Conclusion
The protist community in the Sargasso Sea shows a dynamic seasonality in the epipelagic, responding to hydrographic yearly cycles. Mixotrophic lineages, able to take advantage of the smaller picophytoplankton and heterotroph bacterioplankton, dominate throughout the year; however, autotrophs bloom during the rapid transition between the winter mixing and the stratified summer. Pure heterotrophs have their peak moment at the end of summer, when the base of the thermocline reaches its deepest depth, and likely mixotrophs lose part of their photosynthetic advantage. Below the photic zone, the community, dominated by Rhizaria, is depth-stratified, relatively constant throughout the year. These populations, relying on the vertical flux of organic matter, respond to local hydrographic and biological features such as the oxygen minimum zone. Together, this suggests a dynamic partitioning of the water column, where the niche vertical position for each community changes throughout the year, likely depending on nutrient availability, the mixed layer depth, and other hydrographic features. Future research should address several uncertainties needed to implement these data into biogeochemical models. The most prominent problem to solve is the lack of lineage-level quantitative data (i.e., counts), which would allow a better understanding of the energy flows. Most of the lineages we report on here are 2-20 µm in cell diameter, falling in the range where taxon-specific cell counting is most difficult, between flow cytometry and automated image analyses. A second uncertainty is clarifying the functionality (trophic modes) in those lineages (e.g. MOCH groups with autotrophs, mixotrophs, and heterotrophs), to more accurately associate these strategies with the temporal and spatial patterns in the system. Together with the present data, these would allow better characterization of the carbon flux throughout the water column, and the role of mixotrophs and heterotrophs in controlling the biogeochemical cycles in the oligotrophic oceans.