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.