented by the silicoflagellate genus Dictyocha (order Dictyochales), the genus Florenciella (order Florenciellales), and the pico-eukaryotic (<2 μm cell size diameter) genus Pelagococcus (order Pelagomonadales). Both Dictyocha and Florenciella include known mixotrophic species ( Quéguiner, 2016 ; Li et al, 2021 ), whereas Pelagococcus is thought to be strictly photosynthetic ( Lewin et al, 1977 ). Dinoflagellates were represented by the genus Karlodinium (order Gymnodiniales), which contains mixotrophic species, similar to many of the dinoflagellate genera observed in this study ( Faure et al, 2019 ).…”

Disentangling top-down drivers of mortality underlying diel population dynamics ofProchlorococcusin the North Pacific Subtropical Gyre

“…Mixotrophic nanoflagellates are expected to contribute to the “other” loss of Prochlorococcus , but measurements suggest this is a small contribution relative to the heterotrophic nanoflagellates [21, 42]. Unknown viruses (like RNA viruses), as well as other grazer types including larger consumers may be contributing to the ‘other mortality’ terms in our models.…”