The protist community traces seasonality and mesoscale hydrographic features in the oligotrophic Sargasso Sea

Abstract

Protists represent the majority of the eukaryotic diversity in the oceans. They have different functions in the marine food web, playing essential roles in the biogeochemical cycles. While the available data is rich in horizontal and temporal coverage, little is known on their vertical structuring, particularly below the photic zone. The present study applies V4 18S rDNA metabarcoding to samples collected over three years in conjunction with the BATS time-series to assess marine protist communities in the epipelagic and mesopelagic zones (0-1000 m). The protist community showed a dynamic seasonality in the epipelagic, responding to hydrographic yearly cycles. Mixotrophic lineages dominated throughout the year. However, autotrophs bloomed during the rapid transition between the winter mixing and the stratified summer, and heterotrophs had their peak at the end of summer, when the base of the thermocline reaches its deepest depth. Below the photic zone, the community, dominated by Rhizaria, is depth-stratified and relatively constant throughout the year, although they followed local hydrographic and biological features such as the oxygen minimum zone. The results suggest a dynamic partitioning of the water column, where the niche vertical position for each community changes throughout the year in the epipelagic, likely depending on nutrient availability, the mixed layer depth, and other hydrographic features. At depth, the protist community closely tracked mesoscale events (eddies), where the communities followed the hydrographic uplift, raising the deeper communities for hundreds of meters, and compressing the communities above.

The photic‐aphotic divide is a strong ecological and evolutionary force determining the distribution of ciliates (Alveolata, Ciliophora) in the ocean

“…This pattern is likely explained by abiotic and biotic factors that correlate with depth; for example, in our study, depth was negatively related to temperature, salinity, oxygen, and fluorescence. This is in general agreement with changes in productivity and overall environmental conditions known to affect protist alpha-diversity in the vertical profile, especially from photic to aphotic waters (Blanco-Bercial et al, 2022;Schnetzer et al, 2011;Sogawa et al, 2022).…”

“…The field and laboratory procedures for environmental characterization and DNA metabarcoding have been detailed by Blanco-Bercial et al (2022). In brief, vertical profiles of water temperature, salinity, oxygen concentration, and chlorophyll fluorescence were measured with a CTD from the surface down to a 1000-m depth at the BATS station (31°40′N, 64°10′W) monthly from February 2016 to December 2018.…”

“…For metabarcoding, 4 L of seawater were immediately filtered on 0.22 μm Sterivex filters (MERCK). Details on DNA extraction, PCR amplification of the hypervariable V4 region of the 18S rRNA gene (using primers by Stoeck et al, 2010), library preparation, and Illumina MiSeq sequencing using 2x250 PE V2 chemistry have been published (Blanco-Bercial et al, 2022). Raw reads for a total of 369 samples are available in the Sequence Read Archive (PRJNA769790).…”

“…Continuous variables (sample depth, temperature, salinity, oxygen concentration, and CTD fluorescence) were standardized to scale each variable to a zero mean and unit variance. Categorical variables include layer (11 layers: L0 to L2 and L3 to L10 corresponding to photic and aphotic waters, respectively) and season (mixed, spring, stratified, and fall), as assigned by Blanco-Bercial et al (2022). The environmental metadata is provided in Table S2.…”

“…Here, we study the diversity, taxonomic composition, and phylogenetic partitioning of marine ciliates in photic and aphotic waters by using DNA metabarcoding at the most detailed vertical (12 depths across 1000 m) and temporal (monthly over 3 years) scales so far used for this goal. Taking advantage of a protist dataset obtained at one of the longest oceanic time-series, the Bermuda Atlantic Time-series Study (BATS) in the Sargasso Sea (Blanco-Bercial et al, 2022), and using phylogeny-based approaches, we tested the following hypotheses: (1) ciliate alpha-diversity decreases significantly with depth; (2) beta-diversity shifts significantly from photic to aphotic waters; (3) ciliate lineage distributions are partitioned in the vertical profile, possibly reflecting different trophic strategies and evolutionary histories across depth.…”